Tagging study to determine mortality sources on cod in the Irish Sea : final report.
Date: 2022-08-09
Institutions: European Climate, Infrastructure and Environment Executive Agency
Subjects: European Maritime and Fisheries Fund, Irish Sea, aquatic ecosystem, conservation of fish stocks, fishery management, fishing vessel, report, sea fish

Tagging study to determine 
mortality sources on cod in 
the Irish Sea 

European Maritime and Fisheries Fund (EMFF) 

Written by Mathieu Lundy (AFBI), Victoria Poppleton (AFBI), Jonathan White (Cefas), Emma White 
(Cefas), Steven Beggs (AFBI), Pia Schuchert (AFBI), David Righton (MI), Chris Griffiths (MI), 
Pieter-Jan Schön (AFBI) 
August - 2022 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
   
    
   
“The information and views set out in this study are those of the author(s) and do not necessarily 
reflect the official opinion of CINEA or of the Commission. Neither CINEA, nor the Commission can 
guarantee the accuracy of the data included in this study. Neither CINEA, nor the Commission, or 
any person acting on their behalf may be held responsible for the use which may be made of the 
information contained therein.” 

EUROPEAN COMMISSION 

European Climate, Infrastructure and Environment Executive Agency 
Unit D.3.1 — Sustainable Blue Economy 

Contact: CINEA EMFAF CONTRACTS 

E-mail:  cinea-emfaf-contracts@ec.europa.eu   

European Commission 
B-1049 Brussels 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
EUROPEAN COMMISSION 

Tagging study to determine 
mortality sources on cod in 
the Irish Sea 

Final Report 

Service Contract 
EASME/EMFF/2015/1.3.2.5 

European Climate, Infrastructure and Environment Executive Agency 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Tagging study to determine mortality sources on cod in the Irish Sea 

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Luxembourg: Publications Office of the European Union, 2022 

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Tagging study to determine mortality sources on cod in the Irish Sea 

Table of Contents 

1. 

INTRODUCTION ................................................................................................... 11 

2.  METHODS ............................................................................................................ 13 

  Work package 1: Development of a tagging programme .................................. 13 
2.1.1.  Review of tagging methods ............................................................... 13 
2.1.2.  Tag loss rates ..................................................................................... 17 
2.1.3.  Post tagging mortality ........................................................................ 18 
2.1.4.  Study power ....................................................................................... 18 
2.1.5.  Analysis of pre-existing tagging data ................................................ 19 
2.1.6.  Geographic reconstruction of DST cod movements .......................... 27 
2.1.7.  Virtual recaptures from DST cod movements ................................... 27 
  Spatial distribution from Mark recovery recaptures .......................................... 28 
2.2.1.  Movement of cod between ICES areas .............................................. 32 
2.2.2.  Spatial distribution of data storage tags ............................................. 34 
2.2.3.  Virtual recaptures from DST cod movements ................................... 35 
2.2.4.  Development of a tagging study for 2016 - 2018 .............................. 46 
2.2.5.  Tagging Season .................................................................................. 49 
2.2.6.  Tagging area ...................................................................................... 50 
2.2.7.  Commercial fishery activity .............................................................. 51 
2.2.8.  Reporting recaptures .......................................................................... 53 
2.2.9.  The tagging plan ................................................................................ 53 
2.2.10.  Tagging Charters ............................................................................... 53 
2.2.11.  Research Vessel opportunities ........................................................... 54 
2.2.12.  At-sea observer programme ............................................................... 54 
2.2.13.  Recreational angling events ............................................................... 54 
2.2.14.  Communication and Outreach plan ................................................... 55 
2.2.15.  Commercial fishers: ........................................................................... 55 
2.2.16.  Recreational fishers ........................................................................... 56 
2.2.17.  Press 
2.2.18.  Engagement with stakeholders .......................................................... 57 
  Work package 2: The tagging campaign ......................................................... 58 
2.3.1.  First phase of implementation ........................................................... 58 
2.3.2.  Completed Charters ........................................................................... 58 
2.3.3.  At-sea observer programme ............................................................... 59 
2.3.4.  Research vessel activity ..................................................................... 59 
2.3.5.  Recreational fishers: .......................................................................... 59 
2.3.6.  Tagging in practice: ........................................................................... 59 
2.3.7. 
Interim review of tagging programme – November 2017 ................. 60 
2.3.8.  Redesign actions ................................................................................ 61 
2.3.9.  Research surveys: .............................................................................. 62 
2.3.10.  Recreational tagging volunteers ........................................................ 62 
2.3.11.  Ad-hoc tagging .................................................................................. 62 
2.3.12.  Update to reward scheme .................................................................. 63 

56 

 
Tagging study to determine mortality sources on cod in the Irish Sea 

2.3.13.  Second phase of tagging programme; January 2018 ......................... 63 
2.3.14.  Research Vessel Activity ................................................................... 64 
2.3.15.  At-sea Observer Programme ............................................................. 65 
2.3.16.  Recreational fishers - Shore & Sea angling competitions ................. 65 
2.3.17.  Voluntary Taggers ............................................................................. 66 
2.3.18.  Overview 66 
2.3.19.  Review of the tagging programme .................................................... 66 
  Work package 3: Results & Data analysis ....................................................... 68 
2.4.1.  Review of mark-recapture analysis methods ..................................... 68 
2.4.2.  Data analysis of recapture data .......................................................... 73 
2.4.3.  Fishing mortality sources of Irish Sea Cod ....................................... 78 
2.4.4.  Migration, mixing and spatial range .................................................. 79 
Influence of environmental factors .................................................... 83 
2.4.5. 
Irish Sea cod biology ......................................................................... 85 
2.4.6. 
Implication for assessment of stock status ........................................ 89 
2.4.7. 
  Work package 4: Management advice ............................................................ 93 
2.5.1.  Current management .......................................................................... 93 
Irish Sea cod tagging findings and future management .................... 95 
2.5.2. 

3.  DISCUSSION ....................................................................................................... 97 

4. 

REFERENCES ..................................................................................................... 100 

  Appendix 1. .............................................................................................. 106 

LIST OF TABLES 
Table 1. Results of power analysis (2 x 2 contingency test). .............................................. 19 
Table 2. The number of mark recovery tag recaptures by ICES area of release and seasonal 
quarter of recapture. Lr and Lc refer to length at release and recapture respectively. 
Q4 & Q1 are combined to show number of recaptures during the autumn and winter 
quarters; Q2 & Q3 are combined to show the number of recaptures during the spring 
and summer quarters. ................................................................................................... 22 

Table 3. Released number and mean length (in cm ±SD) of cod tagged with DSTs by 

month and year of release in each ICES management area. Numbers are also given for 
DSTs returned (to date as of January 2012) and tracks successfully reconstructed 
using the Hidden Markov Model (HMM) for all cod at liberty greater than 10 days. 24 

Table 4. Exchange of mark recovery tagged cod between ICES management areas, based 

on area of recapture. Recapture proportions in area of release are shaded in grey. .... 33 

Table 5. Exchange of DST ‘virtual’ cod recapture data between ICES management areas. 

Recapture proportions in area of release are shaded in grey. ...................................... 36 
Table 6. Summary of tagging targets by method, area and season: .................................... 54 
Table 7. Number of fish tagged each year by each tagging source. .................................... 73 
Table 8. Summary statistics by of tagged and released cod by year (2016, 2017 and 2018) 
and release area (ICES management division). All reported values are means. 
Uncertainty is reported as ± 1 standard de6ation. n= sample size. 7a, Irish Sea (ICES 
management division 7a). 7g, Celtic Sea (ICES management division 7g). 6a, West of 
Scotland (ICES management division 6a). .................................................................. 80 

Table 9. Von Bertalanffly plot growth curve parameters for cod the data are derived from 

commercial sampling of Irish Sea cod, fishery independent surveys and the current 
tagging for years 2012-2016. ....................................................................................... 87 

 
 
 
Tagging study to determine mortality sources on cod in the Irish Sea 

Table of Figures 

Figure 1. Examples of common external tags used and attachment site (all within dorsal 

muscle; adapted from McFarlane et al., 1990). ........................................................... 15 

Figure 2. T-bar anchor tag gun being inserted into the dorsal musculature of a mature cod. 
(b) Mature cod double-tagged with external t-bar anchor tags (Bendall & Randall 
2011). ........................................................................................................................... 17 

Figure 3. Release positions of Mark ID tagged cod. Release positions (white circles) in 

ICES sub divisions 6a (6a; West of Scotland), 7a (7a. Irish Sea), 7e (7e western 
English Channel), and 7f-g (7f-g; Celtic Sea). ............................................................ 20 

Figure 4. Mark recovery tag recapture positions of cod released in ICES area 6a (Scottish 
waters). Solid symbols show exact recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. ....................... 29 

Figure 5. Mark recovery tag recapture positions of cod released in ICES area 7a (Irish 
Sea). Solid symbols show exact recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. ....................... 30 

Figure 6. Mark recovery tag recapture positions of cod released in ICES area 7e & 7f 

(Celtic Sea). Solid symbols show exact recapture locations, while shading shows the 
probability density surfaces for 50% (hollow white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for (a) 7e cod recaptured during combined Q4 
& Q1 autumn and winter quarters; (b) 7e cod recaptured during combined Q2 & Q3 
spring and summer quarters; (c) 7f cod recaptured during combined Q4 & Q1 
quarters; (d) 7f cod recaptured during combined Q2 & Q3 quarters........................... 31 

Figure 7. Mark recovery tag recapture positions of cod released in ICES area 7g (Celtic 
Sea). Solid symbols show exact recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. ....................... 32 

Figure 8. Recapture positions of cod tagged with DSTs in (a) ICES area 7a West (Irish 

Sea), (b) ICES area 7a East (Irish Sea), (c) ICES area 7e (western English Channel), 
(d) ICES area 7f (Celtic Sea) and (e) ICES area 7g (Celtic Sea). White cross symbols 
show the release positions of DST-tagged cod, while solid black circles show the 
recapture positions of DST-tagged cod. ...................................................................... 37 

Figure 9. The daily spatial distributions of 81 cod (11,170 days of data) released in the 

following ICES areas: eastern Irish Sea 7a (yellow; 8 cod = 1457 days of data), 
western Irish Sea 7a (red; 24 cod = 4574 days of data), eastern Celtic Sea 7f (white; 7 
cod = 1135 days of data), western Celtic Sea 7g (dark blue; 24 cod = 2533 days of 
data), western English Channel 7e (green; 17 cod = 966 days of data). Solid coloured 
symbols show individual daily geolocated positions reconstructed for each cod at 
liberty. .......................................................................................................................... 38 

Figure 10. The daily spatial distributions of 81 cod (11,170 days of data) by seasonal 

quarter; a) quarter 1, b) quarter 2, c) quarter 3 and d) quarter 4 for each ICES area of 
release; 7a East (yellow), 7a West (red), 7e (green). 7f (white) 7g (dark blue). ......... 39 

Figure 11. The daily spatial distribution of cod by month, released in ICES area 7a (Irish 
Sea). Solid symbols show daily geolocated positions reconstructed for 32 cod 
coloured by month at liberty. Positions shown are for (a) 32 7a cod (2490 days of 
data) during the Q4 & Q1 autumn and winter quarters; (b) 31 7a cod (4047 days of 
data) during the Q2 & Q3 spring and summer quarters. ............................................. 40 

 
 
Tagging study to determine mortality sources on cod in the Irish Sea 

Figure 12. The daily spatial distribution of cod by month, released in ICES area 7e 
(western English Channel). Solid symbols show daily geolocated positions 
reconstructed for 17 cod coloured by month at liberty. Positions shown are for (a) 15 
7e cod (595 days of data) during the Q4 & Q1 autumn and winter quarters; (b) Six 7e 
cod (314 days of data) during the Q2 & Q3 spring and summer quarters. .................. 40 

Figure 13. The daily spatial distribution of cod by month, released in ICES area 7f (Celtic 
Sea). Solid symbols show daily geolocated positions reconstructed for seven cod 
coloured by month at liberty.  (a) Six 7f cod (415 days of data) during the Q4 & Q1 
autumn and winter quarters; (b) Five 7f cod (486 days of data) during the Q2 & Q3 
spring and summer quarters. ........................................................................................ 41 
Figure 14. The daily spatial distribution of cod by month, released in ICES area 7g (Celtic 

Sea). Solid symbols show daily geolocated positions reconstructed for 24 cod 
coloured by month at liberty.  (a) 24 7g cod (839 days of data) during the Q4 & Q1 
autumn and winter quarters; (b) 20 7g cod (1694 days of data) during the Q2 & Q3 
spring and summer quarters. ........................................................................................ 41 

Figure 15. Combined daily position likelihood surfaces (using Hidden Markov model, 

described by Pedersen et al., 2008) of 7a west (Irish Sea) DST cod releases, showing 
probable distribution estimates by seasonal quarter ; a) Q1, b)Q2, c) Q3 & d) Q4. 
Likelihood ‘hot spot’ surfaces are depicted in red (showing locations with highest 
degree of statistical certainty), through to ‘cold spot’ surfaces depicted in blue 
(showing locations with lower degree of statistical certainty). ................................... 42 

Figure 16. Combined daily position likelihood surfaces (using Hidden Markov model, 

described by Pedersen et al., 2008) of 7a east (Irish Sea) DST cod releases, showing 
probable distribution estimates by seasonal quarters; a) Q1, b)Q2, c) Q3 & d) Q4. 
Likelihood ‘hot spot’ surfaces are depicted in red (showing locations with highest 
degree of statistical certainty), through to ‘cold spot’ surfaces depicted in blue 
(showing locations with lower degree of statistical certainty). ................................... 43 

Figure 17. Combined daily position likelihood surfaces (using Hidden Markov model, 
described by Pedersen et al., 2008) of 7g (Celtic Sea) DST cod releases, showing 
probable distribution estimates by seasonal quarters; a) Q1, b)Q2, c) Q3 & d) Q4. 
Likelihood ‘hot spot’ surfaces are depicted in red (showing locations with highest 
degree of statistical certainty), through to ‘cold spot’ surfaces in blue (showing 
locations with lower degree of statistical certainty). ................................................... 44 

Figure 18. DST ‘virtual’ cod recapture positions of cod released in ICES area 7a (Irish 

Sea). Solid symbols show ‘virtual’ recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. ....................... 45 

Figure 19. DST ‘virtual’ cod recapture positions of cod released in ICES area 7g (Irish 

Sea). Solid symbols show ‘virtual’ recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. ....................... 46 
Figure 20. Overview of core fishing areas in the Irish Sea by gear type (Nephrops – black, 

dredging – green, seine – red dashed & demersal (cod/haddock/hake) – blue. These 
areas are defined from kernel density analysis of Vessel Monitoring System (VMS) 
data using gear information from logbooks for the UK fishing fleet (2007 – 2016). . 48 
Figure 21. Irish Sea trends in regulated gear nominal effort (kW*days-at-sea) 2003 – 2016 
for the international fishing fleet active in the Irish Sea. BT – beam trawls, GN – gill 
nets, GT – trammel nets, LL – Longlines, TR – trawls. Data are from the STECF 
(EWG 17-05) Fisheries Dependent Information report. .............................................. 49 

 
Tagging study to determine mortality sources on cod in the Irish Sea 

Figure 22. Commercial landings (t) from logbooks of cod in the Irish Sea by the UK 

fishing fleet. Quarters are coloured; black – quarter 1, red – quarter 2, green – quarter 
3 and blue – quarter 4. ................................................................................................. 50 

Figure 23. Average month commercial landings of cod taken in the Irish Sea by UK 

vessels (2006 – 2012). Symbol size is scaled to the landed weight. Data are derived 
from logbook linked Vessel Monitoring System (VMS) data. .................................... 51 

Figure 24. Distribution of commercial catches of cod above the MCRS during 2013 – 
2016. The data are derived from AFBI observer at-sea data collection programme 
sampling catches on-board commercial fishing vessels. The size of open black 
symbols are scaled to catch rates, red circles indicate sampling location. .................. 52 
Figure 25. Distribution of commercial catches of cod below MCRS 2013 – 2016. The data 

are derived from AFBI observer at-sea data collection programme sampling catches 
on-board commercial fishing vessels. The size of open black symbols are scaled to 
catch rates, red circles indicate sampling locations. .................................................... 52 
Figure 26. Information leaflet distributed amongst angling community ............................. 57 
Figure 27. Hypothetical movement paths between marking location ‘A’ and recapture 

location ‘B’. Panel I. shows a scenario where the marking and recapture interval is 
sufficient to define both direction and speed of movement. Panel II. shows a scenario 
where the marking and recapture interval is sufficient to define direction but 
insufficient to accurately represent fine detail and speed of movement. Panel III. 
shows a scenario where the marking and recapture interval is insufficient to define 
either direction or speed of movement. ....................................................................... 72 

Figure 28. Length frequency distribution of tagged cod by tagging activity. Data is 

aggregated across all sampling events for separate sources. ....................................... 74 
Figure 29. Map of showing haul/release positions and the number of tagged cod released in 
each years of the project 2016 - 2018. Symbols are scaled to the number of fish 
tagged in each fishing haul or event in the case of recreational angling. .................... 75 

Figure 30. Release and recapture positions of all recaptured cod reported until November 

2018. Triangle symbols denote tagging location with circles indicating recapture sites.
 ..................................................................................................................................... 76 
Figure 31. Recaptures of tagged cod in the Irish Sea (IS) and Celtic Sea (CS) in 2016, 2017 
and 2018. By regulated gears bottom trawls and seines with mesh: TR1 ≥ 100 mm; 
TR2 ≥ 70 mm and < 100 mm; TR3 ≥ 16 mm and < 32 mm; other = primarily gills 
nets and beam trawls. Not include recaptured tagged fish caught by anglers, where rod 
and reel fishing making up a small number of the total recaptures. ............................ 77 
Figure 32. Number of recaptures by regulated gear type and agreed Total Allowable Catch 
(TAC) in the Irish Sea (ICES 7a) in 2016, 2017 and 2018. ........................................ 78 

Figure 33. Summary Utilisation Distributions (UD) for tagged mark-recovery cod in 7a 
(Irish Sea) and 7g (Celtic Sea) in 2016 - 2018. Probability kernel density contours 
(KPDF) are stated. Core areas of the stocks are indicated by the 50% kernel contour 
and species range by the 95% contour......................................................................... 82 

Figure 34. Summary Utilisation Distributions (UD) for tagged mark-recovery cod in 7a 
(Irish Sea) and 7g (Celtic Sea) in 2016 - 2018. Probability contours are stated. Core 
areas of the stocks are indicated by the 50% kernel contour and species range by the 
95% contour. ................................................................................................................ 83 

Figure 35. Annual variation in mean monthly thermal habitat (bottom temperatures 

<=12°C) of study area 2015-2017. Habitat scale represents sum of months where 
thermal habitat was <=12°C. ....................................................................................... 84 

Figure 36. Monthly variation in thermal habitat (bottom temperatures <=12°C) of study 
area calculated in 2017. Blue areas represents thermal habitat <=12°C, whereas red 
areas represent bottom temperatures >12°C. ............................................................... 85 

 
Tagging study to determine mortality sources on cod in the Irish Sea 

Figure 37. Combined length frequency of tagged cod showing tagged individuals from all 

sources. ........................................................................................................................ 86 

Figure 38. Von Bertalanffly plot (growth curve) for cod the data is derived from 

commercial sampling of Irish Sea cod, fishery independent surveys and the current 
tagging project from 2017. .......................................................................................... 87 
Figure 39. Comparison of tagging study derived growth rates to Von Bertalanffy modelled 

change in length at age (quarter 1 mean length). The growth rate per year for cod at 
age 2 – 6 was calculated as the change in length within year from the Von Bertalanffy 
method and by growth since tagging or recaptured tagged cod. ................................. 88 
Figure 40. Composition of prey groups by number in cod stomach contents ..................... 89 
Figure 41. Retrospective analysis of stock status, as applied in the ICES annual assessment 
of cod in the division 7a in 2018 (ICES 2018a). Each panel shows comparison of the 
assessment model output for catch, spawning stock biomass (SSB), average fishing 
mortality for cod aged 2 – 4 (Fbar 2-4) and recruitment. The retrospective is applied 
for with the same model removing the final year for 5 years to compare the model 
predictions with subsequent model runs. ..................................................................... 90 
Figure 42. Time series and forecast of Irish Sea cod a) Spawning Stock Biomass (SSB) and 

b) Fishing mortality (F) under the two different hypotheses compared with the model 
used at WGCSE 2018 (ICES 2018). Migration was only included in the 3 most recent 
years (2015-2017). Hypothesis 1 - Blue: Migratory stock that returns to spawning 
sites in 7a (taking into account catches of 4+ year old fish in area 7g), Hypothesis 2 - 
Green: Emigration out of 7a (adjusted M) and Red: WGCSE baseline (stock 
assessment without considering migration). ................................................................ 92 

Figure 43. Cod in Division 7a. Summary of the stock assessment (weights in thousand 
tonnes). The assumed 2018 recruitment value is not shaded. Shaded areas in F and 
SSB plots and error bars in the recruitment plot represent 1 × standard de6ation. 
Uncertainty boundaries are not available for 2018 (ICES, 2018a).............................. 94 

 
1 

INTRODUCTION 

Recently there has been progress toward improving exploitation rates on fish 
stocks through EU regulations and fishery management policy. However, some 
stocks have not responded as expected to management action and their 
conservation status remains a concern. For such stocks a better understanding of 
the interaction between fisheries, the ecosystem and the environment is needed. 
Cod (Gadus morhua) in the Irish Sea (ICES Division 7a) is an example of such a 
stock with a fundamental need to better understand the factors that have an 
influence on the population dynamics. The EASME project “Report of a tagging 
study to determine mortality sources on cod in the Irish Sea 
(EASME/EMFF/2015/010)” provided this opportunity.  

In an effort to recover the Irish Sea cod stock, management measures have been 
implemented since 2000 through a series of EU regulations. These recovery 
measures initially focused on protecting the spawning aggregations of cod 
implementing a closed area for cod from mid-February to end of April in the 
Western Irish Sea (Commission Regulation (EC) No 304/2000). This was followed 
by a series of EU regulated management plans, which introduced numerous 
technical conservation measures, effort limitations and a 25% year on year 
reduction in Total Allow Catch (TAC). Between 2003 and 2018 ICES advised a 
zero catch for cod in the Irish Sea.  

Despite the implementation of conservation and various cod avoidance measures 
the stock showed little or no response, raising questions about unaccounted 
causes of mortality to cod in the study region. Three forms of mortality have been 
assumed to impact this stock: natural mortality, fishing mortality and 
unaccounted mortality. While natural mortality was assumed to be constant and 
fishing mortality had been reduced, the remaining unaccounted (or unexplained) 
mortality resulted in high uncertainty associated with estimated total mortality 
values from the stock assessment and scientific advice. Unexplained mortality 
increased to more than 10 times that of the observed mortality (i.e., commercial 
catch and constant natural mortality), which severely hampered the ability to 
forecast stock development as well as the evidence-based management of the 
stock. 

During 2003 – 2016 a divergence in the perception of the state of the stock 
between the scientific assessment and fishing industry developed (ICES 2016). 
This was exacerbated by the assessment showing continued high total mortality 
and assumed fishing pressure, despite reduced effort targeting cod and technical 
measures to reduce bycatch (ICES 2016). These measures on cod resulted in the 
significantly reduced fishing effort targeting gadoid species in the Irish Sea 
(STECF 2018). 

A number of initiatives have been established to address this with policy, industry 
and scientific stakeholders in particular the WKIrish ICES workshop series (ICES 
2016, ICES 2017a). The project reported on here has a central role in improving 
the understanding of cod population dynamics in the Irish Sea with a focus on 
accounting for all sources of mortality and addressing these. Cod recruitment in 
the Irish Sea has been severely impeded since the late 1990s and with the lowest 
observed value in 2016. Higher recruitment was observed however, in 2009 and 
2013. These stronger recruitments which led to growth in the stock suggest that 
under positive environmental conditions there remains potential in the stock to 
produce higher recruitment resulting in population growth (ICES 2018a).  

11 

 
 
The development of a new assessment model was completed in 2017 (ICES 
2017a). In 2017, the spawning-stock biomass (SSB) was assessed to have 
surpassed Blim (stock size below which there is a high risk of reduced recruitment) 
for the first time since the early 1990s, leading to non-zero catch advice from the 
2017 and 2018 ICES assessments. Despite the assessments showing reduced 
fishing mortality, there is little evidence of improvement in the age structure with 
a paucity of fish 5-years and older in survey datasets and commercial catches 
(ICES 2018). The assessment also shows a year on year downward revision of 
the SSB estimate, suggesting the stock growth forecast is over optimistic, as a 
result of some degree of mortality, or removal from the stock, being missed. This 
study will aid in explaining the uncertainty in the SSB estimates by examining cod 
behaviour and mortality patterns. 

The current tagging study ultimately aims to investigate and better understand 
the unaccounted mortality of mature fish and so aid in the assessment and 
management of the stock. From a management perspective, stock structure 
needs to be understood as well as the range of movement / migration. Species 
that exhibit significant mixing post spawning can be subject to high levels of 
exploitation if spatial ecology, migration and home range are not considered 
(Neat et al., 2014). The ICES Benchmark Workshop series (WKIrish) which aimed 
to provide ecosystem-based advice for the Irish Sea ecosystem, noted that there 
was a requirement to improve our understanding of the level of migration of 
mature fish north and south out of the Irish Sea (ICES 2015). 

Our current understanding of cod movements in the Irish Sea and adjoining areas 
relies on tagging studies pre-2011 (Bedford 1966, Bendell and Randall 2011, 
Bendell et al., 2009, Neat et al., 2014). The current project (a mark-recapture 
tagging project conducted throughout the Irish Sea from March 2016 to 
December 2018) is a more comprehensive and temporally focused baseline study 
for the Irish Sea stock, than previous tagging studies of the region. Initial 
development of the tagging programme was achieved by reviewing the known the 
spatial and temporal occurrence of cod in the Irish Sea to inform sampling design. 
Methods of tagging were reviewed and the most appropriate method selected to 
achieve the project aims. A review of mark-recapture analysis was undertaken to 
identify the analytical methods available to address the project objectives. The 
first phase of tagging included a review of the study design, post-mortem 
examination of recaptured cod, addressed wider dissemination of project 
information and altered the reward scheme in an attempt to increase recapture 
reporting. The analysis of the mark-recapture results has been completed on the 
current recapture results to explore distribution and movements of tagged fish, 
improve understanding of Irish Sea cod biology and inform management through 
exploration of the implications on the stock assessment of the observed 
movements. The results of the project have been presented in the ICES Working 
Group on the Celtic Seas Ecoregion (WGCSE), the ICES Workshop on the Impact 
of Ecosystem and Environmental Drivers on Irish Sea Fisheries Management 
(WKIrish5) and in 2019 at the ICES Benchmark Workshop on Celtic Sea Stocks 
for consideration in the stock assessment model of cod in 7e-k (Celtic Sea cod).  

12 

 
 
 
 
 
2  METHODS 

To achieve the overall project objectives the project was separated into four work 
packages, each of the work package outputs is reviewed below: 

Work  package  1  Development  of  a  tagging  programme  –  A  review  of  tagging 
methods appropriate for cod was carried out along with review of pre-existing work, 
development of a tagging plan, tagging methods and outlining the communication 
and outreach material.  

Work  package  2.  The  tagging  campaign  –  This  was  planned  as  adaptive  3  year 
programme of conventional tagging in the Irish Sea and neighbouring areas by a 
means  of  dedicated  tagging  effort  and  also  taking  advantage  of  the  extensive 
existing and ongoing fisheries observer and research vessel survey programmes. 
Tagging effort was planned for years 1 (2016) and the first quarter of year 2 (2017) 
to allow time for recaptures and data analysis. 

Work  package  3.  Data  analysis  –  Fish  mark-recapture  data  was  analysed  to 
explore: 

  Cod movements and migration patterns in the Irish Sea and neighbouring areas. 

 

The extent of mixing between populations of cod in the Irish Sea and neighbouring areas, 
specifically the Clyde area and the North Channel, and determine the southern boundary of 
the stock as it is necessary to determine if mixing occurs with the west of Scotland and Celtic 
Sea stocks, as described by ICES. 

 

The influence of environmental or other factors and parameters in the cod movements and 
migration patterns. 

  Cod growth and population structure of the stock resident in the Irish Sea. 

 

Fishing mortality of the cod stock in the Irish Sea and potentially for other gadoid stocks in 
the area, and identify the importance of this information for the assessment of the stock. 

Work package 4. Management advice; within this work package the project set out 
to determine, as far as possible, the sources of mortality for Irish Sea cod, identify 
important  information  (such  as  migration  and  changes  in  stock  dynamics)  and 
include  these  objectively  within  the  stock  assessment  and  implication  for  future 
management. This was achieved by independent research and through contribution 
to the routine ICES stock assessment process, benchmarks and workshops. 

2.1  Work package 1: Development of a tagging programme 

Typically tagging studies create – by marking them  – a subset of animals within 
their  population  that  are  known  and  can  be  identified  by  their  tags  or  marks. 
Following a period of time allowing this marked subset of animals to mix with the 
rest of the population, their progress is then followed by means of their recapture 
and reporting, in both space and time to provide information that can be associated 
with the entire population.  

2.1.1  Review of tagging methods 

Conventional  tagging  studies  have  utilised  an  array  of  methodologies  to  provide 
information  on  aspects  of  fish  biology  and  behaviour.  These  methods  rely  on 
marking  and  recapture  events  and  have  become  common  since  the  mid-17th 

13 

 
 
 
century with a mark-recapture method first used for ecological study in 1896 by 
Petersen  to  estimate  plaice  (Platichthys  platessa)  populations  (Petersen  1896). 
Over  time  the  methods  and  application  of  mark-recapture  studies  have  evolved. 
Initially the technique was used to estimate abundance, developing into a method 
to assess survivability and sources of mortality and more recently explore species 
behaviour (Pine et al., 2011). The ability to achieve these applications is linked to 
advances  in  tagging  technology,  with  available  tagging  now  ranging  from 
conventional  passive  non-permanent  ink-marks,  non-digital  plastic  tags,  micro 
coded  wire  tags,  to  new  electronic  tagging  methods  such  as  radio-frequency 
identification  (RFID)  tags  and  tags  recording  and  storing  environmental  and 
positional data such as data storage tags (DSTs) and satellite tags, providing real 
time behavioural data (Thorstad et al., 2013). In some instances chemical markers 
using  methods  such  as  radiochemical  dating  or  marking  with  calcium-binding 
chemicals oxytetracycline, alizarin, or calcein are appropriate, where large numbers 
of individuals can be tagged and ageing and growth is a focus of the study, as such 
techniques laydown a marker in the skeletal component used in ageing. 

External marker or ID tags are simple plastic tags that are used to identify a fish 
upon capture and release, and again upon recapture. The use of such external tags 
for identifying individuals or groups of fish is the oldest recorded and most widely 
used technique within fisheries research. The justification for any type of tag on a 
fish is the future recovery or recapture and the adjoining information gleaned. The 
more  advanced  external  tags  can  now  carry  more  information  on  individual  fish 
released  than  just  an  ID  code,  such  as  the  organisation  undertaking  the  study, 
reporting instructions and information on tag  rewards and in a  range  of colours. 
Extensive literature on external marker or ID tags used within fisheries research is 
readily available in reviews such as Parker et al., (1990), McFarlane et al., (1990); 
Nielsen (1992) and Thorsteinsson (2002) (http://www.hafro.is/catag). New types 
of  tag  are  continuously  being  developed  to  deal  with  conflicts  arising  from 
information requirements on the one hand and practical application (permanency, 
identifiable,  recognisable,  effects  on  fish  behaviour,  etc.)  on  the  other.  The  best 
known  examples  of  external  tags  used  within  commercial  fisheries  research  are 
Petersen discs (Petersen, 1896), Carlin tags (Carlin, 1955), flag tags (Jakobsson, 
1970), anchor tags (Jones, 1976, 1979), and the modifications from these ( 

Figure 1). 

14 

 
 
 
 
 
 
 
 
 
 
 
 
Anchor 

Petersen 
Discs 

Flag 

Carlin 

Figure 1. Examples of common external tags used and attachment site (all within 
dorsal muscle; adapted from McFarlane et al., 1990). 

The Petersen disc was one of the first tag types used to study fish populations 
and is still commonly used today. Although tag construction has changed from 
different types of material and less expensive the application and design of the 
tag have remained unchanged. Petersen tagging involves securing two plastic 
discs either side of the fish under the dorsal fin through the muscle using a wire 
pin and pliers. The sharpened end of the attachment wire is used to pierce 
through the dorsal muscular tissue, with the open end of the titanium wire 
crimped or twisted closed to keep the circular plastic discs at either side of the 
dorsal muscle, in place. A sufficient gap is left to allow for growth, but not excess 
movement which can lead to shedding. 

Petersen discs have been used widely in previous tagging studies on cod (Trout 
1958; Cogswell 1961; Fowler et al., 1991). Disadvantages of using Petersen discs 
is that the flat discs can become fouled with algae and encrusting shellfish. The 
attaching wire is also fixed and inflexible, which can result in restriction of dorsal 
muscle growth if fixed too tightly. Application time for this method can also be 
long due to having to curl and fix the wire pin on either side of the fish. 

Carlin disc tags are essentially an anchored spaghetti tag which carries an 
individual identification disc of the fish and reporting instructions. Carlin tags are 
very common in monitoring and research work and the most popular spaghetti 
tag type used in United States fisheries research for over 100 years, 
predominantly used on salmonid species (Miller et al., 2012). A smaller version of 
the tag is the fingerling tag which has been developed to use on small and 
juvenile fish. Easy to apply the Carlin tag and requires few tools. The Carlin tag 
consists of a plastic disc attached to the fish body with stainless steel wire or 
polythene thread. Between the attachment to the fish and the disc there is an 

15 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
intermediate link which allows tagging of younger fish for visual recapture as 
adults.   

Traditionally made of soft plastic fabric they are now commonly made of PVC 
plates, shaped as long oval rectangles and called ‘flag tags’. The attachment is 
usually with nylon twine inserted directly through the dorsal musculature of the 
fish with a curved needle and secured off as a loop allowing the flag to trail 
behind the dorsal fin. These tags have been used widely on cod within North Sea, 
English Channel, Irish and Celtic Seas (Bedford 1966; Righton & Metcalfe 2002; 
Burt et al., 2006; Righton et al., 2007; Bendall et al., 2009), and are not prone to 
shedding, with exceptional retention rates. However, this approach can make fish 
prone to infection due to the nature of attachment. 

Modern versions of anchor tags such as the Floy t-bar tag are probably the most 
common tags in use today due to their ease of attachment and are highly 
applicable for studies on cod (O’Cuaig & Officer 2007; Bendall & Randall, 2011). 
The t-bar tags are attached to the fish by a nylon monofilament with a T-shaped 
anchor. The tags are inserted with a tagging gun which can be loaded with just 
one tag or a clip of a numerical series of tags, making the tagging of individuals 
or hundreds of organisms quick and easy (Figure 2). Site of attachment is mostly 
at the base of a dorsal fin. To ensure long-term tag retention, it is important that 
anchor tags are inserted inward at an angle next to the dorsal fin to penetrate 
deep enough into the fish that the t-bar interlocks with the skeletal fin rays, and 
that the tag streamer lies alongside the fish when it swims. The tag itself is most 
often a cylindrical plastic tube in various colours with an identification code and 
return instructions written on to it. Due to the ease of attachment tag shedding 
can prove a problem, therefore double tagging is common practice with these 
tags to obtain data to estimate tag shedding rates.  

More recently, conventional tagging methods have been complemented by 
electronic tags, such as data storage tags (DST), that can have a variety of 
sensors; temperature, depth, light, salinity, pressure, pitch and roll, GPS, 
magnetic and compass. Data collected by DSTs can be used to infer locations of 
the animal by interrogation of logged data against know environmental 
parameters - such as recreating movement paths using depth and temperature 
profiles. The tags can required surgical implantation or where the animal is 
sufficiently large can be externally attached. DSTs can provide high frequency 
data, which is particularly useful to monitor movement paths. However, given the 
special attachment methods the numbers of animals that can be tagged during a 
single event is much lower than traditional tagging studies.  

(b) 

16 

 
 
 
 
(a) 

Figure 2. T-bar anchor tag gun being inserted into the dorsal musculature of a 
mature cod. (b) Mature cod double-tagged with external t-bar anchor tags 
(Bendall & Randall 2011). 

Following the review of conventional tagging methods to tag gadoids, anchor tags 
were selected as the most appropriate tagging method to meet the required scale 
of the tagging programme and objectives for this study. This tagging method was 
selected owing to the ease of attachment and applicability. Anchor tags were 
ideally suited to high frequency tagging with low recapture rates. Modern versions 
of anchor tags such as the Floy t-bar tag are probably the most common tags in 
use globally today due to their ease of attachment and they are highly applicable 
for studies on cod (O’Cuaig & Officer 2007; Bendall & Randall, 2011). The tags 
can be quickly inserted making the tagging of individuals or hundreds of 
organisms quick and easy. Given that the current study intended to use a range 
of platforms to tag cod, anchor tags had the added advantage of ease of training 
for volunteers and minimal expense investment for tagging equipment. In 
comparison to electronic DST tagging, it was estimated that more than 150 times 
the number of animals could be tagged for the same cost of tags and 20 times 
the number of cod could be tagged in the same amount of time as compared to 
surgically implanting a DST into a cod. Despite these electronic tagging methods 
being more costly, it was not within the remit of this study. 
2.1.2  Tag loss rates 

Tag  loss  may  occur  with  two  forms  of  tag  loss  from  the  population  considered 
critical to providing unbiased assessments of stock status. Tagging mortality, the 
death of a released fish resulting from the capture and tagging process, can bias 
the  assessment  as  more  tags  are  assumed  to  be  present  in  the  population  for 
recapture  than  are  actually  present.  Similarly,  tag  loss  from  fish  after  they  are 
tagged  (shedding  of  tags)  causes  a  similar  population  over-estimation  bias.  Tag 
loss during the time at liberty was addressed by Commission for the Conservation 
of Antarctic Marine Living Resources (CCAMLR; Ziegler 2013) and by Ó’Cuaig and 
Officer (2007) by the use of double tags.  In the  current  study anchor  tags were 
attached to the left body, at the front and back of the dorsal fin prior to release. 
Marker  tags  may  vary  slightly  in  terms  of  tag  retention/shedding,  biofouling, 
infection  risk,  reporting  rate,  and  so  some  individuals  may  be  double  tagged  to 

17 

 
 
 
provide  information  on  this.  The  rate  of  loss  of  a  single  tag  was  used  to  predict 
double tag loss rates.  

2.1.3  Post tagging mortality 

Studies investigating tagging mortality in cod tagged with t-bar anchor tags found 
the overall mortality to be low for both commercial (e.g., Brattey & Cadigan 2004) 
and  recreational  (e.g.,  Weltersbach  &  Strehlow  2013)  caught  cod.  These  studies 
showed that high survival of post tagging mortality is associated with colder water 
temperature,  shallow  capture  depths  (<180  m)  and  no  visible  physical  damage. 
Brattey  &  Cadigan  (2004)  noted  that  the  general  applicability  of  their  results  to 
other cod tagging studies depends largely on field staff applying consistent methods 
to select only undamaged, healthy specimens from the catch as well as recognizing 
when fish are stressed and avoiding tagging at such times. They estimated that the 
selection  of  only  undamaged  healthy  fish  for  tagging  can  result  in  substantial 
numbers  of  discards  from  individual  catches,  typically  5–20%  from  hand-line 
catches, mostly due to hook damage and bloating, and as much as 80% from trawl 
catches.  Brattey  &  Cadigan  2004  specifically  advised  that  the  temptation  to 
increase the number of released by tagging fish whose condition is poor should be 
avoided.  

The  tagging  methodology  applied  to  this  study  was  in  line  with  these  previous 
findings,  i.e.,  focus  on  fish  condition,  advised  fishing  methodology  and 
environmental  conditions  at  the  time  of  capture.  This  resulted  in  substantial 
discards, with 100% discards of some trawl catches during the study, and resulted 
in much lower than predicted tagging numbers.  

Evidence of post tagging mortality rates were assessed at the time of tagging with 
only undamaged, active fish marked and returned. Retaining fish in tanks on the 
vessels in controlled conditions was used to determine size based mortality rates. 
Some post-tagging mortality is a reality of any tagging study, however, using the 
experience from previous successful tagging studies, every effort was made during 
the current study to reduce the probability of fish death after tagging. This was to 
maximize the survival of tagged fish, to ensure the accuracy of reporting of tagging 
numbers and to increase the potential for good recapture rates. With the focus on 
fish selection, field experiments to hold tagged fish for prolonged periods  of time 
(~2 hours) showed zero short-term post tagging mortality. 

The  results  of  post  mortem  examination  of  recaptured  fish  were  discussed  with 
regard  to  evidence  to  support  the  appropriateness  of  the  tagging  procedure  to 
ensure  viability  of  the  method.  All  recaptures  throughout  the  project  were 
examined to ensure quality was maintained. For all recaptured tagged fish there 
was no evidence of mal-effect of tagging. Re-growth of skin around tagging sites, 
absence of abscess, blood spots and no evidence of necrosis was observed. This 
evidence supported the view that the tagging protocol was effective.  

2.1.4  Study power 

A power analysis of the current study to show inter-stock area movements 
between the Irish Sea and Celtic Sea stock areas was performed. The analysis 
was performed assuming classical levels of statistical significance (p <0.05) and 
with a range of effect sizes (small – large) and power (low – high). The power 
analysis was applied assuming a Chi-square goodness of fit statistic for a 2x2 
contingency analysis (Rodgers and White, 2007), with the null hypothesis of 
independence between tagging release site (Irish Sea vs. Celtic Sea) and 
recapture site (Irish Sea vs. Celtic Sea) (Table 1). The Chi-square Test of 
Independence is a commonly used and accepted statistical method to examine 

18 

 
 
the association between categorical variables (i.e., whether the variables are 
independent or related). It is a nonparametric test. The power analysis, as 
applied, examined the ability of the tagging programme to identify if there are 
statistically different rates of tagging and recapturing of fish moving between the 
Irish Sea and Celtic Sea. The power analysis was applied as described in Cohen 
(1988). The method is common place in analysis of animal movements to allow 
detection between distinct areas or habitats (e.g. Nichols & Kaiser 2005; Linnane 
2005; Grovenberg et al., 2011). The method, whilst course, provides a broad 
overview of movements between management areas. The analysis allows an 
estimate of the sample size required to detect each combination of effect and 
power to be determined. The results indicated the range of recaptures required 
from the current study to explore movements between the adjoining areas was 
between 15 and 1299 recaptures. The power of the analysis is 1 minus the Type 
II error probability, where the Type II error is the error that occurs when the test 
fails to reject a null hypothesis. A small effect indicates smaller differences 
between populations or categorical variables. An effect size of 0.01 indicates that 
the distribution of scores for the treated group overlaps completely with the 
distribution of scores for the untreated group, there is 0% of non-overlap. An 
effect of 0.1 (small) indicates a non-overlap of 7.7% in the two distributions. A 
large effect (0.8) indicates a non-overlap of 47.4%. 

Table 1. Results of power analysis (2 x 2 contingency test). 

Power 

High (0.95) 

Medium (0.75) 

Low (0.50) 

t
c
e
f
f
E

Small (0.10) 

1299 

Medium (0.30) 

Large (0.50) 

144 

52 

694 

77 

28 

384 

43 

15 

2.1.5  Analysis of pre-existing tagging data 

All records of recaptures of Mark ID tagged cod released in ICES areas 6a 
(Scottish waters), 7a (Irish Sea), 7e (western English Channel) and 7f – 7g 
(Celtic Sea) were extracted from the Centre for Environment, Fisheries and 
Aquaculture Science (Cefas), UK ‘Tagfish’ database (archived data sets from1964 
to 2010), Marine Scotland and from Marine Institute, Ireland datasets. The 
majority of cod tagged in areas 6a, 7a and 7e-g were caught in coastal waters by 
commercial trawl (heavy otter, mid-water, bottom pair or beam) or Danish 
anchor siene and purse seine. All cod were measured to the nearest cm and 
externally tagged with a mark recovery tag attached to the dorsal musculature at 
the base of the first dorsal fin. Releases took place mostly during autumn and 
winter (October to March; Quarters 4 & 1), at the sites shown in Figure 3. 

19 

 
 
 
 
 
 
 
 
 
 
Figure 3. Release positions of Mark ID tagged cod. Release positions (white 
circles) in ICES sub divisions 6a (6a; West of Scotland), 7a (7a. Irish Sea), 7e (7e 
western English Channel), and 7f-g (7f-g; Celtic Sea). 

In total, release and recapture information for 2209 tagged cod were available 
(687 in 6a, 1108 in 7a, 141 in 7e, 155 in 7f and 118 released in 7g respectively; 
Table 2). Release and recapture data for tagged cod spanned mostly the 1970’s, 
1990’s and 2000’s. Majority of the mark-recovery data from 6a are from the 
1960’s and 1970’s (54% and 39% respectively). In area 7a (78% of the 
recaptures) are from tagging in the late 1970’s. Data for areas 7e and 7f are 
mostly from the 1990s (53% of 7e cod and 85% of 7f cod recaptures). All the 
data for area 7g cod are from tagging in 2007- 2009. It is important to note that 
the spawning stock biomass of cod in the Irish Sea (area 7a) was around ten 
times larger in the 1970s than in recent years, whereas the stock in area 7e-k 
appeared to be less abundant in the 1970s than in the 1990s but have declined in 
recent years (ICES, 2018a).  

Recapture data were sub-divided into quarters of the year according to the 
recapture season: winter and autumn (Q1 & Q4), spring and summer (Q2 & Q3), 
(see Bedford, 1966). Recaptures were then placed into size categories dependent 
on the length of cod at recapture: smaller than 50cm (classed as ‘juvenile’) or 
larger than 50 cm (classed as ‘adult’). Although the average size and age at 
maturity in Irish Sea cod has declined since the 1990s (Armstrong et al., 2004a), 
at least 50% of the cod of 50cm and over would have been mature. In the Celtic 
Sea, first maturity in female cod during 1986-2002 occurred at 50cm whilst 
around 40% of males were mature at this length (Armstrong et al., 2004b).  

Recapture positions were plotted in ArcView 9.1 for cod released in ICES area 6a, 
7a and 7e-g. The Animal Movement Analysis Extension to Arcview (AMAE: Hooge 
and Eichenlaub, 2000) was used to estimate of the extent of geographical range 
for cod in each area by generating kernel probability density function surfaces 
(KPDF) for 95%, 70% and 50% volume estimates under the three-dimensional 

20 

 
 
 
KPDF surface (see Worton, 1987, Seaman and Powell, 1996; Hooge et al., 2001). 
The KPDF method is more typically used in studies of territoriality and home 
range (Jones, 2005; Righton & Mills, 2006). However, because tag recapture 
locations are analogous to the density and distribution of the locations of single 
individuals over time, the technique is extremely applicable to population level 
mark-recapture data (Righton et al., 2007). 

21 

 
 
 
Table 2. The number of mark recovery tag recaptures by ICES area of release 
and seasonal quarter of recapture. Lr and Lc refer to length at release and 
recapture respectively. Q4 & Q1 are combined to show number of recaptures 
during the autumn and winter quarters; Q2 & Q3 are combined to show the 
number of recaptures during the spring and summer quarters. 

Area  Lr 

(cm) 

Lc 
(cm) 

Q1 

Q2 

Q3 

Q4 

Total  Q4 & 

Q2 & Q3 

Q1 

Autumn 
& 
Winter 

Spring 
& 
Summer 

4a 

<50  <50 

  7 

 45 

 24 

51 

127 

 58 

<50  >50 

 12 

 17 

  4 

 3 

 36 

 15 

 69 

 21 

>50 

132  233  105   54 

524 

186  

338 

Total 

151  295  133  108  687 

259 

428 

7a 

<50  <50  266  61 

23 

12 

362 

278 

84 

<50  >50  6 

5 

3 

2 

16 

8 

8 

>50 

398  234  66 

32 

730 

430 

300 

Total 

670  300  92 

46 

1108  716 

392 

7e 

<50  <50  16 

11 

13 

14 

54 

30 

<50  >50  19 

16 

12 

10 

57 

29 

>50 

1 

9 

5 

15 

30 

16 

24 

28 

14 

22 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 2 (contd.). The number of mark recovery tag recaptures by ICES area of 
release and seasonal quarter of recapture. Lr and Lc refer to length at release 
and recapture respectively. Q4 & Q1 are combined to show number of 
recaptures during the autumn and winter quarters; Q2 & Q3 are combined to 
show the number of recaptures during the spring and summer quarters. 

Area  Lr 

(cm) 

Lc 
(cm) 

Q1 

Q2 

Q3 

Q4 

Total  Q4 & 

Q2 & Q3 

Q1 

Autumn 
& 
Winter 

Spring 
& 
Summer 

7f 

<50  <50  76 

8 

2 

<50  >50  8 

13 

16 

>50 

13 

7 

3 

Total 

97 

28 

21 

7g 

<50  <50  7 

14 

2 

<50  >50  24 

33 

14 

>50 

3 

11 

- 

- 

6 

3 

9 

1 

6 

3 

86 

76 

43 

14 

26 

16 

155 

106 

24 

8 

77 

30 

17 

6 

Total 

34 

58 

16 

10 

118 

44 

10 

29 

10 

49 

16 

47 

11 

74 

The details of Data storage tag (DST) cod releases for each ICES area, by month 
and year are shown in Table 3. . Individuals were brought slowly to the surface to 
avoid rupture of the swimbladder and those in good condition were placed into 
1000 l deck tanks. Fish suitable for tagging with a DST (length > 45 cm) were 
then selected. Cod tagged with DSTs were anaesthetised in a shallow (20 cm) 
bath containing 2-phenoxy-ethanol (0.5ml l-1) before surgically implanting the 
DST into the peritoneal cavity (details in Righton et al., 2006). A spaghetti tag 
attached to the DST was then placed through the lateral body wall by a curved 
needle and a conventional tag was attached to the dorsal musculature at the base 
of the first dorsal fin to act as external identification markers (Righton et al., 
2006). All fish were placed into a recovery tank before release. In order to 
encourage the reporting of recaptured tags a reward scheme was set up 
throughout the commercial fisheries (as for Turner et al., 2002; Righton & 
Metcalfe 2002; Hunter et al., 2004; Righton et al., 2006 & 2007).

23 

 
 
 
 
 
 
 
 
 
 
 
 
 
Table 3. Released number and mean length (in cm ±SD) of cod tagged with DSTs by month and year of release in each ICES 
management area. Numbers are also given for DSTs returned (to date as of January 2012) and tracks successfully reconstructed using 
the Hidden Markov Model (HMM) for all cod at liberty greater than 10 days. 

Area 

Year 

Month 

ICES 

DSTs  Length 

Method of capture 

Rectangle 

7a 

1999 

March 

35 & 36 E4 

19 

64±9 

Long line 

2008 

Feb 

35 & 36 E4 

17 

67±6 

Mid-water trawl 

DSTs 
returned 

Cod tracks 
reconstructed (using 
HMM) 

4 

3 

4 

3 

2009 

Feb – 
Mar 

35 & 36 E4 

80 

72±10 

Mid-water trawl 

18 

17 

2010 

Mar 

36 & 37 E6 

44 

70±13 

Mid-water trawl 

Total:- 

160 

9 

34 

8 

32 

24 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 3 (contd.). Released number and mean length (in cm ±SD) of cod tagged with DSTs by month and year of release in each ICES 
management area. Numbers are also given for DSTs returned (to date as of January 2012) and tracks successfully reconstructed using 
the Hidden Markov Model (HMM) for all cod at liberty greater than 10 days. 

Area 

Year 

Month 

ICES 

DSTs  Length 

Method of capture 

Rectangle 

DSTs 
returned 

Cod tracks 
reconstructed (using 
HMM) 

7e 

2007 

Jan - Feb 
& May 

29 E5 & 28 E7 

169 

72±7 

Fixed gill net & hook and 
line 

27 

17 

Total:- 

169 

27 

17 

7f 

2004 

Mar 

31 E5 & 31 E6 

13 

68±10 

Otter trawl 

2008 

May 

31 E5 & 31 E6 

17 

67±9 

Otter trawl 

2010 

Mar 

30 E4 & 30 E5 

60 

75±8 

Fixed gill net 

Total: 

90 

1 

1 

6 

8 

1 

1 

6 

8 

25 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 3 (contd.). Released number and mean length (in cm ±SD) of cod tagged with DSTs by month and year of release in each ICES 
management area. Numbers are also given for DSTs returned (to date as of January 2012) and tracks successfully reconstructed using 
the Hidden Markov Model (HMM) for all cod at liberty greater than 10 days. 

Area 

Year 

Month 

ICES 

DSTs 

Length 

Method of capture 

DSTs returned 

Rectangle 

7g 

2008 

Mar 

31 E2 & 32 E2-3 

100 

77±9 

Seine net 

2009 

Mar 

31 E2 & 32 E2-3 

141 

71±9 

Seine net 

2010 

Mar 

31 E2 & 32 E2-3 

50 

69±13 

Seine net 

Total: 

291 

10 

18 

10 

38 

Cod tracks 
reconstructed 
(using HMM) 

5 

14 

5 

24 

26 

 
 
 
 
 
 
 
 
 
 
 
2.1.6  Geographic reconstruction of DST cod movements 

Hydrostatic (tidal) data, derived from the sinusoidal pressure cycle recorded in 
the depth data when a fish is at rest on the seafloor, was used to enable the 
geographical reconstruction of an individual cod’s movements (termed 
geolocation). This method is referred to as the Tidal Location Method (TLM, as 
described in Metcalfe & Arnold (1997); Hunter et al., (2004). We used a novel 
Fokker-Planck based method that combines the TLM with a hidden Markov model 
to estimate, for each day at liberty, the non-parametric probability distribution of 
geographic position (Pedersen et al., 2008; Righton et al., 2008) for 81 out of 
100 DST tagged cod recaptured in ICES areas 7a (west & east), 7e, 7f and 7g.  

Daily reconstructed positional data for the 81 cod were grouped and sub-divided 
into quarters of the year as for mark recovery data. All DST cod were released as 
‘adults’ (>50cm). In total 11,170 daily spatial positions were available (7a (east): 
1457 days; 7a (west) 4574 days; 7e 966 days; 7f 1135 days; 7g 2533 days 
respectively). Positions for ‘adult’ DST tagged cod were plotted for 7a and 7e-g 
release areas in ArcView 9.1.  

For each DST depth dataset (average duration 121d), daily position likelihood 
surfaces were generated using the hidden Markov method described in Pedersen 
et al., (2008). The majority of datasets (188 of 213) were analysed without 
constraining model parameters. The remainder were run under using custom 
parameters to ensure that sufficient tidal data were extracted from the depth 
data, and that uncertain position estimates (model error) did not entrain the 
likelihood surfaces across land or into locations that were not consistent with the 
tag temperature record, or which did not match the daily point estimates of 
location. The surfaces were then aggregated by calendar month and normalised. 
Monthly surfaces were then aggregated by tagging region (southern North Sea, 
eastern Channel, western Channel, central North Sea, western Irish Sea, eastern 
Irish Sea, Celtic Sea and Bristol Channel) and renormalized. The surfaces were 
summed for each quarter before plotting on a standard scale.  
2.1.7  Virtual recaptures from DST cod movements 

In order to directly compare mark recovery data with DST geolocated data, a 
single reconstructed DST position per month at liberty was extracted for each 
individual cod track. This gave a sample of 453 geolocations that could be treated 
as ‘virtual recaptures’ and avoid potential bias from over-representation of 
extended migrations from individual cod. 

The number of recaptures by ICES area of release and quarter of recapture are 
shown in Table 4. . Cod remained at liberty on average for six months after 
tagging (±11 months), travelling an average distance from release to recapture 
of 56 km (±83 km). The longest time at liberty was 26 months, by a cod 
recaptured 25 km away from its original release site in 7a during Q1. The 
greatest distance travelled was by a cod at liberty for 590 days, recaptured 700 
km from original release site in 7e during Q1 within ICES management area of 4b 
(off coast of Sunderland, UK; 55˚ 60’N 0˚ 30’W). 

Due to the limited number of returns for cod for 7e – f the recaptures of cod in 
Q4 & Q1 (autumn & winter) were combined, as were the recaptures for Q2 & Q3 
(spring & summer; Table 4. ). These sub-divisions correspond very broadly to 
‘spawning’ (Q4 & Q1) and ‘feeding’ (Q2 & Q3) seasons as given by Wright et al., 
(2006) and Righton et al., (2007), where the definition of “spawning season” 
encompasses gonad maturation, spawning migration and the act of spawning 
itself. 

27 

 
 
2.2  Spatial distribution from Mark recovery recaptures  

In ICES areas 7a and 7g, cod were generally recaptured further away from the 
point of release during Q2 & Q3 than during Q4 & Q1. In contrast, for ICES areas 
7e and 7f, cod were generally recaptured further away from the point of release 
during Q4 & Q1 than during Q2 & Q3.  

  6a releases 

In 6a releases cod showed restricted dispersal close to the area of their release 
sites during Q1 & Q4 (Error! Reference source not found.) while in Q2 & Q3 
showed a much wider dispersal from their release sites into deeper offshore 
waters predominantly to the north west within 6a and north east into 4a (Error! 
Reference source not found.).  

  7a releases 

28 

 
 
 
In 7a releases cod showed highly restricted dispersal close to the area of their 
release sites during Q1 (

a) 

b) 

c) 

d) 

29 

 
 
 
 
Figure 5), while in Q2, Q3 & Q4 showed a much wider dispersal from their release 
sites north into 6a and south to the Celtic Sea within 7f-g (

a) 

b) 

c) 

d) 

Figure 5).   

In relation to site fidelity over time; majority of cod recaptures were within the 
same year of release (52% during Q1&4; 78% during Q2 & Q3), however 48% of 
cod recaptures during Q1 & Q4 compared to 22% of cod recaptures during Q2 & 
Q3 were of cod released within the same season but recaptured the following 
year (or more) after release. 

  7e releases 

In 7e cod were widely dispersed from their release sites over Q1 & Q4 ( 

30 

 
 
 
 
Figure 6). 7e. cod were recaptured to the east through 7d as far into southern 
and central North Sea (4b-c) and north west into the Celtic Sea (within 7f – g). In 
contrast, it was evident that 7e cod were far more constrained in their dispersal 
during Q2 & Q3 ( 

Figure 6), where cod showed a more constrained south west dispersal into Celtic 
Sea (within 7f – j).  

In relation to site fidelity over time; majority of cod recaptures were within the 
same year of release, however 29% of cod recaptures during Q2 & Q3 were of 
cod released within the same season but recaptured the following year (or more) 
after release. 

  7f releases 

In 7f cod recaptures during Q1 & Q4 were widely dispersed south and west into 
7g, 7h and 7e ( 

Figure 6). While during Q2 & Q3, 7f cod showed a more concentrated dispersal 
around the neighbouring boundaries of 7e – h within the Celtic Sea ( 

Figure 6).  

In relation to site fidelity over time; generally cod recaptures were within the 
same year of release, however 53% of cod recaptures during Q4 & Q1 and 26% 
of cod recaptures during Q2 & Q3 were of cod released within the same season 
but recaptured the following year (or more) after release. 

  7g releases 

In 7g, cod recaptures were restricted close to their release sites during Q1 ( 

Figure 7). During Q2, Q3 & Q4, cod dispersed southward away from the coast ( 

Figure 7), yet remained within close proximity of their release.   

In relation to site fidelity over time; majority of cod recaptures were within the 
same year of release, however 36% of cod recaptures during Q4 & Q1 and 7% of 
cod recaptures during Q2 & Q3 were of cod released within the same season but 
recaptured the following year (or more) after release. 

31 

 
 
 
a) 

b) 

c) 

d) 

Figure 4. Mark recovery tag recapture positions of cod released in ICES area 6a 
(Scottish waters). Solid symbols show exact recapture locations, while shading 
shows the probability density surfaces for 50% (centre white), 75% (mid grey) 
and 95% (dark grey) of the recaptures. Data shown are for ‘adults’ recaptured 
during seasonal quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) 
Quarter 4. 

32 

 
 
 
 
 
 
a) 

b) 

c) 

d) 

Figure 5. Mark recovery tag recapture positions of cod released in ICES area 7a 
(Irish Sea). Solid symbols show exact recapture locations, while shading shows 
the probability density surfaces for 50% (centre white), 75% (mid grey) and 
95% (dark grey) of the recaptures. Data shown are for ‘adults’ recaptured 
during seasonal quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) 
Quarter 4. 

33 

 
 
 
 
 
a) 

b) 

b) 

c) 

d) 

Figure 6. Mark recovery tag recapture positions of cod released in ICES area 7e 
& 7f (Celtic Sea). Solid symbols show exact recapture locations, while shading 
shows the probability density surfaces for 50% (hollow white), 75% (mid grey) 
and 95% (dark grey) of the recaptures. Data shown are for (a) 7e cod recaptured 
during combined Q4 & Q1 autumn and winter quarters; (b) 7e cod recaptured 
during combined Q2 & Q3 spring and summer quarters; (c) 7f cod recaptured 
during combined Q4 & Q1 quarters; (d) 7f cod recaptured during combined Q2 
& Q3 quarters. 

34 

 
 
 
 
 
 
a) 

b) 

c) 

d) 

Figure 7. Mark recovery tag recapture positions of cod released in ICES area 7g 
(Celtic Sea). Solid symbols show exact recapture locations, while shading shows 
the probability density surfaces for 50% (centre white), 75% (mid grey) and 
95% (dark grey) of the recaptures. Data shown are for ‘adults’ recaptured 
during seasonal quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) 
Quarter 4. 

2.2.1  Movement of cod between ICES areas 

The exchange of cod between the different ICES management areas and seasonal 
quarters is shown in Table 4. . In ICES areas 6a, 7a and 7g the majority of cod 
were recaptured within their own management area during Q4 & Q1 and Q2 & 

35 

 
 
 
 
 
Q3. For 6a an exchange of cod north east into 4a was apparent during Q1 & Q4 
with greater occurrence throughout Q2 & Q3. For 7a cod shows some exchange 
south into 7e – g and north into 6a during Q2 & Q3 was evident. In 7g cod 
showed a very limited exchange north into 7a during Q4 and south into 7f and 7h 
during Q2 & Q3. In ICES areas 7e and 7f cod showed a much wider spread 
exchange of movement between the different ICES management areas. 7e cod 
showed the most widespread exchange north west and north east across seven 
different management areas during Q4 & Q1, with majority of recaptures caught 
outside of 7e. During Q2 & Q3 7e cod were more widely spread to the west within 
management areas 7f - h of the Celtic Sea and majority of recaptures were again 
caught outside of 7e. For 7f cod showed a widespread exchange west into 
neighbouring management areas 7e, 7g and 7h during Q4 & Q1 and Q2 & Q3 with 
the majority of recaptures caught outside of 7f.  

Table 4. Exchange of mark recovery tagged cod between ICES management 
areas, based on area of recapture. Recapture proportions in area of release are 
shaded in grey. 

Release area 

 Recapture area 

Q1 

6a 

7a 

7e 

7f 

7g 

Q2 

6a 

7a 

7e 

7f 

7g 

5b 

4a 

6a 

7a 

7d 

7e 

7f 

7g 

7h/7j  4b/4c  Tags 

0.10  0.86  0.03  0.00  0.00  0.00  0.00  0.00  0.01  144 

0.00  0.02  0.97  0.00  0.00  0.00  0.01  0.00  0.01  404 

0.00  0.00  0.00  0.15  0.32  0.21  0.24  0.00  0.08  34 

0.00  0.09  0.00  0.00  0.25  0.52  0.09  0.05  0.00  21 

0.00  0.00  0.00  0.00  0.00  0.00  1.00  0.00  0.00  27 

5b 

4a 

6a 

7a 

7d 

7e 

7f 

7g 

7h/7j  4b/4c  Tags 

0.01  0.16  0.76  0.05  0.00  0.00  0.00  0.00  0.00  0.02  250 

0.00  0.00  0.03  0.91  0.00  0.00  0.01  0.05  0.00  0.00  239 

0.00  0.00  0.04  0.00  0.08  0.48  0.16  0.08  0.16  0.00  25 

0.00  0.00  0.05  0.05  0.00  0.10  0.60  0.05  0.15  0.00  20 

0.00  0.00  0.00  0.00  0.00  0.00  0.07  0.89  0.04  0.00  44 

36 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 4 (contd.). Exchange of mark recovery tagged cod between ICES 
management areas, based on area of recapture. Recapture proportions in area of 
release are shaded in grey. 

Release area  Recapture area 

Q3 

6a 

7a 

7e 

7f 

7g 

Q4 

6a 

7a 

7e 

7f 

7g 

5b 

4a 

6a  7a 

7d  7e 

7f 

7g 

7h/7j  4b/4c  Tags 

0.02 

0.28  0.68  0.02  0.00  0.00  0.00 0.00  0.00  0.00  109 

0.00 

0.01  0.05  0.82  0.00  0.00  0.01 0.08  0.00  0.00  69 

0.00 

0.00  0.00  0.00  0.00  0.41  0.24 0.06  0.29  0.00  17 

0.00 

0.00  0.00  0.00  0.00  0.42  0.10 0.16  0.32  0.00  19 

0.00 

0.00  0.00  0.00  0.00  0.00  0.00 1.00  0.00  0.00  14 

5b 

4a 

6a  7a 

7d  7e 

7f 

7g 

7h/7j  4b/4c  Tags 

0.05  0.95  0.00  0.00  0.00  0.00 0.00  0.00  0.00  59 

0.00  0.06  0.76  0.00  0.00  0.00 0.18  0.00  0.00  34 

0.00  0.00  0.00  0.00  0.64  0.18 0.00  0.09  0.09  11 

0.00  0.00  0.00  0.00  0.45  0.22 0.11  0.22  0.00  9 

0.00  0.00  0.011  0.00  0.00  0.00 0.89  0.00 

 0.00  9 

To date, 107 cod (15%) tagged with DSTs have been recaptured, yielding over 
11000 days of data: 34 from releases in 7a, 27 from releases in 7e, 8 from 
releases in 7f and 38 from releases in 7g. Returned cod were at liberty between 1 
– 473 days, and grew up to 43 cm in length during the at liberty period. Release 
and recapture positions of these cod are shown in  

Figure 8. Six DSTs were lost in the gutting process of the cod by fishermen. In 
addition two recovered DSTs failed to download upon return. It was possible to 
reconstruct (in sufficient detail and with sufficient certainty of location) the daily 
movements of 81 cod at liberty >10 days (32 from 7a, 17 from 7e, 8 from 7f and 
24 from 7g). 

2.2.2  Spatial distribution of data storage tags 

The daily spatial distributions of DST cod were similar to results of the mark 
recovery data.  

Figure 9 shows the daily spatial distributions of 81 DST cod recaptures by ICES 
area of release, while  

Figure 9 shows all daily spatial distributions of cod by seasonal quarter for each 
ICES area of release.  

37 

 
 
 
 
 
 
 
 
 
Figure 10 to Figure 14 show the daily spatial distributions for each ICES area by 
month of release and combined season quarters; autumn and winter (Q4 & Q1) 
and spring and summer (Q2 & Q3). These sub-divisions correspond very broadly 
to ‘spawning’ (Q4 & Q1) and ‘feeding’ (Q2 & Q3) seasons as given by Wright et 
al., (2006) and Righton et al., (2007), where the definition of “spawning season” 
encompasses gonad maturation, spawning migration and the act of spawning 
itself. Figure 15 to 17 show the overall combined daily position likelihood surfaces 
of DST cod releases for ICES areas 7a (western Irish Sea), 7a (eastern Irish Sea) 
and 7g (Celtic Sea), using Hidden Markov model, described by Pedersen et al., 
2008. Likelihood ‘hot spot’ surfaces depicted in red (show locations with highest 
degree of statistical certainty), while ‘cold spot’ surfaces in blue (show locations 
with lower degree of statistical certainty).   

For 7a DST cod (n=32) spatial distribution was predominantly confined within the 
Irish Sea primarily in shallower coastal waters throughout December to February 
off north Wales, Dublin and Liverpool becoming more centrally located within 
deeper waters of the central Irish Sea and St George’s Channel from late 
February - March.  During Q2 spatial distribution was considerably more 
dispersed throughout the Irish Sea reaching further north into 6a and south into 
the Celtic Sea within 7g. However cod were much more restricted to the deeper 
waters of the central Irish Sea, North Channel and St George’s Channel and 
generally remained within depths > 80m. During Q3 cod showed preferences for 
similar locations off north Wales and within the North Channel during July and 
September. By Q4 cod began to aggregate in shallower coastal waters off north 
Wales, Dublin and the North Channel.   

For 7e DST cod (n=17) during Q1 a highly widespread distribution across five 
ICES management areas was observed, very similar to that shown by the mark 
recovery data. Distribution west into Celtic Sea within 7f-g together with 
movement east through 7d and into the southern North Sea within 4c was 
evident throughout January – March. No data however were available for Q4 
autumn months October – December. During Q2 & Q3 7e cod showed a more 
constrained spatial distribution remaining within the English Channel within 7d-e. 
Some cod showed preferences to certain locations off the French coast in April 
within 7d, off the south east coast, UK within 7d April – May, yet primarily within 
7e close to the Hurd Deep throughout May – July. No spatial data were available 
for August and September.  

For 7f DST cod (n=8), during Q4 & Q1 dispersion occurred out of the Bristol 
Channel during March together with some limited distribution in October off the 
far coast of Cornwall, UK. During Q2 & Q3 movement was recorded south through 
the Celtic Sea in May, and dispersion on the edge of the English Channel around 
the ICES management boundary between 7e and 7h was particularly evident.  

Finally for 7g DST cod (n=24) during Q4 & Q1 and Q2 & Q3 distribution remained 
fairly constrained within 7g. Some preference to central areas within 7g during 
January – March and April – June was evident and cod appeared to be more 
widely dispersed within 7g during Q1 & Q2.   
2.2.3  Virtual recaptures from DST cod movements 

A total of 453 individual spatial positions were extracted as ‘virtual recaptures’ 
from the full DST dataset (81 cod; 11170 data days), to allow for direct 
comparison between mark recovery data (6a cod predominantly 1960 – 1970’s; 
7a cod 1970’s; 7e & 7f cod 1990’s; 7g cod 2000’s ), with current DST cod 
movement data (1999 – 2010). Table 5.  shows the rate of exchange for DST 
‘virtual’ cod recapture data between ICES management areas. Figure 18 to 19 

38 

 
 
show the DST ‘virtual’ cod recapture positions by seasonal quarter for cod 
released in ICES area 7a (Irish Sea) and 7g (Celtic Sea) respectively. 

Table 5. Exchange of DST ‘virtual’ cod recapture data between ICES 
management areas. Recapture proportions in area of release are shaded in grey.  

Release area 
Q1 
7a 
7e 
7f 
7g 
Q2 
7a 
7e 
7f 
7g 
Q3 
7a 
7e 
7f 
7g 
Q4 
7a 
7e 
7f 
7g 

Recapture area 
7e 

7f 

4a 

6a 

7a 

7d 

0.00  0.02  0.88  0.00  0.03  0.03  0.02  0.02 
0.00  0.00  0.00  0.37  0.37  0.07  0.13  0.03 
0.00  0.00  0.00  0.00  0.00  0.75  0.13  0.12 
0.00  0.00  0.00  0.00  0.00  0.00  0.88  0.12 

7g  7h/7j  4b/4c 
0.00 
0.03 
0.00 
0.00 

0.00  0.02  0.83  0.00  0.0  0.00  0.13  0.03 
0.00  0.00  0.00  0.33  0.67  0.00  0.00  0.00 
0.00  0.00  0.00  0.00  0.17  0.08  0.25  0.5 
0.00  0.00  0.02  0.00  0.00  0.00  0.91  0.07 

0.00  0.03  0.90  0.00  0.00  0.00  0.02  0.05 
0.00  0.00  0.00  0.00  1.00  0.00  0.00  0.00 
0.00  0.00  0.00  0.00  0.29  0.14  0.14  0.43 
0.00  0.00  0.00  0.00  0.00  0.05  0.95  0.00 

0.00  0.00  0.92  0.00  0.00  0.00  0.00  0.08 
0.00  0.00  0.00  0.00  0.00  0.00  0.00  0.00 
0.00  0.00  0.00  0.00  0.00  0.25  0.00  0.75 
0.00  0.00  0.00  0.00  0.00  0.00  1.00  0.00 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

0.00 
0.00 
0.00 
0.00 

Tags 
63 
30 
8 
27 

107 
12 
12 
10 

61 
2 
7 
21 

36 
0 
4 
6 

39 

 
 
 
 
 
 
 
 
 
a) 

b) 

c) 

d) 

e) 

Figure 8. Recapture positions of cod tagged with DSTs in (a) ICES area 7a West 
(Irish Sea), (b) ICES area 7a East (Irish Sea), (c) ICES area 7e (western English 
Channel), (d) ICES area 7f (Celtic Sea) and (e) ICES area 7g (Celtic Sea). White 
cross symbols show the release positions of DST-tagged cod, while solid black 
circles show the recapture positions of DST-tagged cod. 

40 

 
 
 
 
Figure 9. The daily spatial distributions of 81 cod (11,170 days of data) released 
in the following ICES areas: eastern Irish Sea 7a (yellow; 8 cod = 1457 days of 
data), western Irish Sea 7a (red; 24 cod = 4574 days of data), eastern Celtic Sea 
7f (white; 7 cod = 1135 days of data), western Celtic Sea 7g (dark blue; 24 cod = 
2533 days of data), western English Channel 7e (green; 17 cod = 966 days of 
data). Solid coloured symbols show individual daily geolocated positions 
reconstructed for each cod at liberty.  

41 

 
 
 
 
 
 
a) 

b) 

c) 

d) 

Figure 10. The daily spatial distributions of 81 cod (11,170 days of data) by 
seasonal quarter; a) quarter 1, b) quarter 2, c) quarter 3 and d) quarter 4 for 
each ICES area of release; 7a East (yellow), 7a West (red), 7e (green). 7f (white) 
7g (dark blue). 

42 

 
 
 
 
 
 
 
 
a

b

Figure 11. The daily spatial distribution of cod by month, released in ICES area 
7a (Irish Sea). Solid symbols show daily geolocated positions reconstructed for 32 
cod coloured by month at liberty. Positions shown are for (a) 32 7a cod (2490 
days of data) during the Q4 & Q1 autumn and winter quarters; (b) 31 7a cod 
(4047 days of data) during the Q2 & Q3 spring and summer quarters. 

a

b

Figure 12. The daily spatial distribution of cod by month, released in ICES area 
7e (western English Channel). Solid symbols show daily geolocated positions 
reconstructed for 17 cod coloured by month at liberty. Positions shown are for 
(a) 15 7e cod (595 days of data) during the Q4 & Q1 autumn and winter 
quarters; (b) Six 7e cod (314 days of data) during the Q2 & Q3 spring and 
summer quarters. 

43 

 
 
 
 
 
a) 
(d

b) 
(d

Figure 13. The daily spatial distribution of cod by month, released in ICES area 
7f (Celtic Sea). Solid symbols show daily geolocated positions reconstructed for 
seven cod coloured by month at liberty.  (a) Six 7f cod (415 days of data) during 
the Q4 & Q1 autumn and winter quarters; (b) Five 7f cod (486 days of data) 
during the Q2 & Q3 spring and summer quarters. 

a) 
(d) 

b) 
(d) 

Figure 14. The daily spatial distribution of cod by month, released in ICES area 
7g (Celtic Sea). Solid symbols show daily geolocated positions reconstructed for 
24 cod coloured by month at liberty.  (a) 24 7g cod (839 days of data) during the 
Q4 & Q1 autumn and winter quarters; (b) 20 7g cod (1694 days of data) during 
the Q2 & Q3 spring and summer quarters. 

44 

 
 
 
 
a) 

c) 

b) 

d) 

Figure 15. Combined daily position likelihood surfaces (using Hidden Markov 
model, described by Pedersen et al., 2008) of 7a west (Irish Sea) DST cod 
releases, showing probable distribution estimates by seasonal quarter ; a) Q1, 
b)Q2, c) Q3 & d) Q4. Likelihood ‘hot spot’ surfaces are depicted in red (showing 
locations with highest degree of statistical certainty), through to ‘cold spot’ 
surfaces depicted in blue (showing locations with lower degree of statistical 
certainty).   

45 

 
 
 
 
 
a)  

b)  

c)  

d)  

Figure 16. Combined daily position likelihood surfaces (using Hidden Markov 
model, described by Pedersen et al., 2008) of 7a east (Irish Sea) DST cod releases, 
showing probable distribution estimates by seasonal quarters; a) Q1, b)Q2, c) Q3 
& d) Q4. Likelihood ‘hot spot’ surfaces are depicted in red (showing locations 
with highest degree of statistical certainty), through to ‘cold spot’ surfaces 
depicted in blue (showing locations with lower degree of statistical certainty).   

46 

 
 
 
 
 
 
a)  

b)  

c)  

d)  

Figure 17. Combined daily position likelihood surfaces (using Hidden Markov 
model, described by Pedersen et al., 2008) of 7g (Celtic Sea) DST cod releases, 
showing probable distribution estimates by seasonal quarters; a) Q1, b)Q2, c) Q3 
& d) Q4. Likelihood ‘hot spot’ surfaces are depicted in red (showing locations 
with highest degree of statistical certainty), through to ‘cold spot’ surfaces in 
blue (showing locations with lower degree of statistical certainty).   

47 

 
 
 
 
a) 

b) 

48 

c) 

d) 

Figure 18. DST ‘virtual’ cod recapture positions of cod released in ICES area 7a (Irish 
Sea). Solid symbols show ‘virtual’ recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. 

 
 
 
 
a) 

b) 

49 

c) 

d) 

Figure 19. DST ‘virtual’ cod recapture positions of cod released in ICES area 7g (Irish 
Sea). Solid symbols show ‘virtual’ recapture locations, while shading shows the 
probability density surfaces for 50% (centre white), 75% (mid grey) and 95% (dark 
grey) of the recaptures. Data shown are for ‘adults’ recaptured during seasonal 
quarters (a) Quarter 1; (b) Quarter 2; (c) Quarter 3; and d) Quarter 4. 

2.2.4  Development of a tagging study for 2016 - 2018 

The tagging study (2016–2018) aim to provide baseline information on the movement 
rates of cod into adjacent areas and thereby the accessibility / vulnerability of the Irish 
Sea stock to the fisheries in each area. This required cod to be tagged over the greatest 
size range possible (including below minimum conservation reference size (MCRS)), over 

 
 
 
 
50 

a large spatial area to ensure the tagged fish sample reflected all demographics of the 
Irish Sea cod population. Tagging opportunities covering a wide range of sources were 
planned, utilising existing sampling schemes and establish new programmes. The plan 
was designed to achieve a maximum number of tagged cod, using the experience of 
operating fishery dependent and independent sampling plans for cod and previous 
tagging projects by the project partners. Given the existing management measures to 
reduce cod catches in the Irish Sea, the most proven and effective means to catch and 
tag cod based on previous sampling schemes are targeted charters using commercial 
gear specifically developed to target cod in the study area. The plan was developed to 
have targeted tagging events when the highest catch rates of cod could be achieved 
during spawning periods and late summer when aggregation in deep water form. This 
would be augmented by tagging during commercial fishing opportunities throughout the 
year.  

The tagging plan was developed to utilise a range of methods to obtain viable fish for 
tagging, analysis of known sources of information identified that commercial fishing 
activity and targeted fishing for cod using semi-pelagic and demersal gear provide the 
highest catch rates and thus opportunity for tagging.  

Historically, Irish Sea fisheries (Figure 20) were predominantly demersal trawling and 
seining with a high degree of overlap. At present the dominant fishing effort in the area 
arises from the Nephrops fishery, which uses mesh sizes in the range of 70-99 mm. 
Beam trawls operating within the Irish Sea use mesh sizes in the range of 80-119mm, 
targeting sole, plaice, and rays. Cod were target by demersal trawl / seine fishers with 
mesh sizes in the range 100 – 120 mm. Following the cod-recovery plan in 2000, effort 
of the whitefish fleet using fishing gear with ≥100mm mesh sizes has been declining and 
reached a historically low (Figure 21) at the start of the study period. This was the 
dominant fleet targeting cod, with gear specifically designed to catch gadoids, whilst 
other fisheries had low observed by-catch of cod. At present, there is no directed 
commercial fishery for cod permitted in the Irish Sea. The known spawning grounds are 
closed to commercial fishing during February – May, with no vessels using demersal 
fishing gear allowed during this time. Therefore, to exploit this source of fish for tagging 
scientific charters are required. These charters would use fishing gear with mesh size in 
the range 100 – 120mm. To ensure maximised number of viable cod to tag fishing tows 
would be short (30min – 1 hours) to limit damage and stress to fish.  

 
51 

Figure 20. Overview of core fishing areas in the Irish Sea by gear type (Nephrops – 
black, dredging – green, seine – red dashed & demersal (cod/haddock/hake) – blue. 
These areas are defined from kernel density analysis of Vessel Monitoring System 
(VMS) data using gear information from logbooks for the UK fishing fleet (2007 – 
2016). 

 
 
52 

Figure 21. Irish Sea trends in regulated gear nominal effort (kW*days-at-sea) 2003 – 
2016 for the international fishing fleet active in the Irish Sea. BT – beam trawls, GN – 
gill nets, GT – trammel nets, LL – Longlines, TR – trawls. Data are from the STECF 
(EWG 17-05) Fisheries Dependent Information report. 

2.2.5  Tagging Season 

A review of seasonal trends in commercial catches of cod was carried out. The review 
identified an apparent cyclical pattern in the historic landing series. The highest catches 
occur during the end of quarter 1 and the beginning of quarter 2 ( 

Figure 22). This coincides with the spawning period of cod when fishers are likely to 
target pre spawning aggregations. To maximise the catch rates of cod and achieve 
maximum numbers of tagged cod charters of fishing vessels during quarter 1 and early 
quarter 2 were planned for March 2016 and March 2017. A shorter charter was also 
planned to target the summer aggregations in quarter 3.  

 
 
 
  
53 

Figure 22. Commercial landings (t) from logbooks of cod in the Irish Sea by the UK 
fishing fleet. Quarters are coloured; black – quarter 1, red – quarter 2, green – quarter 
3 and blue – quarter 4. 

2.2.6  Tagging area 

Analysis of the spatial distribution of catches (before 2012 when TAC was less 
restrictive) showed a spatial pattern in the catches of cod. This is considered to be 
related to cod moving from deep water feeding areas toward spawning grounds. In 
quarter 1 and early quarter 2 (February – April) cod catches occur across the southern 
extent and inshore areas of their range (Figure 23). These more shallow areas represent 
the known spawning and inshore nursery habitats. Following this period, cod move to 
deeper water regions in the south east of the range and into the Northern Channel. 
Charter of fishing vessels primarily during this period were planned to be carried out in 
these regions. The tagging plan was developed using this information, with a primary 
objective of targeting spawning aggregations during the end of quarter 1 and the 
beginning of quarter 2. 

 
 
 
54 

Figure 23. Average month commercial landings of cod taken in the Irish Sea by UK 
vessels (2006 – 2012). Symbol size is scaled to the landed weight. Data are derived from 
logbook linked Vessel Monitoring System (VMS) data. 

2.2.7  Commercial fishery activity 

Data derived from at-sea observations of the Nephrops fishery show that whilst no 
targeted fishing for cod is permitted, the bycatch catch rates reflect the historic 
commercial catch patterns. Clusters of catches of fish above MCRS are observed in the 
south-eastern area and northern limit of the Nephrops grounds (Figure 24). Cod below 
MCRS are less frequently caught and are more widespread in their distribution (Figure 
26). Catches of cod above and below MCRS shows high levels of internal annual 
variation, with a decreasing trend over time. At the time of developing the initial tagging 
plan (2014/2015) and establishing target tag numbers (Table 6), it was unforeseen that 
the decreasing trend would impact on the availability of cod to tag. Based on estimated 
catches of cod it was expected that commercial fishing activity would generate annual 
values of on average 382 t (2007 - 2015). During the study (2016 – 2018) the annual 
catch estimates have been 127 t, with annual values of 82 t, 84 t and 215 t in 2016, 
2017 and 2018 respectively (ICES 2019).  

 
 
55 

Figure 24. Distribution of commercial catches of cod above the MCRS during 2013 – 
2016. The data are derived from AFBI observer at-sea data collection programme 
sampling catches on-board commercial fishing vessels. The size of open black symbols 
are scaled to catch rates, red circles indicate sampling location. 

Figure 25. Distribution of commercial catches of cod below MCRS 2013 – 2016. The 
data are derived from AFBI observer at-sea data collection programme sampling 
catches on-board commercial fishing vessels. The size of open black symbols are scaled 
to catch rates, red circles indicate sampling locations. 

 
 
 
56 

2.2.8  Reporting recaptures 

Information regarding recaptures was obtained from all sources that recaptured cod 
through ‘normal’ fishery and recreational activity likely to catch cod. A reward scheme 
was developed to encourage reporting of recaptures. Carcases could be left in markets / 
at ports for collection by project staff. Freezer space was provided at ports for fishers to 
return carcases and contact information. Telephone reporting numbers were on tagged 
with dedicated staff to receive and log recaptures.  
2.2.9  The tagging plan 

To achieve the project objectives a number of tagging charters were planned. These 
charters were intended to target cod of a range of sizes normally encountered in 
commercial fishing activity, for tagging. Fishing was planned to be undertaken within the 
western Irish Sea (Area 7a) and North Channel between longitudes 4°50’W and 6°W, 
south of 55°N and north of 53°20’N. Charters would also be carried out in the Celtic Sea. 
Scientists were trained to tag and assess the suitability of fish for tagging. It was 
identified that any opportunity to tag fish in suitable condition should be taken.  

In addition to tagging charters, research vessel activities, at-sea observers and 
recreational fishing events were identified as potential platforms during which cod would 
be tagged. Tagging activities were carried out through: 

 

Tagging Charters 

  Research vessel time 

  At-sea observers 

  Recreational angling events 

2.2.10 Tagging Charters 

Cod spawning aggregations are known in quarter 1. Fishery information supported 
evidence of aggregations of cod in late July/early August 2017 in deeper areas of the 
Irish Sea – these would also be targeted by charter tagging trips. Commercial fishing 
vessels were chartered by both AFBI and the Marine Institute. Initial plans for 6 charter 
trips were made. These charters would cover the entire distribution of know cod 
aggregations in the Irish Sea (7a), North channel (7a/6a) and primary cod aggregations 
in the Celtic sea (7g) and southern Irish Sea (7a). Tagging was carried out in the Irish 
Sea and adjoining sea areas, at the same time of year. This allowed for movement 
between areas to analysed, as per section 2.2.1. Failure to tag in adjoining areas would 
have meant that only one direction movements could be detected. The charter plan: 

  March 2016 – Western Irish Sea & North Channel (10 days) 

  March 2017 – Western Irish Sea & North Channel (10 days) 

  March 2017 – Northern Celtic Sea (5 days) 

  August 2017 – Western Irish Sea (5 days) 

Good catch rates of semi-pelagic commercial vessels with mesh size in the range 100  – 
120mm  have  been  observed  to  be  80-160  kg.h-1.  Taking  the  size  of  fish,  fish  condition 
suitable  for  tagging  and  the  operational  issues  on  board  the  vessels  into  account,  the 
minimum number of fish aimed to be tagged during this charters were 50 per day. These 
tagging targets were in line with achieved tagging numbers from previous tagging projects 
in the study area conducted by the project partners. 

The deployment of this method was severely restricted by the project budget. 

 
 
 
57 

2.2.11 Research Vessel opportunities  

AFBI sea-going scientists were trained to carry out tagging of cod encountered during 
existing research. Time during 5 planned research surveys, to be carried out on the RV 
Corystes in 2018 was identified whereby cod tagging could be carried out through both 
supplementary time or ad-hoc tows between normal survey operations: 

 

Ecosystem Survey (February 2017/2018) – RV Corystes 

  Groundfish survey (March 2017/2018) – RV Corystes 

 

Fishery-Science Partnership (FSP) gadoid survey (March 2017/2018) – Chartered vessel 

  Queen Scallop survey (July 2017/2018) – RV Corystes 

  Groundfish survey (October 2017/2018) – RV Corystes 

Tagging opportunity on the FSP survey has been significant in the past, but given the need 
to collect biological information on the other surveys limits tagging opportunities  to ad-
hoc tows that will be dependent on survey progress. Tagging target from research vessel 
surveys were based on previous observed catch rates. 

2.2.12 At-sea observer programme 

All Marine Institute observers would be provided with tagging kits and asked to avail of 
any tagging opportunities of healthy cod while sampling at sea. AFBI fleet observers 
would be trained to tag cod and would take tagging kits to sea on all observation trips, 
including pot fishing vessels, Nephrops and whitefish directed fishing vessels, to 
undertake opportunistic tagging where possible. 

At the time of the proposal the majority of cod caught in the commercial fisheries were 
discarded, so the plan was heavily dependent on using the extensive at-sea observer 
schemes within the participating laboratories to tag fish. The two dedicated tagging staff 
employed under the project will further increase the at-sea tagging opportunities 
presented from by-catch of cod in commercial trawl fisheries, dedicated pot fishing 
tagging trips and the developing quarter 2 haddock directed fishery in Northern Ireland 
that deployed semi-pelagic gear. Catch estimates of cod in the Irish Sea vs. the TAC 
indicated an average operational surplus of 200 tonnes (2012-2015). The observed 
operational surplus, the anticipated further reduction in the TAC under the cod plan and 
the observed numbers of cod discarded by the respective at-sea observer programmes, 
indicated that this method could yield a significant number of tagged fish. The tagging 
target from commercial fisheries capitalising on the extensive existing observer schemes 
and the further dedicated effort by the project tagging personnel. It was also recognised 
that tagging through the routine at-sea observer programme will continue beyond the 
proposed tagging programme duration of the project.  
2.2.13 Recreational angling events 

It was planned that both shore angling competitions and at-sea angling ‘recreational’ 
charter vessels would be used as tagging opportunities. Initially 10 shore angling 
competition days were identified. Tagging kits would be distributed to recreational 
anglers and training provided. The kits would contain a tagging gun, 20 standard reward 
tags, record sheets, a standard operating procedure (SOP) with pictures and a reward 
poster. It was targeted to have 20 volunteers with kits.  

Due to the uncertainty of catch rates and take-up from recreational fishers in the tagging 
project, tagging targets were conservative using this method. 

Table 6. Summary of tagging targets by method, area and season: 

 
 
58 

Tagging method 

Time 

Area 

Tagging target 

Charter – semi-
pelagic trawl 

Charter – 
demersal trawl 

Charter – semi-
pelagic trawl 

Q1/2 

Irish Sea/North Channel 

1000 

Q1/2 

Northern Celtic Sea 

250 

Q3 

Irish Sea 

250 

Research surveys  Q1-Q4 

Irish Sea/North Channel 

600 

At sea observer 
programme 

Q1-Q4 

Irish Sea/North 
Channel/northern Celtic 
Sea – randomised 
sampling scheme to cover 
fishing grounds and gears 

8500 

Recreational 
fishers 

Q1-Q4 

Irish Sea/northern Celtic 
Sea 

300 

Achievement toward these planned targets is given below in Section 2.3 and was 
reviewed and update during interim project review (Section 2.3.7 & Section 2.3.8).  
2.2.14 Communication and Outreach plan 

The project relied on collaboration with the fishing industry to provide the data to 
develop a better understanding of the current behaviour, biology and stock status of 
Irish Sea cod. Any recaptured tagged cod can provide useful information. It was 
recognised that the project should build on existing science and industry partnerships to 
achieve the project goals. The primary focus of the communication plan was with the 
main fishers active in the western Irish Sea, Clyde prawn grounds of the northern Celtic 
Sea. The main fishing nations in the areas are Ireland and Northern Ireland and account 
for more 90% of the fishing effort in the Irish Sea. The initial plan identified that fishers 
and fish producing organisations should be prioritised as the target audience of 
information material.  

The planned actions and achievements to meet the communication plan are outlined 
below.  
2.2.15 Commercial fishers: 

  Direct contact was made with a proportion of fishermen when planning and executing the tagging 
plan. Fishermen were appraised of the rationale of the project, and the purpose of the tagging 
work.  

  Direct contact with fishermen was made as part of AFBINI / MI Industry Liaison programme. 

Industry Liaison Officers raised awareness of the project, and informally discussed fishing practices 
with to gain a greater understanding of the issues. 

 

Indirect contact was made through the FPOs and regional fisheries offices. Industry leaders were 
provided with information about the project, and asked to pass on this information or contact 
details if they receive any enquiries about the work. 

  Media press articles were produced to communicate with stakeholders and the public. 

 
59 

 

Posters detailing the tagging programme were produced and sent to fishing ports and FPOs. The 
posters produced in English, French, Dutch and Welsh and distributed in French, Belgian, Dutch 
and Welsh ports.  

  A short summary of the project was translated into the languages of the majority of the crew 

working on local fishing vessels; mainly Egyptian, Polish, Filipino and Russian for distribution in 
ports, and vessel wheelhouses. 

2.2.16 Recreational fishers 

  Consultation was made with fishermen when planning and executing the tagging plan. Fishermen 

were appraised of the rationale of the project, and the purpose of the tagging work. 

  AFBINI / MI distributed information packs to regional angling groups, including information 

regarding opportunity and training to become voluntary taggers. 

 

Posters detailing the tagging programme were sent to recreational fishing organisations 
throughout Ireland, Isle of Mann and west coast of the UK. 

  Attendance by project staff at shore angling competitions provides not only tagging potential but 

also invaluable opportunity to speak to anglers directly about the project and to tailor the 
conversation to how they as stakeholders can be affected and/or become involved.  

2.2.17 Press  

 

 A Marine Times Article was published detailing the aims of the project, returns process and reward 
scheme; see March issue of the Marine Times newspaper. 

  Off The Scale Article in March 2018: Detailed experience of AFBI and MI taggers with the angling 
community, some preliminary results and release and recapture maps, details of the returns 
process and reward scheme – Link:  

 

 

 

 

http://www.offthescaleangling.ie/issue21/issue21.html#&panel1-13%20  

Irish Angling Update in April 2018: The monthly newsletter published on the Irish Angling Update 
website which is emailed to their subscribers mentioned the tagging project in conjunction with an 
angling club competition, showing the involvement with the angling community – Link: 
http://fishinginireland.info/news/sea-reports/good-fishing-at-ballyhack-for-meapia-sea 
anglers/?utm_source=Irish+Angling+Update&utm_campaign=b9f6f2fb2b-
EMAIL_CAMPAIGN_2018_04_19&utm_medium=email&utm_term=0_9f79bf242a-b9f6f2fb2b-
263548273 

Project staff attended and presented at the Swords Angling Expo on 17th February 2018. The 
presentation was well received and the expo proved to be a good networking event providing 
contacts for future events.  

The project poster and information was provided to Sea Angler magazine in February 2018 who 
shared the content on their Facebook page. This was shared further by interested anglers.  

  Angling NI published a press release pushed by the AFBI Corporate Communications department in 

April 2018. Link:http://www.gofishingni.co.uk/tag-a-cod-shore-angling-competition/ 

  A flyer (Figure 26) was distributed amongst anglers at shore competitions with maps detailing 

inshore release sites and information on the data collected from a returned fish. 

 
 
 
60 

Figure 26. Information leaflet distributed amongst angling community 

2.2.18 Engagement with stakeholders 

The results of the projects have been communicated to the NI-Gear Trials steering 
group, which includes representatives from fishery policy, fish producing organisations 
and fishers. The steering group is involved in a range of fishery selectivity improvement 
studies. At all quarterly meetings of the steering group scientist involved in the cod 
tagging study have been present. This allowed continual discussion of the results of the 
projects with both industry and policy stakeholders. Updates of the project have also 
been presented to the Seafish Northern Ireland Advisory Committee (SNIAC). The 
committee has membership form fish producing organization, government policy, 
scientific institutes and other NGO bodies. The committee meet 3 times a year. At all 
meeting of SNIAC scientists involved in the cod tagging study have been present to 
provide formal and informal updates of the project. The schedule meetings provide a 
regular forum to discussion the progress of the project form inception to delivery. A 
project update was presented at the ICES Working Group for the Celtic Sea Ecoregion 
(WGCSE) in May 2018 and in May 2019 (unpublished) at which the assessment scientists 
for cod in the Celtic Seas Ecoregion were present. The project was also included in the 
WKIrish 5 workshop, attended by scientists and industry representatives and a 
presentation given to the ICES data collation workshop (WKCeltic, 2019) which will 
develop new assessment methods for cod in the Celtic sea, ICES Area 7b-k. 

 
 
 
 
61 

2.3  Work package 2: The tagging campaign 

The project proposal was for a 15-month intensive tagging programme to start in quarter 
1 2016 and continue through 2017. Although there was a charter during the first quarter 
of 2016 and tagging carried out during the ‘Fishery Science Partnership’ (FSP) scientific 
survey  a  delay  of  12  months  in  starting  the  full  tagging  programme,  (e.g.  training 
volunteers, attending recreational events) was caused by delay in recruitment of project 
staff. During the first phase 38 days-at-sea were carried out on chartered fishing vessels 
(30 planned), during which 2433 cod were tagged. At-sea commercial fishery observations 
(552  observed  hauls)  provided  2  tagged  cod,  70  cod  were  tagged  during  44  days  of 
scientific survey days and 70 cod tagged from recreational angling events.  

2.3.1  First phase of implementation 

The first phase of the programme included training and equipping fishery observers to tag 
cod, organization of ad-hoc tagging during research vessel operations and engagement of 
recreational anglers. During the first quarter of 2016 AFBI staff concentrated on setting 
up initial procedures, tools and communication material to allow the project to be initiated. 
To take advantage of reports of a good abundance of cod in the area a short dedicated 
charter was conducted, staffed by existing fishery observers, with experience in previous 
tagging studies. All charters were completed on commercial fishing vessels with a track 
record in targeted fishing of whitefish in the Irish Sea or neighbouring sea areas. Short 
tows  are  preferred  to  increase  likelihood  that  captured  cod  are  in  good  condition  and 
suitable  to  allow  for  tagging  and  release.  Large  mesh  (100  –  120mm)  demersal  trawls 
targeting schooling cod in the mid-water (5 – 10m foot rope off bottom) or seine nets were 
the preferred method to catch cod in viable condition for tagging. Tagging achievement 
on these is detailed below: 

2.3.2  Completed Charters 

  Charter 1: Northern Irish Sea – March 2016 (AFBI) 

The charter was conducted in three short trips (influenced by weather conditions) during which 32 
short hauls were fished. Catch rates were good and a total of 963 cod were tagged during the 
exercise. The charter covered the western Irish Sea grounds, including transitional areas, i.e., 
North Channel. All the cod tagged were above the MCRS. The tagging followed the defined 
Standard Operating Procedure (SOP) shared with all project partners. The vessel used a mid-water 
trawl, as used in commercial fishing, to catch cod. 

  Charter 2: Northern Irish Sea – February / March 2017 (AFBI) 

The charter targeted cod of a range of sizes normally encountered in commercial fishing activity. 
Fishing occurred for up to 12 hours per day. The activity allowed for approximately five variable 
short tows (i.e. from 30 min to several hours) throughout the designated fishing area. Fishing was 
undertaken within the western Irish Sea (Area 7a) and North Channel between longitudes 4°50’W 
and 6°W, south of 55°N and north of 53°20’N. Fishing was conducted in a way, which samples 
across the entire area. Four fishing trips were undertaken over 15 days during which 645 cod were 
tagged. 

  Charter 3: Southern Irish Sea/Celtic Sea – April 2017 (MI) 

The charter was conducted on board a vessel using a seine net. The number of cod tagged was 
lower than expected owing to low catch rates and damage from other species in the net. The 
condition of each fish was examined carefully to ensure that only viable fish were tagged and 
released. 166 cod were tagged. Spatially, the charter covered the southern neighbouring area of 
the Irish Sea. 

 
 
 
62 

  Charter 4: North/mid Irish Sea – April 2017 (MI) 

The charter was conducted over two days, before the MI staff member and skipper agreed to 
postpone the charter owing to low catch rates of cod. In total 25 cod were tagged out of a total of 
39 caught. Using a demersal trawl. 

  Charter 5: Southern area of spawning grounds in Western Irish Sea – July 2017 (MI) 

This charter used the method and vessel as described in Charter 4, but fished in the Irish Sea, 
south of spawning grounds. During the trip 173 cod were tagged. 

  Charter 6: Deep water aggregations south east of Nephrops grounds – August 2017  

The charter used a mid-water trawl net to target cod of a range of sizes normally encountered in 
commercial fishing activity. Fishing occurred for up to 12 hours per day. The activity per day 
should allow for approximately five variable short tows (i.e., from 30 min to several hours). Five 
days were undertaken across two fishing trips during which 461 cod were tagged. It was noted 
that a higher pre-tagging mortality rate of cod was observed during the trip; a possible 
explanation for this was the higher air temperature causing added stress, fishing in deeper water 
and other biological factors such as parasite load. Intact specimens (15 fish) were retained and for 
post-mortem but yielded no conclusive results. 

2.3.3  At-sea observer programme 

Observers aimed to carry out tagging during routine observations. Observers reported that 
fish  were  not  frequently  caught  and  when  caught  were  rarely  in  suitable  condition. 
Practicalities  with  handling  holding/recovery  tanks  during  commercial  operations  also 
proved  problematic  on  most  vessels  when  vessels  are  engaged  in  commercial  fishing 
activity.  Due  to  all  these  factors,  tagging  through  the  observer  schemes  yielded 
significantly  less  tagging  than  originally  anticipated.  Although  these  non-designated 
tagging charters provide a platform to tag cod throughout the year.  

2.3.4  Research vessel activity 

Tagging during surveys on-board the RV Corystes was conducted although low numbers 
of  suitable  fish  were  encountered.  Utilising  vessel  time  within  the  existing  survey 
programme required identification of time which coincide with suitable fishing conditions. 
During  the  on-going  survey  programme  14  cod  were  tagged.  During  the  Northern  Irish 
Fishery-Science Partnership survey (NI-FSP) in the both 2016 and 2017 carried out in the 
western Irish Sea opportunistic tagging of cod was prioritised. In 2016 12 cod were tagged 
with 44 tagged in 2017. The survey is used to assess the biomass of cod spawning stock 
biomass in the region. The fishing characteristics are akin to the commercial fishing activity 
that historically targeted cod with tow duration of 4 – 6 hours.  

2.3.5  Recreational fishers: 

During the first phase of the project tagging personnel developed a working relationship 
with  the  recreational  angling  community,  attending  shore  competitions  and  sea  angling 
boat trips in an attempt to tag cod, with four events providing 57 tagged cod. This also 
provided  an  opportunity  to  train  and  recruit  volunteer  taggers.  Two  volunteers  tagger 
successfully tagged 5 cod during the first phase of the tagging programme.  

2.3.6  Tagging in practice: 

During  charters,  multiple  holding  tanks  with  continuous  supply  of  fresh  oxygenated 
seawater  (at  times,  up  to  4)  were  used.  Cod  were  transferred  to  holding  tanks,  as  a 
priority, to ensure the maximum number of cod from the haul were in a healthy condition 
for tagging. Cod permitted acclimatisation and recovery time within holding tanks before 

 
63 

tagged and where possible, held for a time post-tagging in the holding tank before being 
released back into the sea. Only when a fish was successfully tagged and released alive 
were they added to the dataset to be reported. Fish in the holding tank with inflated swim 
bladders, upturned and unable to right themselves, or fish with ‘low energy’ levels despite 
excessive  scarring  or  wounds  present  were  not  tagged.  Some  healthy  tagged  fish  were 
observed being attacked and/eaten by sea birds after release, these fish were not included 
in the “tagged fish” dataset. Some catches during charters (specifically Charter 6 in 2017) 
included  fish  with  heavy  sea-lice  (external  parasites)  infestations,  with  observed  low 
energy levels, and so tagging was not performed. Samples were taken to the lab for post-
mortem  analysis  but  results  proved  inconclusive.  Some  upturned  fish  were  deflated  by 
scientists on board by piercing the swim bladder and allowed time to recover. This proved 
worth-while as two recaptures to date have been of individuals recorded as having their 
swim bladder pierced.  

Project staff attending shore and sea-angling used a similar protocol during angling events. 
It was  observed that fish caught at angling events tend to be in excellent condition for 
tagging given their short duration out of the water and small change in pressure owing to 
fish coming from shallower depths. Fish were rejected for tagging if injury had been caused 
by unhooking (even if the fish was  still viable), to further reduce the  possibility of post 
tagging mortality. This was to provide quality assurance and reporting accuracy.  

In preparation for tagging, a tagging work-station was set-up and the available deck tanks 
filled  with  seawater,  ready  for  the  reception  of  live  cod.  The  deck  tank  set-up  would 
ultimately vary between vessels. Cod were manually removed from the trawl catch, gill 
nets or hooks and placed in deck tanks. If possible the codend of the trawl was lowered 
into  a  filled  deck  tank  and  emptied.  The  deck  tanks  received  a  regular  and  frequent 
exchange of clean seawater. 

The expectation was that all cod considered to be lively and in good condition would be 
tagged  with  an  external  mark  ID  tag.  Individuals  suitable  for  tagging  were  measured 
before being placed onto a suitable surface such as a V-shaped foam block, where their 
gills and head were be covered with a wet towel (kept wet with fresh seawater). If possible, 
tissue samples were taken for future genetic analysis. Training of vessel crew in good cod 
handling  methods,  recording  and  tagging  and  release  procedures  was  emphasized  to 
maximize the chances of future live commercial discarding and survival of cod. 

2.3.7  Interim review of tagging programme – November 2017 

At a project meeting attended by all project partners in November 2017, the tagging 
approach and methods were reviewed. In the initial proposal, it had been anticipated 
that tagging could be carried out during at-sea observations within the existing fleet 
observer programme by national fisheries laboratories on both white-fish directed boats 
and the Nephrops fishery. Owing to long haul durations and practical constraints with 
limited space for holding tanks on board, extensive tagging during commercial 
operations proved prohibitively challenging. It was identified that commercial charters 
proved extremely successful. After an extensive and necessary review by all project 
partners, the tagging programme was redesigned and the focus placed on extending the 
commercial charters, with maximum budget committed to this tagging activity.  

An analysis to evaluate the level of confidence in the current programme’s ability to 
derive robust conclusions was applied. This analysis provided an assessment of the level 
of confidence that could be attributed to the observed patterns of recaptures and 
potential sources of cod mortality. The analysis used a time series of tag release events, 
recapture reports and simulated fishery removals. The results of the Lincoln-Petersen 
mark-recapture method (Pollock 1982) for estimating population size were examined. 
The analysis was applied using numbers of fish and converted to a biomass estimate. 
The catches by fishers were calculated from at-sea observed catch rates of cod. These 

 
64 

rates were applied to the reported fishery activity, from logbooks, over the course of the 
tagging study. 

The analysis diagnostics were used to review the confidence in the project and tagging 
plan to deliver fundamental population level insights in the cod population in the study. 
The results of the analysis were initially discussed with the EASME at the review meeting 
in December 2017, with further detailed discussion in January 2018. These diagnostics 
support that the implemented tagging programme could provide robust information 
about the Irish Sea cod population. Simulations of tagging effort in 2018 support that 
increased tagging effort in 2018 would continue to improve confidence in the population 
parameters derived from the study. Following an initial analysis of the recapture data 
collected at that stage and qualitative evaluation of tagging success, a tagging plan for 
2018 was developed.  

The results of post mortem examination of recaptured fish were discussed with regard to 
evidence to support the appropriateness of the tagging procedure to ensure viability of 
the method. All recaptures throughout the project were examined to ensure quality was 
maintained. For all recaptured tagged fish there was no evidence of mal-effect of 
tagging. Re-growth of skin around tagging sites, absence of abscess, blood spots and no 
evidence of necrosis was observed. This evidence supported the view that the tagging 
protocol was effective and all staff taking part in tagging activities were properly trained. 
Tagging equipment was taken on all AFBI fishing surveys on the research vessel RV 
Corystes by trained staff during in 2017, to allow for opportunistic tagging of cod. The 
‘ground fish surveys’ in March and October and the Queen Scallop survey in June were 
identified as surveys with a high likelihood to yield most tagging opportunity. 
2.3.8  Redesign actions 

As commercial charters proved extremely successful and had provided the main source 
of tagged cod (963 tagged fish from the first charter alone), further charters were 
planned by both AFBI and the Marine Institute in Q1 of 2018 to coincide with the 
spawning season of Irish Sea cod. During the AFBI charter in late July/early August 
2017, observers reported increased numbers of cod being unsuitable for tagging. Some 
cod were landed in order to carry out biopsies and lab tests to determine the cause of 
poor suitability. It was considered that March provided greater numbers of suitable fish 
for tagging, therefore all available budget was committed to charter days in Q1, with 
August/September charters being second choice should any budget be left over.  

Additional charters: 

  March 2018 – Western Irish Sea & North Channel (5 days) 

  March 2018 – Northern Celtic Sea (5 days) 

  August 2018 - Western Irish Sea (5 days) 

It was planned that the charter programme would be extended until August 2018. In 
March 2018 AFBI would target cod in the North Channel and the Marine Institute in the 
Celtic Sea, with an overlap of tagging by AFBI and the MI on the Irish Sea cod spawning 
grounds. This spatial overlap was to ensure that the majority of effort was targeted on 
the largest aggregation of cod in the Irish Sea. All charters would be staffed by two 
experienced observers. In August 2018 AFBI would carry out a further charter to target 
cod in deep water aggregation in the Western Irish Sea.  

Furthermore, that all charters would be staffed by two members of personnel, using 
contract observers where necessary, to ensure that the largest possible number of 
healthy cod were processed and tagged. During year one of the tagging programme it 
quickly became apparent that space for and logistics of deck tanks on board the 

 
65 

Nephrops fleet specifically proved challenging. This however, worked well on charter 
trips, where deck space for multiple holding tanks and a suitable workstation set-up was 
taken into consideration during vessel selection. The redesigned plan of November 2017 
this was communicated to the contracting authority in December 2017 and January 
2018. It was paramount to ensure that only viable cod with a high probability of survival 
should be tagged and released.  
2.3.9  Research surveys: 

A dedicated cod tagging trip was planned on the AFBI research vessel RV Corystes in late 
January 2018. It was intended that the AFBI project scientist would lead a 3 day cruise 
from January 31st to February 3rd 2018, along with trained personnel. Any ship time 
available for cod tagging whilst the RV Corystes was at sea, was committed to be utilised 
where possible. It was anticipated that any time available on other research cruises 
would have cod tagging added to the sailing orders as an additional duty of the cruise 
plan, either by way of a supplementary day or ad-hoc tows in between sampling stations 
of the relative survey. Where time and resources allowed on the AFBI survey schedule, 
fishing for cod and subsequent tagging was planned to be carried out. Tagging 
equipment and trained personnel were to be present on every cruise where potential cod 
tagging may occur.  

The Northern Ireland Fisheries Science Partnership survey, conducted in March, provided 
some opportunistic tagging in 2016 and 2017, with some survey stations located at the 
cod spawning grounds. Tagging was, however, an additional task to be carried out where 
possible on top of the main survey tasks. To allow for more fish to be tagged more 
efficiently and maintain them in better condition, it was proposed that a dedicated 
tagger, most likely a contracted observer would also staff the NI FSP in March 2018, with 
the aim to improve on the previous year’s numbers of tagged cod.  

2.3.10 Recreational tagging volunteers 

A recreational tagging programme was also proposed for interested stakeholders to get 
involved in the tagging project. Tagging kits containing a tagging gun, 20 standard 
reward tags, record sheets, a SOP with pictures and a reward poster were put together 
to be distributed amongst suitable stakeholders in a plan to establish a voluntary tagger 
initiative.  

2.3.11 Ad-hoc tagging 

The Marine Institute has a well-established observer programme in the Irish Sea and 
Celtic Sea for offshore and inshore vessels. Trained observers were provided with 
tagging kits and asked to avail of any tagging opportunities of healthy cod while 
sampling at sea, with the permission of the skipper. AFBI fleet observers were trained 
and continued to take tagging kits to sea on all observation trips, including pot fishing 
vessels, Nephrops and whitefish directed fishing vessels, to undertake opportunistic 
tagging where possible.  

Opportunities to tag during the commercial inshore fishery were investigated. Project 
staff contacted pot-fishermen to carry out observation trips on their vessels in a bid to 
tag any cod caught. The Marine Institute’s inshore team were enlisted to provide 
contacts and possible tagging trips alongside their regular sampling trips.  

The tagging programme review also identified the need to allocate more effort towards 
generating tagging opportunities targeting cod below MCRS, to ensure the sample of cod 
tagged was reflective of the Irish Sea cod population. Project staff attended national 
angling competitions, which provided some cod for tagging and received reports of good 

 
 
 
66 

inshore fishing during the winter months. An extensive schedule was developed for 
project staff to attend shore competitions and sea angling boat trips in an attempt to 
increase tagging numbers and target fish below the MCRS.  

2.3.12 Update to reward scheme 

After review of initial data the rate of recaptures was lower than expected < 3% 
compared to rates exceeding 27% (Burt et al., 2006). It was discussed that a potential 
reason for this was that while the commercial fishing skippers are locals, the majority of 
crew are foreign, often with limited English language skills, therefore the posters may 
have been having only a limited impact. Skippers had been contacted directly by project 
staff, but this did not guarantee that the information was passed on to crew. There had 
also been some feedback from the industry that the EUR 25 reward was not viewed as 
sufficient incentive to encourage crew to look for tags. To increase the likelihood of crew 
actively looking for tags, as well as the likelihood of skippers passing on project 
information to crew, the decision was made to increase the tagged return incentive. 
Previously there had been a EUR 25 reward for red tags and EUR 75 for pink, blue or 
yellow tags. An increased incentive of EUR 1000 for every 20th tagged cod returned to 
the project (for a period of time at project staff’s discretion) was added in December 
2017. It was expected that this would increase the incentive to check for tags and for 
skippers to pass on project information to their crew. Furthermore, the opportunity was 
taken to increase additional media coverage published when a EUR 1000 reward was 
received in the relevant fishing news press, social media, AFBI and MI websites.  

2.3.13 Second phase of tagging programme; January 2018  

 

Following the interim review of the project, by project partners in November 2017, an 
updated tagging plan for 2018 was adopted. The focus of intensive charter tagging trips 
was considered to be best applied in quarter one, focused in the western Irish Sea but 
exploring other areas also. Extensive tagging using other methods, at lower intensity 
would still be applied outside quarter one and some shorter charters were planned for 
quarter three when fish concentrate in deep water. Every opportunity to tag cod was taken 
and extensive resources and staff were provided to the project in-kind. 

  Charter 7: Irish Sea spawning grounds and Celtic Sea – February/March 2018 (MI)  

 

 

The first trip was carried out at the end of February in the Celtic Sea Conservation area 
(statistical rectangle 32-E3). The charter was conducted using a demersal trawl to target 
schooling cod. After 12 hours of searching for cod within the closed area, the vessel moved 
outside the conservation area to finding cod at low densities. The vessel returned to 
normal fishing activity (non-chartered time) and allowed any cod that came on-board to be 
tagged. The vessel was out for a total of three days – two days of which were dedicated to 
cod tagging – and tagged a total of 41 cod in the Celtic Sea over 13 hauls.  

In mid-March this charter was re-commenced following reports of cod catch by the vessel. 
Four days were spent fishing in the Irish Sea where 225 cod were tagged and released 
over 28 hauls. Weather disrupted the trip and the vessel returned to port until conditions 
improved. During the 3rd trip, three days fishing resulted in 170 tagged cod over 17 hauls, 
weather was again the cause of return to port. The vessel was contracted for one further 
day of tagging and the vessel owner committed to providing a further day of dedicated cod 
tagging and accommodating MI staff member(s) on-board during regular fishing trips later 
in the summer to tag any viable cod caught. The remaining two days of tagging on-board 
the charter vessel were to be arranged with the vessel owner.  

  Both phases observed that dense cod aggregations were scarce in the western Irish Sea 

during the first quarter of 2018. 46% of the cod tagged during the charter programme 
were tagged over four days of fishing effort in the North Channel where cod were observed 
in dense aggregations and were ideal for tagging (in good condition when caught). Every 
effort was made to tag as many viable fish as possible. 

 
 
 
67 

  Charter 8: North Channel and Western Irish Sea – February / March 2018 (AFBI)  

  A total of 975 cod were successfully tagged and released over four dedicated commercial 
charter trips in February and March 2018. Four days were spent in the North Channel 
where 647 cod were tagged over 18 hauls. 10 days were spent in the Western Irish Sea 
where 328 cod where tagged over 39 hauls. During the four trips over 89 % of the cod 
caught were tagged, mortality rates were considered low. Two dedicated cod tagging staff 
were present on each of the trips and the vessel provided plenty of deck space, allowing 
for four holding tanks when needed. Tagging was carried out from 25/02/2018 - 
21/03/2018 when progress was impacted by poor weather conditions. Trips were ended 
prematurely on two occasions due to storms and cod gradually dispersed, resulting in few 
cod caught per haul towards the end of March. 14 days were completed before catch rates 
decreased to a point where continuing with the charter was no longer deemed to be 
providing sufficient catch rates. The remaining six days were reserved for use in 
August/September to carry out further tagging.  

  Charter 9: Western Irish Sea – August 2018 (MI) 

  Dedicated cod tagging was carried out by the Marine Institute from 28/08/2018 – 

30/08/2018, where 85 cod were tagged over 13 hauls. The vessel used demersal trawl 
fishing gear. Fishing was concentrated in the Western Irish Sea but cod catches were 
deemed to be low. In total, 85 cod were tagged.  

  Charter 10: Northern Irish Sea – September 2018 (AFBI) 

  A further 203 cod were tagged and released in early September 2018 during a dedicated 

commercial trip. Two dedicated cod tagging staff tagged fish over 16 hauls in the northern 
Irish Sea. The vessel was used previously on dedicated tagging charter on the current 
project and so crew were experienced in survey operations and aiding the scientific staff in 
preparing the deck tanks as before. Finclip samples were taken from tagged fish. Cod were 
in good condition to be tagged and mortalities were retained for biological analysis.  

2.3.14 Research Vessel Activity 

A biological survey during which all viable cod was planned for tagging was conducted on 
AFBI’s  RV  Corystes  from  31/01/2018  -  02/02/2018.  Both  AFBI’s  cod  tagging  project 
manager and the MI cod tagging technician were present as well as two additional AFBI 
scientists. After review of historical cod landings data and consultation with local skippers, 
six  hauls  were  fished  over  two  days  on  cod  grounds,  however,  low  catch  rates  were 
observed. The survey experienced adverse weather conditions resulting in one survey day 
being  lost.  Other  whitefish  vessels  were  also  operating  in  the  fishing  areas  and  yielded 
similar  low  catch  rates  of  cod.  This  aided  the  decision  to  delay  dedicated  commercial 
charter days until the end of February as cod aggregations had not begun to show at this 
time. 

  During the project review in November 2017, four research surveys planned for the RV 

Corystes and chartered survey vessels in 2018 were identified where cod tagging could be 
carried out through either supplementary time or ad-hoc tows between normal survey 
operations: 

 

Ecosystem Survey (February 2018) RV Corystes: This survey coincided with the first few 
dedicated charter dates and therefore cod tagging staff were unavailable for the Ecosystem 
survey. Time on the commercial vessel rather than the Ecosystem survey would provide 
more opportunity for tagging cod.  

  Groundfish Survey (March 2018) RV Corystes: Three dedicated cod tagging days were 

conducted within the time allocated for this survey, since the survey objectives were met 
on time. The groundfish survey was completed in good time, allowing for the vessel to 
return to port to pick up cod tagging personnel and carry out three more days of semi-
pelagic fishing from 23/03/18 - 26/03/18. Five hauls were fished in the Western Irish Sea 
and two hauls in the North Channel where 18 cod were tagged. Cod aggregations had 
dispersed. 

 
 
68 

  Despite providing some tagging opportunity in previous years, the Queen Scallop Survey 

on board the RV Corystes in July 2018 and the Groundfish Survey in October 2018 did not 
catch any suitable cod for tagging. 

 

Fishery-Science Partnership Gadoid Survey (March 2018) – MFV Sparkling Sea: The MFV 
Sparkling Sea was charted to carry out this survey, accompanied by a member of the fleet 
observation staff as a dedicated tagger to maximise on tagging opportunity. Fishing tow 
durations averaged of six hours and therefore not all cod were in suitable condition for 
tagging, however, 113 healthy cod were tagged. 

2.3.15 At-sea Observer Programme 

A  target  of  20  pot  sampling  trips  from  January  2018  to  August  2018  were  planned. 
Progress was reviewed on an ongoing basis. Fish hauled in pots are normally in excellent 
conditions  for  tagging.  Despite  conversation  with  pot  fishing  skippers  to  gain  more 
understanding of numbers caught in pots, these fishing trips did not yield much success. 
When time allowed, project staff continued to carry out pot fishing trips for opportunistic 
tagging,  however  the  target  of  20  days  was  lowered  in  priority.  In  total  13  pot  fishing 
observation days were carried out, yielding 4 tagged cod.  

One  pot  vessel  skipper  showed  an  interest  in  contributing  to  the  project  by  utilizing  a 
tagging  kit  so  after  a  tagging  demonstration  onshore,  the  vessel  utilized  the  kit  when 
opportunities  arose,  resulting  in  16  voluntary  tagged  cod  during  pot  fishing  activities. 
Various other methods were trialled and tested to capitalise on all tagging opportunities, 
including using a vivier to hold cod caught in fishing pots and purchase of cod traps. Every 
effort to tag cod at any given opportunity was seized during the duration of the tagging 
scheme.  

AFBI  fleet  observers  continued  to  take  tagging  kits  to  sea  on  all  observation  trips  (584 
hauls  observed)  including  pot  fishing  vessels,  Nephrops  and  whitefish  directed  fishing 
vessels  to  undertake  opportunistic  tagging  where  possible.  Tagging  opportunity  on 
commercial fishing vessels remained low due to the long towing duration and the inability 
to use holding tanks of adequate size (deck space restrictions) yielding only a single cod 
for tagging in the second phase. The Irish Sea was ‘closed’ to targeted whitefish activity 
during February to April. The targeted haddock fishery which can yield cod began on May 
1st,  however  an  increased  cod  TAC  for  the  Irish  Sea  further  limited  fish  available  for 
tagging. In total 15 cod were tagged on commercial trips of this nature.  

2.3.16 Recreational fishers - Shore & Sea angling competitions 

The  tagging  programme  review  also  identified  the  need  to  allocate  more  effort  towards 
generating tagging opportunities targeting cod below MCRS, to ensure the sample of cod 
tagged was reflective of the Irish Sea cod population. Cod tagging personnel developed a 
working  relationship  with  the  recreational  angling  community,  attending  shore 
competitions and sea angling boat trips in an attempt to tag cod. This proved the second 
most successful tagging activity after commercial charters in terms of numbers as well as 
suitable  fish  condition.  Significant  effort  was  directed  towards  capitalising  on  this 
opportunity, particularly in the last year of the project. 

During phase 2 a total of 29 angling competitions were attended with 267 cod tagged. The 
recreational  angling  clubs  were  engaged  to  assist  in  the  project.  Whilst  project  staff 
attended shore competitions on beaches along the East Antrim coastline, feedback from 
anglers was that historically good cod catches were seen off the more exposed coastlines, 
rocky outcrops, near deeper ledges and piers along this particular stretch of coastline, in 
winter and during the April/May “spring run”  of  cod. Cod are not frequently targeted in 

 
 
 
69 

these competitions. To test the ability of this approach to provide opportunity, a “Tag a 
cod”  competition  was  organised  by  project  staff  and  co-hosted  Sea  Angling  Club.  This 
competition took place on 28th April 2018. The competition was attended by 47 anglers, 
fishing  for  5  hours  from  rocky  outcrops  and  piers  using  methods  targeted  at  catching 
gadoids. In total 3 cod were caught and tagged. During the summer months in 2018, 16 
trips,  through  a  combination  of  angling  boat  charters  and  individual  trips  were 
accompanied by tagging staff.  

2.3.17 Voluntary Taggers 

Fourteen  tagging  kits  were  provided  to  interested  anglers  and  one  potting  fisherman 
tagging  66  cod  during  phase  two  of  the  tagging  programme.  The  Ulster  Wildlife  Trust 
(UWT)  secured  funding  for  an  elasmobranch  tagging  study  to  be  commenced  in  the 
summer/autumn  of  2018.  The  AFBI  and  the  UWT  project  coordinator  planned  to  run 
tagging training in a collaborative capacity to multiple anglers at various training sessions 
around the country.  

2.3.18 Overview 

During the second phase 32 days-at-sea were carried out on chartered fishing vessels (15 
planned),  during  which  1699  cod  were  tagged.  Charters  for  this  phase  were  originally 
planning  for  15  tagging  days,  but  were  increased  to  32  –  the  maximum  that  could  be 
achieved  with  maximum  budget  committed.  This  was  increased  to  attempt  to  increase 
tagging  numbers,  since  other  methods  yielded  low  numbers  despite  significant  effort. 
Achieve of at-sea commercial fishery observations (584 observed hauls) provided 1 tagged 
cod, 131  cod were tagged during 16 days of scientific survey days and 333 cod tagged 
from recreational angling events, including 66 by voluntary taggers.  

2.3.19 Review of the tagging programme  

Conceptually this project was initiated as a large scale tagging programme attempting to 
tag both juvenile and adult cod. The project proposal therefore considered all possible 
opportunities for tagging to maximise tag opportunities from existing sampling 
programmes as well as funding a number of charters through the project (focusing on 
cod closure periods, for example). Tag numbers were originally estimated based on 
catch rates observed at that time during the entire suite of sampling activities and was 
informed by tagging achievement in previous projects. The aim was to maximise all 
tagging opportunities with assumptions made on catch rates on commercial vessels, 
potential charter vessels and research surveys. The availability of fish to tag is, of 
course, completely dependent on the stock abundance, commercial gear selectivity and 
effort. The significant uncertainties associated with this was a known entity at the time 
of project commissioning. 

The project proposal was for a 15-month intensive tagging programme to be conducted 
during year one of the project and the first quarter of the second year. Although there 
was a charter and tagging during on-going surveys during the first quarter of the 2016, 
the delay in the recruitment of the dedicated tagging personnel resulted in delay of 
intensive tagging programme by 12 months, but when initiated this continued for 20 
months. Dedicated tagging personnel remained in post for the duration proposed in the 
project plan. 

Lower than expected recapture rates were observed during the initial stage of the 
project. The project was initiated and resources campaigned for by the fishing involved, 
therefore, buy-in and awareness was thus high from the onset of the tagging activities 
and industry cooperation was considered unlikely to have a negative impact on return 
rates. Given the significant implication of low return rates within the context of high total 
mortality for the stock, focus was shifted completely for fish condition to mitigate the 

 
70 

possible influence of post tag mortality on the results. This change in emphasis was 
supported by feedback received on review of the initial project reports by EASME. The 
result of the focus on quality and fish condition, rather than quantity clearly effected the 
absolute numbers tagged. 

As a result, some tagging activities, e.g. at-sea observer tagging during commercial 
operations, did not yield the tag numbers estimated. The tagging effort increased on less 
intrusive capturing techniques, such as pot and recreational fisheries, but this resulted in 
low tag numbers (<10) in relation to tagging effort. Chartering commercial vessels 
proved to be the most effective method and produced the highest proportion of fish in 
good condition. Being a costly part of the tagging programme, the initial project plan 
allowed for 30 charter days. Despite the budgetary implications, the project plan had to 
be amended to ensure robust delivery of the scientific objectives of the project. 
Completed charter days more than doubled to 69 days over the duration of the project. 

At the review stage an analysis was applied to examine the ability of the project to 
deliver robust population level parameters. The analysis was carried out at the interim 
project review stage to inform the plan for tagging in the second stage of the tagging 
program. The purpose therefore was to use expected tagging rates based on lessons 
learnt during the initial stage. 

An absolute assumption of direct proportionality between tag numbers and tagging effort 
would be incorrect. In the first instance there is no consideration given to tagging quality 
or success. For example, tagging all cod by-catch from the commercial fishery regardless 
of fish conditions would have resulted in a larger number of tagged fish, however even 
within a fisheries management context the assumption would be for zero survival of 
these fish. Alternatively effort could have been biased toward nursery areas to generate 
high catch numbers, however, this may have impacted the ability of the project to meet 
its objectives, in particular, with regard to elucidating the steep age structure of Irish 
Sea cod. Similarly, in a scenario where the population abundance declines during a 
tagging project, an increase in tagging effort will yield less tags compared to a scenario 
of stable population. Tagging effort, in fact, exceeded that initially planned over the 
duration of the project. Whilst lower tagging numbers were achieved compared to the 
original estimate, this is a reflection of the changing behaviour within the fishery, 
management and control, and natural variability of the population beyond the control of 
the project. 

 
 
 
71 

2.4  Work package 3: Results & Data analysis  

2.4.1  Review of mark-recapture analysis methods 

Typically  tagging  studies  create,  by  marking  them,  a  subset  of  animals  within  their 
population that are known and can be identified by their tags or marks. Following a period 
of time allowing this marked subset of animals to mix with the rest of the population, their 
progress is then followed by means of their recapture and reporting, in both space and 
time  to  provide  information  that  can  be  associated  with  the  entire  population.  Tagging 
studies  have  historically  utilised  an  array  of  methodologies  to  provide  information  on 
aspects  of  fish  biology  and  behaviour.  These  methods  rely  on  marking  and  recapture 
events  and  have  become  common  since  the  mid-17th  century  with  a  mark-recapture 
method first used for ecological study in 1896 by Petersen to estimate plaice (Platichthys 
platessa) populations (Petersen 1896). Over time the methods and application of mark-
recapture studies have evolved. Initially the technique was used to estimate abundance, 
developing  into  a  method  to  assess  survivability  and  sources  of  mortality  and  more 
recently explore species behaviour. The ability to achieve these applications is linked to 
advances  in  tagging  technology,  with  available  tagging  now  ranging  from  passive  non-
permanent ink-marks, non-digital plastic tags, micro coded wire tags, to radio-frequency 
identification  (RFID)  tags  and  electronic  tags  recording  storing  environmental  and 
positional  data  such  as  data  storage  tags  (DSTs)  and  satellite  tags,  providing  real  time 
behavioural  data.  In  some  instances  chemical  markers  using  methods  such  as 
radiochemical dating or marking with calcium-binding chemical oxytetracycline, alizarin, 
or calcein are appropriate, where large numbers of individuals can be tagged and ageing 
and growth is a focus of the study, as such techniques laydown a marker in the skeletal 
component used in ageing (Wilson et al., 2011). 

Conventional mark-recapture methods can inform biologists about the two specific time 
points in the life of a fish – when caught and tagged, and when recaptured – in some cases 
multiple  recapture  events  on  the  same  individual  can  occur.  In  addition  to  collecting 
spatiotemporal data regarding the location of tagging and recapture the condition, age, 
body mass, length and other biological parameters can be collected at both the initial and 
re-capture  events,  and  information  inferred  as  to  the  behaviour  and  biology  of  the  fish 
during the period at liberty.  

Within any tagging study there are assumptions which under pin the analysis. These must 
be recognised to ensure valid interpretation of results: 

  Marked animals are not affected (either in behaviour or life expectancy) by being marked, 

and the marked individuals can always be attributed to a marking event. 

  Marked animals are representative of the entire population.  

 

The probability of capturing a marked animal is the same as that of capturing any member 
of the population.  

Mark and recapture is a method commonly used in ecology to estimate an animal 
population's size. The principle used to calculate the population size is based on the 
theory that when a sample from a population are caught, marked, returned to the 
population, allowed to completely re-mixing and are re-sampled, the number of marked 
individuals in the second sample will have the same ratio, as the total number of marked 
fish in the population. From this, the population size can be estimated. This is the most 
basic form of the analysis. Other intricacies and assumptions are however, made within 
a range of analytical techniques.  

Populations can be classified as either open or closed. A closed population remains 
unchanged over the period of study whereas an open population changes because of 
some combination of birth, death and emigration. Closed models include the Lincoln–

 
72 

Petersen model and are most appropriate for short-term studies (Otis et al., 1978; Seber 
1982). 

In its simplest form the population estimate for a mark-recapture study is derived: 

Rearranged to: 

 𝑀/𝑁 = 𝑥/𝑛 

𝑁 = 𝑛𝑀/𝑥 

Where N is the population size, M is the number of marked individuals in the first catch, 
x the number of marked individuals in the second catch and n the total number of 
observed individuals in the second catch. 

These methods assume that the population does not change over the period of study – 
no immigration or emigration, births or deaths – which is termed a closed population. It 
is therefore essential that this period is short compared to life expectancy. Further, 
individuals must not be able to enter or leave the study area. Methods in which the 
probability of capture is not assumed constant require a series of occasions (at least 
two) on which animals previously marked are recaptured, marked again, and released.  

A number of methods have been developed to estimate population parameters for open 
populations, of which the most general is the Jolly-Seber method (Williams et al., 2002). 
Simply tagging offers both a full Jolly-Seber analysis and a reduced model in which 
probability of capture and survival are assumed to be constant over the period of the 
experiment. Before either of these methods is attempted, it is important to note that 
they are unlikely to produce good estimates unless a high proportion of the population is 
tagged. If only a small proportion of the population are captured, then closed methods 
should be used and effort is best expended in ensuring that the population can be 
assumed closed for the duration of the experiment. Pollock (1982) detailed ‘the robust 
design’, a combined analysis design using aspects of both closed and open models. 
Within this method a series of discrete, closed population studies are linked to provide 
an estimate population of abundance with an open-population model to estimate survival 
(analysis of mark-recapture data to estimate aspects of survivability are dealt with 
below). The main advantages of the robust design is that it allows for unequal capture 
probabilities and also temporary emigration of fish (Williams et al., 2002).  

The present study used the ‘robust design’ (Pollock 1982), with multiple tagging events 
and continuous reporting of recaptures to inform the tagging study and ensure validity of 
the results. A population assessment was applied using the Lincoln-Petersen mark-
recapture method for estimating population size was applied. This method allows an 
estimate of population size to be derived from the current tagging programme, whilst 
making an assumption about losses from the population due to natural processes. The 
method was used to assess the power of the tagging study as a diagnostic comparison 
with the ICES assessment of Irish Sea cod SSB.  

One of the objects of tagging experiments is to estimate the survival rate of the 
individuals in the tagged population. The final estimate may be expressed in terms of the 
proportion of individuals that survive for one time step (e.g. a year) or as an 
instantaneous coefficient of total mortality. Typically, methods for estimating survival 
rates are divided into two groups (Jones 1979): 

Methods for estimating survival rate when tagging have been done on one occasion only. 
If tagging is carried out at single event the survival rate can be determined from the rate 
of decline of the tagged individuals. Recaptures can be made during a number of focused 
recapture  attempts  to  record  the  tagged  individuals  surviving  in  the  population.  This 

 
73 

method is considered appropriate when a large proportion of the population can be tagged 
and  recapture  rates  are  expected  to  be  high.  These  methods  are  more  often  applied  in 
focused, small-scale studies on discrete subunits of a population (e.g. Weber et al., 2016).  

If  the  expectation  of  obtaining  tagged  fish  in  subsequent  samples  is  small,  continuous 
monitoring  of  recaptures  over  an  extended  period  is  appropriate.  This  is  frequently  the 
study design applied with mark-recapture experiments on commercially exploited, widely 
distributed fish stocks (Fabrizio et al., 1999).  

Both scenarios rely on the ability to calculate the proportion of the total numbers of 
tagged individuals remaining in the population at successive times. Additionally, 
regression estimates, assuming a constant mortality rate with the decline in the numbers 
of tagged individuals can be expected to approximate to an exponential curve (Beverton 
and Holt 1956, Paulik 1963, Gulland I969). 

In the case that populations are large and/or widely distributed or where non-permanent 
methods of marking are used, multiple tagging events are often required to ensure a 
sufficiently large proportion of the population is marked at a given point in time. In this 
case the data analysis must classify the marked-recaptured individuals suitably. Often 
this classification relates to the probability of recapture. When the probability of 
encountering an individual on a number of occasions is high, a decision on data analysis 
should reference: 

  All occasions of marking & recaptured  

 

 

The last occasion of marking  

The occasion on of first released.  

Within the current project it was expected that multiple recaptures of the same 
individual would be rare with release of recaptured fish unlikely. The key objective of the 
current study was to establish sources of mortality – this predicated that the analysis of 
the recapture (and mortality) would be limited to the cause of the non-natural mortality. 

Tagging studies can provide valuable data for measuring growth under the assumption 
that the marking process does not affect growth. The individual growth trajectories from 
tagging data aid in identifying the key sources of variation. This can be especially 
important for species that cannot be aged or that can only be aged with difficulty. In this 
case multiple repeated sampling at regular time intervals are used to establish length (or 
weight) with age, where age is inferred from the cumulative time since marking. A 
growth curve can be fitted to these data using methods such as the Von Bertalanffy 
curve. In cases where sampling is carried out over unequal time intervals, Gulland and 
Holt (1959) described a graphical estimation method with conversion to a linear 
equation.  

In situations where individuals can be easily aged, the growth increments from tagging 
studies, irrespective of sampling interval, provide a means to validate existing growth 
models (Hamel et al., 2014). Alternately, mark–recapture techniques to estimate growth 
rates (and ages) are particular well-suited for long-lived species. These methods can be 
used to validate both the periodicity of growth increments and the absolute age. An 
integrated analysis of these complementary data sources provides the most reliable 
approach for understanding growth (Pine et al., 2012). When fish age is known at 
marking, either directly or inferred (i.e. young fish), absolute age validation can be 
performed when fish are recaptured. In the case that the age is unknown at the time of 
marking, multiple recapture events (a minimum of 2) will be needed to inform on growth 
rates. It is unlikely that conventional markers, such as implanted tags, will be a viable 
method in these cases and other methods will be needed – these experiments often rely 
on chemical markers using methods such as radiochemical dating or marking with 
calcium-binding chemical oxytetracycline, alizarin, or calcein.  

 
 
74 

To inform knowledge of Irish Sea cod biology it was planned that recaptured cod would 
be sampled biologically in order to determine sexual maturity, stomach content, age (by 
obtaining otoliths for future age determination studies) and stock identity (by obtaining a 
finclip for future genetic studies). During tagging activities only lively cod were tagged 
and recorded in the dataset, finclips were taken for ongoing genetic studies, while 
biological sampling was carried out on all remaining cod captured. The length of cod at 
the time of marking and recapture was recorded. Age and length change would be used 
to validate and refine existing growth models of Irish Sea cod derived through traditional 
sampling. Additionally to improve knowledge and understanding of cod catches it was 
planned that the characteristic of the catches continuing tagged of cod by a vessel would 
be recorded and submitted with each fish when a tagged cod was recaptured. In 
practice, this was rarely the case, most recaptured tagged cod were returned with the 
recapture location, date and vessel details only. 

Information derived from mark-recapture studies is essential in developing spatial 
management programs such as marine protected areas or defining the spatial limits and 
management areas of a stock. Within the analysis suite of mark-recapture records, a 
number of parameters can be derived describing the movement of marked individuals. 
These parameters can explore aspects of animal behaviour, such as speed and direction 
of movement. The simplest way of demonstrating the pattern of movement of a group of 
recaptured marked fish is pictorially. The position of recapture of each fish can be plotted 
on a chart in relation to its position of liberation and its mean direction and distance of 
travel can be indicated. To achieve accurate derivation of quantitative parameters it is 
necessary to have a recapture method with sufficient interval, which will be determined 
by directionality and speed of movement, to ensure the marking and recapture locations, 
represented by a straight line, give a reliable indication of the overall direction and 
distance of movement (Figure 27). For highly mobile species, this can dictate that 
conventional marking methods are not appropriate and data storage tags (DSTs), which 
log locational information at a higher rate than directional change occurs, should be used 
as these can show all movements between the time of marking and tag recovery.  

Movement information from DSTs can provide in-depth data of animal behaviour, such 
as vertical daily movements within the water column in relation to feeding behaviour or 
predator avoidance. It must be noted however, that these are non-independent, auto-
correlated observations, and must be balanced in comparison to the independent 
observations of recaptures from conventional tags. This is particularly the case when 
analysing selectivity behaviour, such as habitat selection, when assessment of choice is 
the aim of the study (Boyce, 2010). Alternatively the use of isotopic and elemental 
markers may be used as forms of ‘natural tagging’, to determine natal origins and to 
assess large-scale movement patterns of larvae, juveniles and adult fish over large areas 
(Elsdon et al., 2008). 

 
75 

Figure 27. Hypothetical movement paths between marking location ‘A’ and recapture 
location ‘B’. Panel I. shows a scenario where the marking and recapture interval is 
sufficient to define both direction and speed of movement. Panel II. shows a scenario 
where the marking and recapture interval is sufficient to define direction but 
insufficient to accurately represent fine detail and speed of movement. Panel III. shows 
a scenario where the marking and recapture interval is insufficient to define either 
direction or speed of movement. 

Tagging and analysis of recapture data provides a method to inform population ranges. 
Population‐level estimates of species’ distributions can reveal fundamental ecological 
processes, facilitate conservation and assist in the appropriate definition of management 
units. Data from DSTs may also be applied to explore individual animals ranging or 
movement patterns. This information can be useful when defining local level 
conservation or exploring the potential impact of an activity at a site scale. Methods for 
the analysis of this data can be both non-statistical and statistically derived. Non-
statistical methods can include mapping location information and provide limits of the 
distribution, such as observed range boundaries in terms of latitudinal or longitudinal 
ranges – a Minimum Convex Polygon (MCP), bounding 100% of all recapture and 
marking location. Alternatively, analytical methods can quantify the probability of 
utilisation distribution (UD) – the probability that an animal spends time in given areas. 

Within the UD there are distinct areas which are defined by the ‘characteristic’ of their 
usage. These are defined by the frequency, or density of use (Burt 1943): 

 

The home range or core of an animal or group of animals is the area that is inhabited while 
engaging in daily activities.  

  Geographic range is the geographic extent of the distribution of a species. 

These areas are frequently estimated using kernel density analysis. The geographic 
range is often defined as the 95% or 90% kernel densities (Borger et al., 2006). Use of 
50% of locations is considered as a core area of the range, as the area most frequently 
used by an animal or population within its whole home range. 

The use of mark-recapture data can elucidate a wide array of information ranging from 
population estimates to species behaviour. The design of any study and the question of 
interest will dictate the appropriate analysis methods but must be interpreted with 
respect to the assumptions of the marking study to accurately reflect individual or 

 
 
76 

population level parameters. The use of UD was applied in the current study to 
determine the distributional range of Irish Sea cod and compared with the spatial 
overlap of stocks in adjoin areas.  
2.4.2  Data analysis of recapture data 

Multiple tagging opportunities were utilised during this study including: chartered 
commercial fishing vessels, research survey vessels, at-sea observer programmes and 
recreational fisher events. Charters proved to have provided by far the most effective 
method to maximise tagging rates (fish tagged and released per day). Charters focused 
on the known spawning aggregations during Q1 in the study region and mid to late 
summer and utilised semi-pelagic trawling, bottom otter trawling and purse seine 
fishing. The second most successful tagging activity was through recreational angling 
events. Project staff attended shore competitions, angling boat trips and charters, as 
well as recruiting voluntary taggers from the angling community who could continue 
tagging ad-hoc outside of competitions. The project also organised a cod tagging shore 
competition. Tagging at angling events targeted fish below the minimum conservation 
reference size (MCRS), in inshore waters, to further strengthen the range of spatial 
coverage and size distribution of Irish Sea cod included in the tagging programme 
(Figure 28).  

In total, 4743 cod have been tagged (Table 7. ) details of the capture events are 
provided in Appendix 1. The coverage of the tagging scheme was more comprehensive 
spatially, temporally and by fishing method, than any previous tagging study of the 
region (Figure 29 & Figure 30).  

Table 7. Number of fish tagged each year by each tagging source. 

Year  Charter 

Shore 
Angling 

Sea 
Angling 

Scientific 
Survey 

Commercial  Voluntary 

2016 

2017 

2018 

963 

1470 

1699 

0 

95 

77 

0 

8 

0 

58 

152 

131 

Total 

4132 

172 

160 

189 

12 

6 

1 

19 

0 

5 

66 

71 

 
 
 
 
 
 
 
 
 
 
77 

Figure 28. Length frequency distribution of tagged cod by tagging activity. Data is 
aggregated across all sampling events for separate sources. 

Planned cod tagging charters focused on the known cod spawning aggregations in the 
Western Irish Sea (7a), with an intentional overlap by AFBI and the MI’s charter vessels 
to obtain good coverage in this area. The North Channel (6a) and Northern Celtic Sea 
(7g) were also targeted as “spill over areas” but to a lesser extent. There are no known 
cod aggregations in the North Channel outside of the Firth of Clyde cod box, which 
exhibits fishing restrictions in Q1 is a recognised spawning region. There is also 
significantly less fishing effort from whitefish directed fishing vessels. The time spent at 
various locations reflected all these factors, while ensuring fishing was conducted in a 
way which sampled across the entire area and targeted cod of a range of sizes normally 
encountered in commercial fishing activity. The coverage of tagging release locations 
achieved complete coverage of the known fishery data as reviewed in section 2.2.7. 

 
 
78 

Figure 29. Map of showing haul/release positions and the number of tagged cod released 
in each years of the project 2016 - 2018. Symbols are scaled to the number of fish tagged 
in each fishing haul or event in the case of recreational angling. 

To date 187 recaptures have been reported (3.92% return rate), with 172 of these with 
complete datasets sufficient for data analysis (3.61% return rate). Recapture rates 
remain lower than anticipated – a recapture rate of 10% may be expected based on 
previous studies – despite increased reward incentives and full implementation of the 
extensive communication plan. The recapture of 172, with 161 used in the analysis 
suggests the study has the ability to detect a medium size effect with high power based 
on the applied power analysis. 

The current spatial analysis records supports that the range of Irish Sea cod extends as 
far south as 50.5˚ N, overlapping with Celtic Sea cod and extending beyond the 
southern boundary of the Irish Sea. Within the current tagging project no movements of 
cod from the Celtic Sea to the Irish Sea have been observed. Recapture records are 
providing further insights into the behaviour, biology and range of Irish Sea cod (Figure 
30). This supports the historic review of tagging data, although shows a more extensive 
distribution in the Celtic Sea than previously observed. 

 
 
79 

Figure 30. Release and recapture positions of all recaptured cod reported until 
November 2018. Triangle symbols denote tagging location with circles indicating 
recapture sites. 

Recaptured tagged cod were predominantly caught by the ‘TR1’ fleet (fishing vessels 
targeting demersal fish with a mesh size in the range of 100 – 120mm) in the Irish Sea 
in 2016 (Figure 31), however, the total number of recaptures was low. The number of 
recaptures increased in 2017, as did the gear types contributing to the data set. 
Recapture numbers increased significantly throughout 2018, the vast majority being 
returned to the project by the small number of whitefish directed fishing vessels 
operating as TR1s in the Irish Sea (Figure 31). The proportion of recapture records by 
gear type was; gillnet: 0.6%, rod and line: 4.5%, otter trawl targeting Nephrops 14.0%, 
demersal trawl targeting fish 69.2% , seine netting 6.7% and 9% by beam trawl.  

 
 
 
 
80 

60,00%

50,00%

40,00%

30,00%

20,00%

10,00%

0,00%

TR1-IS

TR1-CS

TR2-IS

TR2-CS

Other-IS

Other-CS

2016

2017

2018

Figure 31. Recaptures of tagged cod in the Irish Sea (IS) and Celtic Sea (CS) in 2016, 
2017 and 2018. By regulated gears bottom trawls and seines with mesh: TR1 ≥ 100 mm; 
TR2 ≥ 70 mm and < 100 mm; TR3 ≥ 16 mm and < 32 mm; other = primarily gills nets 
and beam trawls. Not include recaptured tagged fish caught by anglers, where rod and 
reel fishing making up a small number of the total recaptures.  

Initial tag return rates were expected to be in line with other studies at around 10%. This 
expected  rate  was  also  informed  by  the  high  fishing  effort  indicate  by  the  scientific 
assessment  available  at  the  time.  Within  the  project  a  tag  return  rate  of  3.92%  was 
observed. Discussion with stakeholders and an adapted communication plan was put in 
place to ensure continued support from industry and awareness of reporting tagged fish 
caught. Other contributing factors were assessed to include tagging during closed season 
and area, limiting tag returns in the immediate period after tagging and observed reduction 
in  fishing  effort  and  bycatch  of  cod  in  the  main  fisheries  active  in  the  region.  The 
combination  of  low  fishing  mortality  and  tagging  occurring  when  fishing  activity  is 
prohibited,  within  the  immediate  area,  could  have  contributed  to  lower  than  expected 
recaptures. Rates of tag loss were monitored. Approximately 17.5% of recaptures during 
the current study were returned with only one tag. From this loss of both dorsal tags is 
estimated to occur in 2.8% of study animals. Tagged fish returned with only one tag intact 
had an average time at liberty of 224 days, with some exceeding 550 days. It is likely that 
tag loss occurs immediately after release, before the tag has become securely embedded 
into the dorsal tissue.  

To address the ability of the tagging campaign to support fishery management and 
provide insight into species biology, simulations of tagging effort informed by the initial 
results were used to plan tagging events and demonstrate the potential power of the 
study. A Lincoln-Petersen mark-recapture method (Seber, 2002) for estimating 
population size was applied. The method allowed an estimate of population size to be 
derived from the tagging programme, whilst making an assumption about losses from 
the population owing to natural processes. The analysis was applied using numbers of 
fish and converted to biomass estimates. Applying the method to a time series of 
tagging events and recaptures allowed for improvement and changes in uncertainty to 
be evaluated. The analysis, diagnostics and comparison with the current ICES 
assessment (ICES, 2018) of the size of the spawning stock biomass (SSB) suggested 
that the current tagging programme was able to provide robust information about the 
Irish Sea cod population. To forecast the potential effect of planned tagging scenarios, 
the analysis was extended to simulate marking and recaptures in 2018. Three scenarios 
were simulated:  

  Cessation of tagging and no further recapture reporting.  

  Continued tagging with continued reporting.  

 

Increased tagging effort with continued reporting.  

 
 
81 

The simulation results supported continuing the tagging effort in 2018, indicating that 
confidence in the population parameters derived from the study would be increased.  

Results of the projects have been communicated to the Northern Ireland Gear Trials 
steering group, which includes representatives from fishery policy, fish producing 
organisations and fishers, and to the Irish Fisheries Science Research Partnership 
(IFSRP) in the Republic of Ireland, a collaboration between representatives from fish 
producing organisations and fishers, and scientists of the Marine Institute and BIM. A 
project update was presented at the ICES Working Group for the Celtic Sea Ecoregion 
(WGCSE) in May 2018 and in May 2019 at which the assessment scientists for cod and 
gadoids in the Celtic Seas Ecoregion were present. The project was also included in the 
ICES WKIrish 5 workshop, attended by scientists and industry representatives. 

2.4.3  Fishing mortality sources of Irish Sea Cod  

An increased in the number of recaptured tagged cod returned to the project was seen in 
2018 (Figure 32). This may in part be explained by an increased TAC in the Irish Sea for 
cod in 2018, the first year since 1999 that ICES advice was for a catch greater than zero 
tonnes. Commercial skippers reported fishing on marks showing mixes of cod and 
haddock, which they would have previously avoided due to minimize cod bycatch. 
Statistical analysis of recaptures by gear type showed a significant difference in causes 
of mortality within the recapture data (The chi-square statistic: 21.21 p < 0.0001) with 
the Irish whitefish directed TR1 fleet in the Irish Sea providing the biggest contribution 
to cod mortality.  

TR1
TR2
TAC (tonnes)

695

86

146

6

5

2016

10

146

5

2017

2

2018

100

90

80

70

60

50

40

30

20

10

0

800

700

600

500

400

300

200

100

0

Figure 32. Number of recaptures by regulated gear type and agreed Total Allowable 
Catch (TAC) in the Irish Sea (ICES 7a) in 2016, 2017 and 2018.  

 
 
 
2.4.4  Migration, mixing and spatial range 

The recapture records used in the analysis include 140 fish which were tagged and 

released within the Irish Sea (ICES management division 7a) and 21 fish which were 
tagged and released within the Celtic Sea (ICES management division 7g). Summary 
statistics by release year (2016, 2017 and 2018) and release area are reported in  

82 

Ye
ar 

T
o
t
al 
n 

Rel
ea
se  

ar
ea 
n 

Rele
ase 
leng
th 
(cm
) 

Time 
at 
libert
y 
(day
s) 

Re
ma
in 
in 
IRE 

Re
mai
n in 
CEL 

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

20
17 

6
5 

20
18 

8
0 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
74 

CE
L = 
6 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 
10.5
) 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

- 

- 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 
management 
division 7k 

Mean 
velocit
y  

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

Dist
anc
e 
trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 
45.8
) 

7. Given the markedly different management measures that have been in place 

during the study period for individual years, in particular for Irish Sea cod, the 
distributional analysis of recapture (see Table 8 and Figure 33 / Figure 34). A time at 
liberty threshold was not imposed (min/ max number of days since release) on the 
recapture data. Given annual management of stocks and different management between 

 
83 

the two divisions (7a and 7g) the study period provides an opportunity to explore how 
management may effect recapture rates between the two divisions. 

The extent of geographical range for cod in Irish and Celtic Sea is described using 
utilisation distributions (UDs) (Figure 33 & Figure 34). UDs are defined as the probability 
of locating a tagged individual within an area (Worton 1987). The UDs are calculated for 
each release area and each release year. All UDs are calculated using the kernel 
probability density function (KPDF) approach using the adehabitatHR package in R. As in 
previous studies the 95%, 70% and 50% probability contours are extracted (see Bendall 
et al., 2009) described as the range (95%) and the core area (50%) of cod. Range 
provides a spatial representation of all broad scale area utilised by tagged cod (Downs & 
Horner 2008; Dean et al., 2014). Whereas, core area is assumed to reflect the areas 
where individuals spend the most time conditional on our observations (Downs & Horner 
2008; Dean et al., 2014). The KPDE method is more typically used in studies of 
territoriality and home ranges (Righton & Mills 2008). However, because tag recapture 
locations are analogous to the density and distribution of the locations of single 
individuals over time, the technique is applicable to population-level mark-recapture data 
(Righton et al., 2007; Bendall et al., 2009).  

Values of distance travelled (km; d) were calculated as the straight-line distance 
between release and recapture locations, along the surface of a sphere, using the Great 
Circle equation in R. Mean velocity was calculated by dividing d by the time spent at 
liberty (number of days) ( 

Ye
ar 

T
o
t
al 
n 

Rel
ea
se  

ar
ea 
n 

Rele
ase 
leng
th 
(cm
) 

Time 
at 
libert
y 
(day
s) 

Re
ma
in 
in 
IRE 

Re
mai
n in 
CEL 

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

20
17 

6
5 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

- 

- 

Mean 
velocit
y  

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

Dist
anc
e 
trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

 
IRE 
= 
74 

CE
L = 
6 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 
10.5
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

20
18 

8
0 

).  

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

84 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 
management 
division 7k 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 
45.8
) 

 
 
 
 
 
 
 
 
 
 
 
 
Table 8. Summary statistics by of tagged and released cod by year (2016, 2017 and 2018) and release area (ICES management division). All 
reported values are means. Uncertainty is reported as ± 1 standard de6ation. n= sample size. 7a, Irish Sea (ICES management division 7a). 7g, 
Celtic Sea (ICES management division 7g). 6a, West of Scotland (ICES management division 6a). 

Year 

Total 
n 

Release  

area n 

Release 
length 
(cm) 

Time at 
liberty 
(days) 

Remain 
in IRE 

Remain 
in CEL 

IRE to 
CEL 

IRE to 
WSC 

Other 

(notes) 

All 

161 

IRE = 
140 

CEL = 21 

IRE = 
68.3 (± 
15.7) 

CEL = 
53.4 (± 
11.9) 

IRE = 
174.3 
(± 
149.9) 

CEL = 
114.4 
(± 
136.6) 

113 fish 

20 fish 

23 fish 

3 fish 

(80.7%) 

(95.0%) 

(16.4%) 

(2.1%) 

1 fish released in CEL 
moves west into ICES 
management division 
7j 

1 fish released in IRE 
moves south through 
the CEL and is 
recaptured in ICES 
management division 
7k 

Distance 
travelled 
(km) 

Mean 
velocity  

(km day-
1) 

IRE = 
80.6 (± 
111.2) 

CEL = 
52.4 (± 
59.8) 

IRE = 0.9 
(± 1.9) 

CEL = 0.6 
(± 0.6) 

 
 
 
 
 
 
 
 
 
 
 
 
Table 8 (contd). Summary statistics by of tagged and released cod by year (2016, 2017 and 2018) and release area (ICES management division). 
All reported values are means. Uncertainty is reported as ± 1 standard de6ation. n= sample size. 7a, Irish Sea (ICES management division 7a). 
7g, Celtic Sea (ICES management division 7g). 6a, West of Scotland (ICES management division 6a). 

Year 

Tot
al n 

Relea
se  

area n 

Release 
length 
(cm) 

Time at 
liberty 
(days) 

Remai
n in 
IRE 

Remain 
in CEL 

IRE to 
CEL 

IRE to 
WSC 

Other 

(notes) 

2016 

16 

IRE = 
16 

IRE = 
62.0 (± 
5.9) 

IRE = 
219.8 (± 
285.7) 

11 fish  

(68.8%
) 

- 

3 fish 

2 fish 

(18.8%) 

(12.4%) 

- 

- 

44 fish 

15 fish 

(88.0%
) 

(100.0
%) 

5 fish 

1 fish 

(10.0%) 

(2.0%) 

2017 

65 

2018 

80 

IRE = 
50 

CEL = 
15 

IRE = 
74 

CEL = 
6 

IRE = 
67.4 (± 
20.0) 

CEL = 
48.0 (± 
7.3) 

IRE = 
70.3 (± 
13.5) 

CEL = 
67.0 (± 
10.5) 

IRE = 
235.8 (± 
156.6) 

CEL = 
106.7 (± 
158.8) 

IRE = 
122.9 (± 
61.9) 

CEL = 
133.5 (± 
58.7)  

58 fish 

(78.4%
) 

5 fish 

15 fish 

(83.3%) 

(20.2%) 

- 

1 fish released in CEL moves 
west into ICES management 
division 7j 

1 fish released in IRE moves 
south through the CEL and is 
recaptured in ICES 
management division 7k 

IRE = 
93.4 (± 
116.3) 

CEL = 
120.7 (± 
45.8) 

IRE = 1.0 (± 
1.2) 

CEL = 1.1 (± 
0.6) 

Distance 
travelle
d (km) 

Mean 
velocity  

(km day-1) 

IRE = 
99.1 (± 
121.0) 

IRE = 
55.9 (± 
97.2) 

CEL = 
25.1 (± 
39.7) 

IRE = 0.9 (± 
0.9)  

IRE = 0.9 (± 
2.7)  

CEL = 0.4 (± 
0.4) 

 
 
 
Cod remained at liberty on average for approximately five months (166.5 days ± 149.2 
days;  

Ye
ar 

T
o
t
al 
n 

Rel
ea
se  

ar
ea 
n 

Rele
ase 
leng
th 
(cm
) 

Time 
at 
libert
y 
(day
s) 

Re
mai
n 
in 
IRE 

Re
mai
n in 
CEL 

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

20
17 

6
5 

20
18 

8
0 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
74 

CE
L = 
6 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 
10.5
) 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

- 

- 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 
management 
division 7k 

Mean 
velocit
y  

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

Dist
anc
e 
trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 
45.8
) 

), travelling an average distance from release to recapture of 77.0 km (± 106.2 km) at a 
mean velocity of 0.89 km per day (± 1.7 km day-1). The longest time at liberty was a 
little over 30 months (939 days), by a cod recaptured only 6.0 km away from its original 
release site in the Irish Sea. The greatest distance travelled was by a cod at liberty for 
150 days, recaptured within the Celtic Sea, 403.6 km from its original release site in the 
central Irish Sea.  

The following four broad-scale movement patterns were observed ( 

Ye
ar 

T
o
t

Rel
ea
se  

Rele
ase 
leng

Time 
at 
libert

Re
mai
n 

Re
mai

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

Dist
anc
e 

Mean 
velocit
y  

 
 
al 
n  ar

ea 
n 

th 
(cm
) 

y 
(day
s) 

in 
IRE 

n in 
CEL 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

20
17 

6
5 

20
18 

8
0 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
74 

CE
L = 
6 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 
10.5
) 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 
45.8
) 

- 

- 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 
management 
division 7k 

): 
  Cod released in the Irish Sea and recaptured within the Irish Sea (80.7% - 113 out of 140 

fish). 

  Cod released in the Irish Sea and recaptured in the waters off the west coast of Scotland 

(2.1% - 3 out of 140 fish). 

  Cod released in the Irish Sea and recaptured in the Celtic Sea (16.4% - 23 out of 140 fish).  

  Cod released in the Celtic Sea and recaptured within the Celtic Sea (95.0% - 20 out of 21 

fish).  

The rate of recapture of cod released in 7a and recaptured in 7g is much higher than 
observed in the review of existing data 16.4% in the current study compared to 3.7% in 
the review. However, the historic review and current study both support the spatial 
distribution and range overlap of Irish Sea and Celtic Sea cod (Figure 34). 

 
 
 
 
Figure 33. Summary Utilisation Distributions (UD) for tagged mark-recovery cod in 7a 
(Irish Sea) and 7g (Celtic Sea) in 2016 - 2018. Probability kernel density contours 
(KPDF) are stated. Core areas of the stocks are indicated by the 50% kernel contour 
and species range by the 95% contour. 

 
 
 
 
 
 
 
 
 
 
 
 
Figure 34. Summary Utilisation Distributions (UD) for tagged mark-recovery cod in 7a 
(Irish Sea) and 7g (Celtic Sea) in 2016 - 2018. Probability contours are stated. Core 
areas of the stocks are indicated by the 50% kernel contour and species range by the 
95% contour. 

2.4.5  Influence of environmental factors 

Cod in the Irish Sea are considered to be at the upper thermal boundary of the species, 
and their growth and surplus production (i.e. including the reproductive component) are 
among the highest found throughout the range. Cod stocks are not observed much above 
annual mean bottom temperatures of 12°C (Drinkwater, 2005).  

Analysis was applied to explore the thermal habitat suitability of the Irish Sea and 
adjoining sea regions based on assumptions of thermal limits. Literature has been used 
to identify optimal sea bottom temperatures for different ages of cod, developing a 

 
 
 
 
 
baseline habitat suitability model for cod in the Irish Sea and neighbouring areas. The 
model demonstrates the continual thermal link between the Irish Sea and adjoining 
areas. The theoretical model of habitat suitably was tested for predictive capacity with 
the empirical results of the current recapture study. The spatial pattern of this optimal 
habitat and overlap with the observed recapture results validates the habitat suitability 
(Figure 35). At present the study identifies a small area in 7a of suitable thermal habitat 
which mirrors the core area of the 7a recapture data and clearly identifies a much larger 
area of suitable habitat in the Celtic Sea within which the recapture data is recorded. 
Additionally the analysis of monthly habitat suitability supports the hypothesis this 
environmental driver would have strongest effect in late quarter 3 and early quarter 4 
(Figure 36).  

Figure 35. Annual variation in mean monthly thermal habitat (bottom temperatures 
<=12°C) of study area 2015-2017. Habitat scale represents sum of months where thermal 
habitat was <=12°C. 

 
 
 
 
 
 
Figure 36. Monthly variation in thermal habitat (bottom temperatures <=12°C) of study 
area calculated in 2017. Blue areas represents thermal habitat <=12°C, whereas red areas 
represent bottom temperatures >12°C. 

2.4.6  Irish Sea cod biology 

It was critical that tagging was applied to fish across the size distribution reflective of 
what is being caught from the stock. By tagging over a broad spatial area, using various 
gear types and tagging activities throughout the year, a large range of cod lengths and 
ages were tagged (Figure 37).  

 
 
 
 
 
d
e
g
g
a
t
d
o
c

f
o

.

o
N

450

400

350

300

250

200

150

100

50

0

3
1

6
1

9
1

2
2

5
2

8
2

1
3

4
3

7
3

0
4

3
4

6
4

9
4

2
5

5
5

8
5

1
6

4
6

7
6

0
7

3
7

6
7

9
7

2
8

5
8

8
8

1
9

4
9

7
9

0
0
1

3
0
1

6
0
1

9
0
1

2
1
1

Length (cm)

Figure 37. Combined length frequency of tagged cod showing tagged individuals from 
all sources. 

Otolith age analysis was carried on fish returned with intact carcasses. All fish that 
moved between ICES Divisions ‘transient fish’ were aged as 3+ years. 23 cod were 
tagged and released in the Irish Sea and recaptured in the Celtic Sea (16.4% of cod 
released in the Irish Sea). Of those aged, all were 4+ years old cod. There were 4 fish 
(2.9% of cod tagged and released in the Irish Sea) displaying movement from the Irish 
Sea to management area 38-E4/North Channel. Otolith age analysis showed all these fish 
were 3+ years old.  

The body length of cod at the time of release ranged from 18 cm to 103 cm with an 
average value of 66.3 cm (± 16.6 cm;  

Ye
ar 

T
o
t
al 
n 

Rel
ea
se  

ar
ea 
n 

Rele
ase 
leng
th 
(cm
) 

Time 
at 
libert
y 
(day
s) 

Re
mai
n 
in 
IRE 

Re
mai
n in 
CEL 

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

20
17 

6
5 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

- 

- 

Mean 
velocit
y  

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

Dist
anc
e 
trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

 
 
 
 
 
 
 
 
 
20
18 

8
0 

IRE 
= 
74 

CE
L = 
6 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 
10.5
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 
management 
division 7k 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 
45.8
) 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

). Time spent at liberty is found to be similar across stocks (t = 1.8, df = 27.7, p value = 
0.08), whereas body length at the time of release is significantly larger in those cod 
released in the Irish Sea compared to those released in the Celtic Sea (t = 5.1, df = 
31.6, p value < 0.0001;). That said, sample size is skewed towards the Irish Sea.  

Recaptured cod were dissected to collect stomachs and further biological data including 
sex and maturity. Otoliths were extracted, mounted and ages read. Stomach content 
analysis was performed to identify prey items and create a database of cod diet. Within 
the project it has been noted that a number of recaptured carcasses are of smaller length 
than at the time of tagging. This is a known effect of carcass freezing and storage. 
Fishers often freeze tagged cod prior to reporting the fish and freezing is the 
recommended preservation method prior to scientific analysis. A species and area specific 
shrinkage factor is required to account for this phenomenon.  

An up-to date growth model was derived for Irish Sea cod for 2017 (Figure 38). This 
model has been updated with data from the current project and other sampling sources 
including fishery independent and fishery dependent sources. This was applied using the 
methods as performed in the annual ICES stock assessment (ICES 2017a). The growth 
parameters for 2017 were: Linf 2017   = 174.51 cm, K2017 = 0.126 and t02017 = -0.568 

In comparison, Table 9 shows growth parameters estimated for the years 2012 to 2016. 
These and the most recent cod growth parameters suggest that cod growth rate has 
been reducing (K value) with individuals expected to reach a larger asymptotic length 
(Linf). It is likely that this results from the presence of older, larger, fish in the population 
as a result of the maturation of the 2013 cohort, which was relatively strong compared to 
previous cohorts of cod in the Irish Sea.  

Table 9. Von Bertalanffly plot growth curve parameters for cod the data are derived 
from commercial sampling of Irish Sea cod, fishery independent surveys and the current 
tagging for years 2012-2016. 

Year 

Parameter 

2012 

2013 

2014 

2015 

2016 

K 

Linf 

t0 

0.409 

0.356 

0.402 

0.211 

0.202 

102.04 

108.476 

115.61 

134.27 

147.25 

0.041 

-0.038 

0.393 

-0.146 

0.232 

 
 
 
 
 
 
Figure 38. Von Bertalanffly plot (growth curve) for cod the data is derived from 
commercial sampling of Irish Sea cod, fishery independent surveys and the current 
tagging project from 2017. 

An experiment to examine shrinkage due to freeze of cod carcasses revealed an on 
average 2.42% (s.d. 1.26) decline in length due to freezing. This factor was used to 
correct the lengths of recaptured cod stored or returned frozen. Using the days at liberty, 
annual growth increments were estimated for aged recaptured cod. Comparison of 
tagging study derived growth rates (converted form daily increments to yearly growth) 
with the age based Von Bertalanffy model show similar rates of growth at age (Figure 
39). From the tagging study, annual growth of cod aged two was 26.8 cm per annum, 
decreasing to 6.5 cm per annum at age 6.  

Figure 39. Comparison of tagging study derived growth rates to Von Bertalanffy 
modelled change in length at age (quarter 1 mean length). The growth rate per year for 

 
 
 
 
cod at age 2 – 6 was calculated as the change in length within year from the Von 
Bertalanffy method and by growth since tagging or recaptured tagged cod. 

Stomach content analysis was carried out along with the field work activities and 
dissection of returned cod carcases. Full analysis of these is ongoing, with 577 cod 
stomachs processed and 1069 prey items identified, this data is being incorporated with 
the foodweb ecosystems models being developed through the ICES WKIrish process. 
There have been 92 individual prey items recorded in cod stomachs. Of the cod stomachs 
processed from the cod tagging project, 8 were empty. 5 prey items comprised almost 
75% of the diet by weight Pasiphaea sivado (19.6%), Nephrops norvegicus (17.8%), 
Melanogrammus aeglefinus (15.7%), Merlangius merlangus (13.3%) and Clupea 
harengus (7.9%). 

The stomach contents data will be included in an ongoing project looking at trophic 
interactions of Celtic and Irish Sea fish stocks in an attempt to improve fisheries 
management advice. A breakdown of the percentage abundance of prey groups 
encountered in this analysis is given in Figure 40, showing that fish are the most 
common prey group taken in terms of numbers followed by shrimps. The most common 
fish species taken were sand eel sp., sprat, whiting and herring. Nephrops was the third 
most common prey item by number. Whilst, outside the scope of the current project this 
analysis will contribute to the overall project objectives and regional management 
objectives towards the ongoing development to the ecosystem understanding to the 
region.  

Figure 40. Composition of prey groups by number in cod stomach contents 

 
 
 
 
 
 
 
2.4.7  Implication for assessment of stock status  

The quality of the Irish Sea cod assessment and the lack of a short term forecast have 
severely impacted the ability to provide sound management advice and maximise yield of 
the stock since the mid 2000s. This was primarily due to the uncertainty in the mortality 
estimates for the stock. Total mortality rates for the stock have been high throughout the 
time period for which information is available for the assessment (since the 1960s) and 
the stock is characterised by the paucity of older fish in the population. As the 
information that inform the stock assessment increased and quality improved, the high 
mortality rate translated into an unallocated removals estimate form the assessment 
model (essentially due to a mismatch in the mortality signal for the surveys and catch 
over time). Unallocated removals could potentially include components due to increased 
levels of emigration, discarding, survey catchability, natural mortality, changes in fishery 
selectivity, as well as misreported landings. 

The 2017 ICES benchmark accounted for most of the sources that could result in the 
unallocated mortality estimates, but lacked updated information on emigration. The 
resulting assessments (ICES 2017b, ICES 2018a) have changed the perception of fishing 
mortality showing a marked decrease since 2010 to a current F <0.10. The assessment 
also indicates an increase in the Irish Sea cod population and spawning stock biomass 
(SSB). This increase is greatly reliant on the 2013 year-class, which had relatively strong 
recruitment compared to recent years. The 2013 cohort accounted for approximately 
60% of the SSB in 2017 (ICES 2017b, ICES 2018a). There is still some uncertainty and 
bias in the assessment with a year on year downward revision of SSB and a change in 
the magnitude of mortality over the assessment period (Figure 41; ICES 2018a). The 
downward revision in SSB is apparent in retrospective comparisons between modelled 
and observed SSB (Figure 41).  

 
 
 
 
Figure 41. Retrospective analysis of stock status, as applied in the ICES annual 
assessment of cod in the division 7a in 2018 (ICES 2018a). Each panel shows comparison 
of the assessment model output for catch, spawning stock biomass (SSB), average fishing 
mortality for cod aged 2 – 4 (Fbar 2-4) and recruitment. The retrospective is applied for 
with the same model removing the final year for 5 years to compare the model 
predictions with subsequent model runs.  

Stock identification and the biological containment of a stock within a defined 
geographical area is clearly important for stock assessment and the sustainable 
management of the stock within a spatially matched management area. ICES (2012) 
reviewed historical tagging studies to evaluate the stock identity of Irish Sea cod and 
concluded that although there is evidence for limited seasonal migrations into 
neighbouring regions, most fish will stay within their management area and there is no 
need to change the assessment units. Genetic evidence, however, indicated that cod 
spawning in the Firth of Clyde, Irish Sea, Celtic Sea, and western Channel form a single 
breeding unit (Heath et al., 2014). Such larger meta-population structures are often the 
outcome of genetic studies, where the data do not allow for differentiation if there are 
any mixing between stock units (even at a very low rate as suggested by historic tagging 
studies). The migration rate of mature fish from the Irish Sea to the Celtic Sea observed 
in this study is an order of magnitude larger than anything previously observed. The 
tagging study suggests a ‘migration’ of approximately 15% to 20% of mature fish from 
the Irish to the Celtic Sea. The review of historic tag records shows that this can be 
variable over time but with a comparable result (18%) observed in quarter 4. 

The observed movement supports two hypotheses:  

1.  There is a migratory part of the Irish Sea stock which returns to the spawning grounds 
in the Irish Sea. Cod show considerable spawning ground fidelity, however, within the 
current tagging project no movements of cod from the Celtic Sea to the Irish Sea have 
been observed. 

2.  A  proportion  of  mature  fish  emigrate  permanently  out  of  the  Irish  Sea,  due  to 
environmental  conditions,  carrying  capacity  or  re-homing,  to  spawning  areas  in  the 
Celtic Sea. This might be Celtic Sea fish that spent their juvenile years in the warmer 
Irish Sea, which enhances fast growth and they then return to the Celtic Sea at a later 
stage, or Irish Sea fish which migrate into the Celtic Sea owing to the carrying capacity 
and or environmental factors in the Irish Sea. 

Neither hypothesis can be truly be rejected, but provide potential scenarios which should 
be included in management considerations and sensitivity analysis for both Irish Sea and 
Celtic Sea cod. To initially assess the impact of both hypotheses on stock assessment and 
management both options are being investigated. The tagging results support that: 

  Only fish aged 4+ are migrating from the Irish to the Celtic Sea. 

  23% of 4+ cod caught in ICES area 7g are cod that have migrated from the Irish Sea. 

  19% of 4+ year old fish are migrating from the Irish Sea, i.e. fish tagged in Irish Sea re-

captured in Celtic Sea. 

  No fish have been observed to migrate from the Celtic Sea to the Irish Sea.  

An age structured population model was applied following the peer reviewed ICES Irish 
Sea cod stock assessment model (ICES 2017a, ICES stock annex). The implications of 
current management have been incorporated into the analysis by applying short term 
forecasts. Forecasts are using TAC for catches in 2018 (intermediate year) and catches 
under FMSY in the Irish Sea for 2019 and 2020.  

 
 
 
 
 
 
The assumptions applied within the assessment model to describe mortality were: 

  Catch reallocation - 23% of catches of cod >60cm in area 7g were re-assigned to area 

7a, following the assessment methods for plaice in 7d (ICES). 

 

Increased natural mortality M - Permanent migration out of the Irish Sea was simulated 
as an increase in natural mortality M of individuals age 4+ (fish 4+ contribute the majority 
to length >60cm).  

Here migration is included from 2015 onwards to reflect the current project estimates of 
movement out of Division 7a. Landing and discard data for ICES area 7g for 2015 and 
2016 were taken from the STECF report on Fishery Dependent Data 2017. Landings and 
discards for 2017 were the average of 2015 and 2016 as data were not available at time 
of report (STECF Fishery Dependent data report 2018). 

Results of Irish Sea cod Spawning Stock Biomass (SSB) and Fishing mortality (F) under 
the two different hypotheses were compared with the model used at WGCSE 2018 (ICES 
2018) (Figure 42). The annual downward revision of the SSB and continued truncated 
age structure in Irish Sea cod remains unexplained despite low fishing pressure 
experienced by cod in the Irish Sea. The results of the current tagging study incorporated 
into the assessment framework provides an explanation for this, given the observed 
movements in particular for larger cod (>60cm).  

a) 

b) 

Figure 42. Time series and forecast of Irish Sea cod a) Spawning Stock Biomass (SSB) 
and b) Fishing mortality (F) under the two different hypotheses compared with the 
model used at WGCSE 2018 (ICES 2018). Migration was only included in the 3 most 
recent years (2015-2017). Hypothesis 1 - Blue: Migratory stock that returns to spawning 
sites in 7a (taking into account catches of 4+ year old fish in area 7g), Hypothesis 2 - 
Green: Emigration out of 7a (adjusted M) and Red: WGCSE baseline (stock assessment 
without considering migration). 

While all three scenarios (i.e. the original one used to produce the ICES advice, and the 
two scenarios including migration) show a downward trend in future SSB owing to low 
recruitment in years following 2013, the downward trend in SSB is considerably faster 
under the assumption that cod migrate permanently out of the Irish Sea. This supports 
observed retrospective annual downward revision of SSB as shown in the current annual 
assessment of the stock abundance and adjusts the observed uncertainty in the forecast.  

Management Strategy Evaluation 

To investigate the impact of permanent migratory behaviour of Irish Sea cod out of the 
Irish Sea, a MSE (management strategy evaluation) was conducted. The MSE was used 
to evaluate possible future management scenarios from a starting point where the status 
of the stock is assumed to be known (i.e. the assessment for a given year). This 
scenarios allow a comparison of management based on the current assessment model 

 
 
 
 
 
and assuming stock mixing and migration as observed. Three starting years are chosen 
for the MSE, 2015, 2016 and 2017. The management scenarios are a range of F values, 
from 0.01 to 0.36 (the current FMSY lower range).  SSB, catches and F are projected up to 
2025 and a number of statistics are applied to compare between assessments including 
migration or not. Those statistics are:  

  Risk 1: Mean probability for the second half of the projection that SSB falls below Blim 

Mean( P(SSB<Blim)) 

  Risk 3: Maximum probability for the second half of the projection that SSB falls below Blim 

MAX( P(SSB<Blim)) 

  Risk 4: Mean probability for years 2019 - 2025 of the projection that SSB falls below Blim 

Mean( P(SSB<Blim)) 

  Average catch in years 2019-2025 

All risks should be below 0.05 to assure sustainability; risk 3 < 0.05 is seen as a 
conservative measure. 

In the simulations migration is accounted for by an additional 20% natural mortality for 
cod aged 4-6 years. Figures 1 to 3 show the differences in recruitment, SSB, catch and F 
for starting years 2015 to 2017 setting the TAC at the lower FMSY range (0.36).  Both 
scenarios (original and migration) have large confidence intervals, however, SSB is 
projected to be slightly larger without migration. While the projections have a strong 
year-effect, originating from random recruitment events, including migratory behaviour 
in the assessment makes the projections more stable and less dependent on occurrence 
of those events. The maximum sustainable F becomes more centred between 0.11 and 
0.21 without taking in the migratory behaviour it has a much larger range between 0 and 
0.3. Estimated average catches for the period 2019-2025 taking into account the 
migratory behaviour of fish is below the estimated average catches without migration 
due to the more stable estimation of F.  Both simulations scenarios support that the 
current F reference points, estimated from historic patterns of SSB and recruitment, 
exceed those that based on the MSE with a starting position of the stock in its current 
state whilst minimising the risk of stock biomass falling below reference points.   

2.5  Work package 4: Management advice 

2.5.1  Current management  

The first management agreements date back to the 1600’s allowing access to fish in the 
Irish Sea. Reliable records of landings date from the late 1960s. Landings have fluctuated 
markedly  and  diminished  since  the  late  1980s.  Following  advice  in  1999  from  the 
International  Council  for  the  Exploration  of  the  Sea  (ICES),  in  2000  the  European 
Commission  implemented  a  series  of  iterative  management  policies  in  the  form  of  a 
recovery plan. These initially closed two areas in the Irish Sea to protect spawning over 
their  known  grounds  between  mid-February  and  the  end  of  April  each  year,  and 
subsequently  detailed  specifications  of  banned  fishing  gears  (Kelly  et  al.,  2006).    The 
specifications  aimed  to  protect  spawning  cod  while  allowing  the  continuation  of  other 
fisheries.  The  closed  areas  and  gear  specifications  were  subsequently  redefined  in  early 
2001. In 2004 the “cod recovery plan” defined a target biomass at Bpa = 10,000 tonnes, 
along with a plan to achieve a 30% annual increase in stock biomass and limit TAC changes 
by 15% (Kelly et al., 2006).  

The 2017 ICES assessment showed that the stock had increased to be above Blim (6,000 t) 
but remained below the current Bpa value of 8616 t (ICES 2017c). In 2018 the assessment 
again above Blim but remained below the current Bpa and noting “The current assessment 
shows  a  significant  downward  revision  of  SSB”  (ICES  2018d).  In  2019  the  annual 
assessment was changed to a different framework, largely based on the “strong tendency 
for the assessment to overestimate SSB” (ICES 2019).  

 
 
 
 
 
A number of paths are available for future management of the Irish Sea cod stock. Under 
current  management  the  stock  appears  to  have  responded  favourably  in  particular  with 
regard to reduction of  fishing pressure which,  all-be-it that they have taken almost two 
decades to affect an observed response. 

Since the inception of the Irish Sea Cod Tagging Project, the cod stock in the Irish Sea 
has seen some recovery (Figure 43; ICES, 2018a) with catch advice, following the MSY 
approach, of 1073 t for 2018 and 807 t for 2019. These represent the first ICES advised 
catches above zero tonnes since 1999. Within the assessment model of Irish Sea cod 
there is uncertainty of the assessment to reliably inform SSB. An increase in the stock 
biomass has been driven by slightly improved recruitment in isolated years, but it also 
implies that the implemented management measures, by reducing effort generally and 
especially during spawning aggregations, have given space for the stock to recover from 
the high exploitation levels experienced during the 1980s and 1990s - notably the 
reduction in F in 2010 (Figure 43) following the implementation of Cod Long Term 
Management plan under Council Regulation (EC) No. 1342/2008. The observed 
retrospective downgrading of cod SSB within the Irish Sea assessment model highlights 
aspects of unknown behaviour and biology for the stock and its current management.  

Figure 43. Cod in Division 7a. Summary of the stock assessment (weights in thousand 
tonnes). The assumed 2018 recruitment value is not shaded. Shaded areas in F and SSB 
plots and error bars in the recruitment plot represent 1 × standard de6ation. 
Uncertainty boundaries are not available for 2018 (ICES, 2018a). 

Within the 2013 reformed Common Fisheries Policy (CFP), multiannual plans became a 
priority to form the framework for management of fish stocks by fishery and sea basin. A 
regional approach was introduced taking into account the specific features of each sea 
basin. The overall objective of the multiannual plans (MAP) is to restore and maintain fish 
stocks above levels capable of producing the maximum sustainable yield (MSY). The 
Basic Regulation also defines the content of the plans, which must include quantifiable 
targets (such as fishing mortality rates and spawning stock biomass), and safeguards to 
ensure that quantifiable targets are met. 

Multi-annual plans have been developing as a means of safeguarding the state of stocks 
by setting catch opportunities in line with sustainability targets. In August 2016 the 
Commission proposed a multi-annual plan for demersal fish stocks in the North Sea, in 
February 2017 a multi-annual plan for small pelagic stocks in the Adriatic Sea, in March 
2018 a multi-annual plan for demersal stocks in the western Mediterranean and a 
Multiannual plan for fish stocks in the western waters and adjacent waters, and for 

 
 
 
 
fisheries exploiting those stocks (2018/0074 (COD)). It is this proposal which 
incorporates cod in the Irish Sea, under Article 1.1 (5). Article 3 sets the objectives, to 
achieve the objectives of the CFP applying the precautionary approach to fisheries, 
aiming to ensure exploitation of living marine above levels which can produce MSY, 
implementing the ecosystem-based approach to fisheries management. Article 4 details 
targets, aiming to achieve fishing mortality in line with the ranges of FMSY as soon as 
possible on a progressive incremental basis by 2020. A multiannual plan for demersal fish 
stocks in the Western Waters and adjacent waters is at an advanced stage to be adopted 
in EU regulation. While this plan has yet to be adopted, its intentions in line with 
sustainability goals.  

The plan incorporates a number of principles and objectives of relevance to this study: 

  Restore and maintain fish stocks above levels capable of producing MSY 

 

 

Ensure fishing is environmentally sustainable in the long term 

Establishment of appropriate reference points for management (scientific advice should 
particularly include ranges of FMSY and biomass reference points, i.e. MSY Btrigger and Blim) 

  Reference to mixed fisheries considerations 

  Should be based on multi-species considerations 

 

Implement the ecosystem approach to fishery management 

  Consistence with geographical distribution in biology and fishery terms 

  Measures shall be taken in accordance with the best available scientific advice. 

Results from this study support these objectives, both directly and indirectly. The 
observed linkage and mixed stock area in area VIIg have implications for the stock 
assessment of both Irish Sea and Celtic Sea stocks. In turn this will impact on known 
reference points (no longer accepted by ICES (ICES 2019)) for the Irish Sea and 
potentially adjacent stocks. In particular, the results questions the appropriateness of the 
current management areas and their independence. 

Mixed fishery and multispecies models are not as developed for the Irish Sea as for the 
North Sea or Celtic Sea. The timely completion of the study will greatly informed the 
development of these models. A mixed fishery model of the Irish Sea is in development 
through the ICES MixedFish-Methods group and there is significant movement towards 
ecosystem based management for the Irish Sea through the ICES workshops. The results 
from this study has been presented to the ICES community and are already partly 
integrated into the development of an ecosystem based approach to management (ICES 
2018c) by informing the likely energy movement out of the Irish Sea food web 
ecosystem model. The project results have been presented in the initial work toward the 
2019 benchmark process of the Celtic Sea gadoid stocks. The mixed fishery model 
development, the ecosystem modelling process and the Celtic Sea benchmark are 
ongoing processes which can use the information from the current project. Here we 
outline a road map for this and detailed elements to improve the management of the 
Irish Sea cod stock in the multiannual plan. 
2.5.2  Irish Sea cod tagging findings and future management 

The results and findings of the 2016 – 2018 Irish Sea cod tagging project are clearly 
important to the development of future management of the stock through development 
of the stock assessment and management controls. Recommendations will influence 
management through a number of routes, integral to these is the current framework for 
scientific advice. In 2000 the STECF reviewed tagging programmes in the Irish sea and 
concluded that while some cod moved from the Irish Sea into the Celtic Sea they 
constituted a very small proportion of the Celtic Sea stock and furthermore that no cod 
tagged in the Celtic Sea were recovered in the Irish Sea (Anon., 2000). Results of the 
current study suggest that the proportion of cod migrating from the Irish Sea into the 
Celtic sea is of importance in terms of removal from the Irish Sea stock: 

 
 
 
Management recommendation: Stock assessment as currently undertaken by ICES must 
to account for the proportion of the stock leaving the Irish Sea stock and entering the 
Celtic Sea stock. Model development must explore the sensitivity of the model and advice 
to stock mixing in VIIg. Through this advice to the European Commission, on the 
structure and content of management regulations pertinent to fisheries exploiting the 
Irish Sea cod stock. Management Strategy Evaluation should be used to define 
appropriate reference points taking into consideration mixing and the current stock 
status.   

In relation to regulatory controls, the current system of defining Total Allowable Catches 
is the most direct way of controlling the fishing mortality on cod. For the immediate 
future this will continue to take the form of single stock catch advice, though may move 
in the medium term to multi-species or ecosystem based assessments. Development of 
multi-species assessments should continue to be encouraged through ICES. In 2019 the 
ICES working group WKCELTIC will review the findings of the current tagging study into 
account at the benchmarking process for the cod stock in the Celtic Sea. 

Biomass estimates form the tagging project support annually assessed SSB estimates. 
The whilst the stock is considered to be above Blim its size below Bpa suggest that 
currently (2018-2019) the Irish Sea stock remains in a delicate state. All management 
measures to limit cod mortality outside sustainable limits should be taken:  

Management recommendation: The TAC should be maintained as exclusively for by-
catch. This should aim at preventing a resumption of an unsustainable cod targeted 
fishery until the stock recovery is confirmed.  

An increase in fishing pressure is probable given that fishing opportunities increased in 
2018 and 2019, although the uptake of TAC in 2018 was low. The Landings Obligation, 
which in 2019 will require all TAC species that are caught in the Irish Sea to be landed 
may also result in increased reporting of cod catches, however, current estimates of 
discards are considered to be sufficient to assess catch. The increased fishing 
opportunities and landing obligation may result in enhanced port sampling, by giving 
opportunity to sample the entire size distribution of the stock in ports.  

With a bycatch only fishery significant catch proportion is generated by the Nephrops 
fishery. The experience of tagging staff during the project indicate that whilst cod catch 
rates within the predominant Nephrops fishery is low the viability of cod for release was 
insufficient due to low vitality levels. This was considered to be due to long tow duration. 

Management recommendation: Discard survivability of cod within the Nephrops fishery is 
low and must be included / discounted against TACs and landed. Technical measures are 
regarded to have contributed to low catch rates. Given low survivability of bycatch, 
monitoring of catch rates must ensure that bycatch is maintained at a minimum level. 

The most successful method to obtain cod to tag during the current project was through 
target demersal and midwater trawling using trawls with mesh 100 – 120mm. 
Furthermore, during quarter one / early quarter two when cod were form spawning 
aggregations tagging rates were highest.  

Management recommendation: Management of mortality on cod should be continued 
through gear restrictions. Current gear specific technical measures should be maintained 
and further developed. 

Development of multi-species assessments is likely in the next decade. The ICES 
Working Group on Multispecies Assessment Methods (WGSAM) carry out a multi-species 
assessment in the North Sea and will now consider the a model for the Irish Sea in 2019. 
An Irish Sea mixed fishery model will be developed by ICES Mixed-Fish Methods group in 
2019. This will help develop understanding of predator-prey interactions and advise on 
the ecosystem approach to fisheries management. There has been significant recent 
progress in understanding multi-species dynamics and interactions using Ecopath with 

 
 
 
Ecosim (ICES, 2018) through the WKIrish workshop series at which the current work has 
been presented and initial results reported. Progress on the development of true multi-
species stock assessments and advice has yet to mature. Managers through the CFP have 
identified a need for multispecies management but this will require dedicated resources 
and clear guidance from managers and stakeholders to develop properly.  

Ongoing modelling studies at AFBI of cod habitat suitability and the recapture results 
from this study support a thermal linkage and driver to cod distribution. More likely to 
provide the next advisory assessments are ecosystem based assessments. These 
comprise multiple trophic levels, incorporating abiotic and biotic impacts across the food 
web from primary production through primary, secondary and tertiary consumers, 
incorporating state of the environment in combination with the concept of a carrying 
capacity of the ecosystem for the resource of interest, be they targeted fisheries or 
conservation interests such as cetaceans, elasmobranchs and birds. Coupled with impacts 
of fishing pressure, spawning stocks biomass, recruitment estimates and estimated 
reference points, these models are likely to be developed further and may provide an 
implementable ecosystems basis for resource management. 

Management recommendation: The ecosystem approach being development with 
predator-prey linkage and food-web dynamics must include appropriate environmental 
drivers to allow change dynamics to be addressed. 

Through the diffuse distribution of the projects findings. These may include the 
incorporation of the results of the study into development of Spatial Resource 
Management Plans for the Irish Sea; through public bodies and resource managers such 
as national governments, county and local councils. Whilst these are diffuse influences 
may arise from project publicity, project reporting, scientific and natural resource 
management networking (including science-government advisory processes, 
collaborative industry-science partnerships and science eNGO partnerships), and through 
the research network of scientific peer reviewed literature, conferences and meetings. 

The actions required to achieve these management measures for the Irish Sea cod stock 
are: 

  Review of the ICES cod 7a stock assessment and consideration of the need to account for 
mixing with the Celtic Sea stock based on the results of this project. Both assessments 
have very significant retrospective bias revisions and associated uncertainties. Further 
focused analysis using alternative methods, such as otolith microchemistry to resolve stock 
origins and quantify inter-annual variation in seasonal migration patterns.  

 

TAC management needs to take account of assessment uncertainties. A more cautious 
management regime is appropriate where by-catch only fisheries are allowed and cod 
avoidance measures are maintained, until the stocks are rebuilt for several years.  

  Realignment of the spatial coverage of management measures to incorporate the spatial 

extent of aggregations of cod found in the project over the spawning period and catch and 
sampling data. 

  Develop a time series under the changing landscape of the Landings Obligation, the 

increased fishing opportunities for cod and the Western Waters MAP assuming it is 
implemented. 

  Realignment of the time period of closures, to coincide with the observed timing of 

spawning aggregations. 

 

 

Extension of the time of the restrictions, in line with the ranges of timing of spawning. It is 
currently extremely difficult to accurately model times of spawning, with enough lead time 
to translate into legislation/ regulation, and with possible changes in the temperature and 
current regime in the Irish Sea. It may therefore be better to extend the period to ensure 
the spawning events are covered within the time frame. 

Introduction of more selective gears will be needed in the Irish Sea. The selectivity 
objective will need definition but should fundamentally aim to reduce the catch of cod 
below MCRS.  

  Review of data collection under the Landings Obligation, non-zero TAC and Western Waters 

MAP and benchmarking of the ICES cod 7a stock assessment in light of the data. 

 
 
 
3 

DISCUSSION 

The aim of the current study was to explore sources of mortality for Irish Sea cod. 
Through the project we establish a new baseline understanding of the current 
movements and distribution of Irish Sea cod. Previous movement studies have relied on 
recapture data sampled pre-2011 (Brander 1975; Connolly & Officer 2001; Bendall et al. 
2009; Neat et al., 2014). Migration of Atlantic cod has been reported in other cod stocks 
around Britain and to a greater extent in the North Atlantic. Incorporation of these 
movement patterns into the assessment process is of fundamental importance to define 
the population structure of the stock and to allow forecast of stock abundance and derive 
management options.  

The results of the study indicate that inter-annual movement of cod from Division 7a to 
Division 7g is 16.4% sd = 4.5%. Recaptured cod remained at liberty on average for 
approximately five months (166.5 days ± 149.2 days;  

Ye
ar 

T
o
t
al 
n 

Rel
ea
se  

ar
ea 
n 

Rele
ase 
leng
th 
(cm
) 

Time 
at 
libert
y 
(day
s) 

Re
mai
n 
in 
IRE 

Re
mai
n in 
CEL 

IRE 
to 
CEL 

IRE 
to 
WSC 

Other 

(notes) 

20
16 

1
6 

IRE 
= 
16 

IRE 
= 
62.0 
(± 
5.9) 

IRE = 
219.8 
(± 
285.7
) 

11 
fish  

(68
.8
%) 

- 

3 
fish 

2 
fish 

(18.
8%) 

(12.
4%) 

- 

- 

44 
fish 

15 
fish 

(88
.0
%) 

(10
0.0
%) 

5 
fish 

1 
fish 

(10.
0%) 

(2.0
%) 

20
17 

6
5 

20
18 

8
0 

IRE 
= 
50 

CE
L = 
15 

IRE 
= 
74 

CE
L = 
6 

IRE 
= 
67.4 
(± 
20.0
) 

CEL 
= 
48.0 
(± 
7.3) 

IRE 
= 
70.3 
(± 
13.5
) 

CEL 
= 
67.0 
(± 

IRE = 
235.8 
(± 
156.6
) 

CEL 
= 
106.7 
(± 
158.8
) 

IRE = 
122.9 
(± 
61.9) 

CEL 
= 
133.5 
(± 
58.7)  

58 
fish 

(78
.4
%) 

5 
fish 

15 
fish 

(83.
3%) 

(20.
2%) 

- 

1 fish released in 
CEL moves west 
into ICES 
management 
division 7j 

1 fish released in 
IRE moves south 
through the CEL 
and is recaptured 
in ICES 

Dist
anc
e 
trav
elle
d 
(km
) 

IRE 
= 
99.1 
(± 
121.
0) 

IRE 
= 
55.9 
(± 
97.2
) 

CEL 
= 
25.1 
(± 
39.7
) 

IRE 
= 
93.4 
(± 
116.
3) 

CEL 
= 
120.
7 (± 

Mean 
velocit
y  

(km 
day-1) 

IRE = 
0.9 (± 
0.9)  

IRE = 
0.9 (± 
2.7)  

CEL = 
0.4 (± 
0.4) 

IRE = 
1.0 (± 
1.2) 

CEL = 
1.1 (± 
0.6) 

 
 
 
 
10.5
) 

management 
division 7k 

45.8
) 

), travelling an average distance from release to recapture of 77.0 km (± 106.2 km). An 
up-to date growth model was derived for Irish Sea cod for 2017 (Figure 38). This model 
has been updated with data from the current project and other sampling sources 
including fishery independent and fishery dependent sources. The growth parameters 
were: Linf 2017= 174.51 cm, K2017 = 0.126 and t0
cod growth from published sources (ICES 2018a) parameters suggest that cod growth 
rate has been reducing (K value) with individuals expected to reach a larger asymptotic 
length (Linf). This was considered to be linked to maturation of the 2013 cohort, which 
was relatively strong compared to previous cohorts of cod in the Irish Sea. The annual 
assessment of Irish Sea cod to provide advice has identified a retrospective bias – with 
an over estimation of the spawning stock biomass. The results of the current tagging 
study were incorporated into the assessment framework and show a reduced 
retrospective bias therefore providing an explanation for the annual downward revision of 
the SSB, given the observed movements in particular for larger cod (>60cm) outside the 
limits of the current assessment area.  

2017 = -0.568. These and the most recent 

A review of historic tagging study data has been conducted by Bendall et al., (2009) and 
Neat et al., (2014) who found that fish tagged in the Celtic Sea remain exclusively in 
ICES management divisions 7f, 7h and 7g and display no evidence of northwards 
migration into the Irish Sea (ICES management division 7a). Conversely, cod tagged in 
the Irish Sea have been shown to migrate both southwards into the Celtic Sea (Connolly 
& Officer 2001; Bendall et al., 2009; Neat et al., 2014) and northwards into the waters 
off the west coast of Scotland, most notable the North Channel and the Firth of Clyde 
(Brander 1975; Neat et al., 2014). This is supported in by the recapture results of the 
current study (Figure 30). The reasons why Irish Sea cod undertake such migrations 
were previously largely unknown. From the current project the a corroborative 
environmental / habitat suitability driver is thought to define the distribution and 
movements of Irish Sea cod based on habitat suitability modelling of thermal habitat 
(Section 3.1.4). At present the study identifies a small area in 7a of suitable thermal 
habitat which mirrors the core area of the 7a recapture data and clearly identifies a much 
larger area of suitable habitat in the Celtic Sea, within which the recapture data are 
recorded. Additionally the analysis of monthly habitat suitability supports the hypothesis 
this environmental driver would have strongest effect in late quarter 3 and early quarter 
4 (Figure 36), as shown from the historic tagging review. 

It cannot be discounted that there are two distinct, sedentary stocks in the Irish and 
Celtic Seas, which might be complemented by a third, migratory component, as seen in 
Skaggerak and Kattegat cod (Svedang et al., 2007). Comparative to the Irish Sea, adult 
cod abundance in the eastern Skagerrak and Kattegat has dramatically declined since the 
1980s (Andre et al., 2016). For the first time we now have robust estimate of the 
contribution of this stock component to the Irish Sea cod stock – by combining the ICES 
stock assessment model with the observed movement data. A hypothesis that cod 
populations in the Skagerrak and Kattegat comprise a mixture of resident and migratory 
stocks was supported by a tagging study (Svedang et al., 2007). Kattegat and Gullmar 
Fjord showed an abundance of cod displayed resident behaviour, whereas fish tagged 
and released off the eastern Skagerrak coast predominantly migrated during the 
spawning season towards the North Sea. The study observed the majority of these fish 
returning to the tagging site by late spring. In the eastern Skagerrak region, small 
spawning aggregations of cod have resulted in observed low adult cod abundance 
coinciding with high juvenile abundance (Svedang 2003). It is thought that the majority 
of these juvenile cod result from passive movement from spawning grounds further 
offshore. The observed low abundance of adult cod resulted from juvenile and/or 

 
 
 
maturing fish of a certain size exhibiting return migration behaviour to the North Sea. 
This raises questions regarding the degree of connectivity between marine fish 
populations should both a migratory and resident cod stock exist in the Irish Sea and to 
what degree they may fluctuate over time.  

A significant change in TAC for 2018 and 2019 has already seen a higher amount of cod 
landings. This had a marked effect on the recapture rates observed in the project which 
were lower in the first two years but increased as the targeted whitefish ‘TR1’ vessel 
began to catch increasing amounts on cod in 2018 in response to TAC increase. The 
source of recapture records between years demonstrate how changes in management 
and in particular catch advice can result in markedly different sources of mortality (Figure 
32).  

Within the current ICES assessment catches are summarised for ‘otter trawls’ targeting 
Nephrops and demersal fish, ‘Scottish Seines’, ‘Mid-water trawl’, ‘Beam trawls’ and 
‘others’ (ICES 2019). Results of the current project support that these categories include 
the main sources of cod mortality with all reported recaptures derived from Nephrops 
and demersal fish fisheries. However, there is a bias with the recaptures toward demersal 
trawl fisheries which contributed 73% of recaptures in the Irish Sea compared to 8% by 
Nephrops directed trawls, whilst landings in 2018 were comprised 65% by demersal 
fisheries and 25% by Nephrops directed trawl fisheries. Recaptures by recreational 
fishers in the Northern Irish Sea were limited, with a total of 10 recaptures made by 
recreational anglers, 8 of which were in ICES rectangle 33E4 and all but two above 
minimum landing size (7 were returned alive). Recreational fishing for cod tended to be 
shore based and mostly in the southern Irish Sea. Whilst the recreational catches are not 
considered within the annual assessment, the high return rate and shallow depth from 
which the cod are caught suggest the mortality rates from these catches are low.  

Since 2003 landings in the Irish Sea stock assessment have been adjusted to exclude 
those taken from the southern rectangles (33-E2, 33-E3 and 33-E4), as they have been 
believed not to be part of this stock but rather of the stock in divisions 7e–k (western 
English Channel and Celtic Sea). Thus, the assessment and the advice exclude these 
southern rectangles for the present stock but include them in the assessment and advice 
for cod in ICES divisions 7e–k. No tagged Irish Sea cod or tagged Celtic Sea cod were re-
captured in those rectangles. 

Despite large reductions in whitefish trawling effort since the early 2000’s, Irish Sea cod 
have shown only, recent, moderate indications of recovery – linked to rare strong recruit 
events in isolate years. The input into management advice from this project will be 
undertaken under the ICES framework of annual advice in line with the requirements of 
the CFP. The initial results of the project have been presented to the ICES Working Group 
for the Celtic Seas in May 2018 (ICES 2018) and in May 2019 (unpublished). It is 
through this communication and working directly with the ICES assessment groups that 
the conclusions from the study can be incorporated in the scientific assessment of the 
stock and placed in the context of current management and future management plans. 
The current study clearly indicates a greater mixing of the stocks than in previous 
observed. The implications of current management will be incorporated into the 
development of scenarios for both the 7a and 7b-k cod stocks.  

From the late 1960s to early 2000s, the Irish Sea underwent an environmental regime 
shift, with accompanying changes in ecological processes and productivity (ICES 2015). 
From a physicochemical position this was exhibited as changes in average nutrient 
loadings and seasonal mean temperatures. These physical oceanographic changes were 
accompanied by an ecological shift, effecting primary and secondary production of 
phytoplankton and zooplankton, through the food chain to finfish, crustacean and 
gastropod shell fish communities. The effects of such a regime shift on the gadoid 
assemblage will have included reductions in spawning and recruitment, growth and size. 
These changes may be the reason why there have been shifts in the dominant species for 
fisheries in the Irish Sea during the 1970s and 1980s, cod and whiting, and into the later 
part of the century to the present Nephrops dominated system. Further, it may be 

 
 
 
postulated that the dramatic reductions seen in the herring, cod, whiting arose 
fundamentally not because of fishing pressure, but owing to the change to the 
physicochemical balance of the system.  

This is not to down weight the significant effect of high fishing mortality on the stock. 
Under the stress of a changing ecosystem balance, high fishing pressure would have 
further accelerated stock declines. The causes of the regime shift may be aligned with 
land use patterns, increasing use of agricultural fertilizers, pesticides and herbicides, 
channelization of waterways and increases in urban run-off, and climate change. While 
such a regime shift is not necessarily directly taken into account in fisheries 
management, it is indirectly taken into consideration when making management 
decisions. Future management plans should be based on improved process 
understanding and be robust to the likely future regime scenarios. Policies which are not 
in line with the environmental limitations is unlikely to achieve their aims.  

The series of WKIRISH ICES workshops are also engaged in investigating ecosystem 
based management, of which any Irish Sea cod focused management plan should be 
cognisant. These methods will explore the food web and fishery interactions to develop a 
more holistic approach to fishery management. To this end the recommendations of 
WKIRISH need to have bearing upon Irish Sea cod management. 

The results from the current project, in conjunction with historic analysis of spatial 
distributions of Irish and Celtic Sea cod, provide details of widespread exchanges of cod 
across different management areas. Through population forecasts models it is possible to 
provide management option ranges under scenarios supported by the recapture data, 
exploring the sensitivity of catch management options, toward FMSY, of migration and 
stock mixing scenarios for the Celtic Sea and Irish Sea stocks. Analysis of recapture 
records and biological data has provided new insights into the ecology of Irish Sea cod. 
Results show an area of stock mixing within cod of Celtic Sea and Irish Sea origin (Figure 
33 & Figure 34). There is no evidence of long distance movements of cod tagged in the 
Celtic Sea moving north into Irish Sea. Scenario testing using population assessment 
models demonstrates that these migration patterns could have significant implication for 
the assessment of biomass of the Irish Sea cod stock and correct for the retrospective 
bias of SSB forecast as seen in the current ICES assessment (Figure 42). 

The current study clearly indicates a greater mixing of the stocks than previously 
observed. It is possible that the migratory component of the population fluctuates over 
time and with longer term environmental conditions, and other factors such as strong 
recruitment enhancing density dependent migration. It is recommended that a method to 
monitor the contribution of the two spawning populations should be developed as used 
for management of herring in the North Sea and Western Baltic. Cod released in the Irish 
Sea and recaptured within the Irish Sea was 80.7%. Whilst cod released in the Irish Sea 
and recaptured in the Celtic Sea was 16.4%, only a single recapture of a cod released in 
the Celtic Sea was recaptured outside the Celtic Sea, in an inshore southern Area of the 
VIIa. Knowledge of such broad-scale movement is critical to fisheries management as 
unaccounted movements could expose parts of the stock to inappropriate management 
regimes in other regions but also fish that remain in reproductively isolated units (as 
observed in the Celtic Sea; Bendall et al., 2009; Neat et al., 2014) will be prone to 
localised depletion and are less likely to recover from high levels of historical exploitation 
(Neat et al., 2014). Movement beyond stock boundaries will expose individuals to 
unknown and often unconsidered sources of natural mortality (Cook et al., 2015; 
Alexander et al., 2015). Secondly, because current fisheries management considers the 
Irish Sea to be a discrete, self-contained unit, so movement beyond the spatial 
boundaries of ICES management Division 7a will result in individuals being open to 
exploitation as part of a separate fishery.  

 
 
 
 
4 

REFERENCES  

Alexander, K.A., Heymans, J.J., Magill, S., Tomczak, M.T., Holmes, S.J., and 
Wilding, T.A., 2015. Investigating the recent decline in gadoid stocks in the west of 
Scotland shelf ecosystem using a foodweb model.  ICES Journal of Marine Science, 
72(2): 436-449. 
André, C., Svedäng, H., Knutsen, H., Dahle, G., Jonsson, P., Ring, A,K., Sköld, M. 
and Jorde, P.E., 2016. Population Structure in Atlantic Cod in the Eastern North Sea 
Skagerrak Kattegat: Early Life Stage Dispersal and Adult Migration. BMC Research 
Notes 9 (February): 1–11.  
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4.1  Appendix 1. 

Appendix 1. Overview of tagging events in both the initial and revised tagging phase. The number of fish tagged and below the minimum 
conservation reference size (MCRS) is given.  

Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

24/03/2016 

AFBI 

01/04/2016 

AFBI 

1 

1 

Charter 
1 

Charter 
1 

03/05/2016 

AFBI 

1 

NI-FSP 

11/03/2017 

AFBI 

14/03/2017 

AFBI 

1 

1 

Charter 
2 

RV 
Corystes 
Groundfi
sh 
Survey 

Charter 

475 

Charter 

488 

Scientific 
Survey 

12 

Charter 

135 

0 

0 

0 

0 

4 

13 

4 

14 

2 

5 

4 

16 

W. Irish Sea 
and North 
Channel 

W. Irish Sea 
and North 
Channel 

Mid Irish Sea 
haddock fishing 
grounds 

W. Irish Sea 
and North 
Channel 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Scientific 
Survey 

7 

0 

15 

- 

Various survey 
stations across 
the Irish Sea 

‘Rockhop
per’ otter 
trawl 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

17/03/2017 

AFBI 

1 

NI-FSP 

Scientific 
Survey 

11 

21/03/2017 

AFBI 

1 

Charter 
2 

Charter 

37 

28/03/2017 

AFBI 

1 

NI-FSP 

Scientific 
Survey 

30 

29/03/2017 

AFBI 

1 

Charter 
2 

Charter 

368 

01/04/2017 

AFBI 

1 

NI-FSP 

Scientific 
Survey 

3 

0 

0 

0 

0 

0 

3 

4 

1 

2 

5 

12 

7 

18 

2 

2 

06/04/2017 

AFBI 

1 

Charter 
2 

Charter 

105 

1 

3 

8 

W. Irish Sea 
and North 
Channel 

W. Irish Sea 
and North 
Channel 

W. Irish Sea 
and North 
Channel 

W. Irish Sea 
and North 
Channel 

W. Irish Sea 
and North 
Channel 

Western Irish 
Sea spawning 
grounds and 
North Channel 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

14/04/2017 

MI 

17/04/2017 

MI 

06/06/2017 

AFBI 

03/07/2017 

MI 

05/07/2017 

AFBI 

1 

1 

1 

1 

1 

Charter 
3 

Charter 
4 

Charter 

166 

39 

5 

23 

Charter 

25 

1 

2 

10 

Southern Irish 
Sea/Celtic Sea 

Southern area 
of spawning 
grounds in 
Western Irish 
Sea 

Pot-
fishing 
Observer 

Commerci
al 
Observati
on 

2 

2 

1 

- 

Down Coast 

Charter 
5 

RV 
Corystes 
Queen 
Scallop 
Survey 

Charter 

173 

4 

3 

24 

Scientific 
Survey 

5 

0 

9 

- 

North/Mid Irish 
Sea 

Various survey 
stations across 
the Irish Sea 

Mid-
water 
seine net 

Mid-
water 
seine net 

Pot 
bycatch 

Mid-
water 
seine net 

Bottom 
trawl 
queen 
scallop 
net 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

31/07/2017 

AFBI 

11/08/2017 

AFBI 

23/08/2017 

MI 

05/09/2017 

AFBI 

25/09/2017 

MI 

06/10/2017 

MI 

1 

1 

1 

1 

1 

1 

Charter 
6 

Charter 
6 

Charter 

31 

Charter 

430 

0 

0 

1 

3 

4 

18 

South-east of 
Nephrops 
grounds 

South-east of 
Nephrops 
grounds 

Mid-
water 
trawl 

Mid-
water 
trawl 

Pot-
fishing 
Observer 

Commerci
al 
Observati
on 

1 

0 

1 

- 

Western Irish 
Sea 

Pot 
bycatch 

RV 
Corystes 
Groundfi
sh 
Survey 

Angler - 
self 
tagging 

Angler - 
self 
tagging 

Scientific 
Survey 

2 

0 

8 

- 

Various survey 
stations across 
the Irish Sea 

‘Rockhop
per’ otter 
trawl 

Voluntary 

2 

Voluntary 

3 

1 

2 

1 

1 

- 

- 

Waterford 

Wexford 

Rod and 
line 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

Angling 
boat 

Sea 
Angling 

8 

4 

08/10/2017 

MI 

13/10/2017 

MI 

14/10/2017 

MI 

19/11/2017 

MI 

30/11/2017 

AFBI 

03/12/2017 

MI 

16/12/2017 

AFBI 

1 

1 

1 

1 

1 

2 

2 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Angling 

Shore 
Angling 

Shore 
Angling 

Observer 
on TR1 

Commerci
al 
Observati
on 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Angling 

Shore 
Angling 

1 

0 

- 

- 

- 

Waterford 

Rod and 
line 

Cheekpoint, 
Waterford 

Rod and 
line 

Passage East, 
Waterford 

Rod and 
line 

37 

34 

18 

18 

0 

2 

3 

1 

0 

- 

Youghal, Cork 

Rod and 
line 

0 

0 

5 

Mid Irish Sea 
haddock fishing 
grounds 

Mid-
water 
trawl 

12 

12 

0 

6 

6 

0 

- 

- 

Ballyhack, 
Wexford 

Rod and 
line 

Cushendun, 
Antrim 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

17/12/2017 

05/01/2018 

MI 

MI 

13/01/2018 

AFBI 

21/01/2018 

MI 

27/01/2018 

AFBI/MI 

28/01/2018 

AFBI/MI 

29/01/2018 

AFBI/MI 

2 

2 

2 

2 

2 

2 

2 

Shore 
Fishing 

Shore 
Angling 

Shore 
Fishing 

Shore 
Angling 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Competit
ion 

Shore 
Angling 

Shore 
Angling 

Shore 
Angling 

Shore 
Angling 

Shore 
Angling 

20 

20 

5 

2 

2 

1 

1 

1 

5 

2 

2 

1 

1 

1 

0 

0 

0 

- 

- 

- 

Arthurstown 

Arklow North 

Rod and 
line 

Rod and 
line 

Cushendall, 
Antrim 

Rod and 
line 

0 

- 

Arklow South 

Rod and 
line 

0 

0 

0 

- 

- 

- 

Kilmuckridge, 
Wexford 

Rod and 
line 

Kilmuckridge, 
Wexford 

Rod and 
line 

Kilmuckridge, 
Wexford 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

10/02/2018 

AFBI 

17/02/2018 

MI 

18/02/2018 

MI 

24/02/2018 

MI 

25/02/2018 

MI 

25/02/2018 

AFBI 

2 

2 

2 

2 

2 

2 

Shore 
Competit
ion 

Cod 
specific 
Shore 
Competit
ion 

Shore 
Competit
ion 

Charter 
7 (Trip 
1) 

Shore 
Competit
ion 

Charter 
8 

Shore 
Angling 

Shore 
Angling 

Shore 
Angling 

9 

9 

0 

- 

Waterfoot, 
Antrim 

Rod and 
line 

12 

11 

0 

- 

Cheekpoint, 
Waterford 

Rod and 
line 

16 

16 

0 

- 

Ballyhack, 
Wexford 

Rod and 
line 

Charter 

41 

Shore 
Angling 

6 

Charter 

143 

0 

6 

0 

3 

13 

Celtic Sea 

Demersal 
trawl 

0 

0 

Passage East, 
Waterford 

Rod and 
line 

3 

12 

Western Irish 
Sea spawning 
grounds 

Mid-
water 
trawl 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

Scientific 
Survey 

113 

Charter 

225 

Charter 

608 

Charter 

156 

Charter 

68 

0 

1 

0 

0 

0 

05/03/2018 

AFBI 

2 

NI-FSP 

13/03/2018 

MI 

05/03/2018 

AFBI 

12/03/2018 

AFBI 

19/03/2018 

AFBI 

2 

2 

2 

2 

23/03/2018 

AFBI 

2 

Charter 
7 

Charter 
7(Trip 2) 

Charter 
7 (Trip 
3) 

Charter 
8 

RV 
Corystes 
Groundfi
sh 
Survey/
Cod 
Tagging 

0 

21 

Various survey 
stations across 
the Irish Sea 

Mid-
water 
trawl 

4 

28 

Western Irish 
Sea spawning 
grounds 

Demersal 
trawl 

5 

22 

North Channel 

3 

13 

3 

12 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

Mid-
water 
trawl 

W. Irish Sea 
and North 
Channel 

Western Irish 
Sea spawning 
grounds 

Western Irish 
Sea spawning 
grounds and 
North Channel 

Scientific 
Survey 

18 

0 

3 

7 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

26/03/2018 

MI 

07/04/2018 

MI 

15/04/2018 

MI 

22/04/2018 

MI 

24/04/2018 

AFBI 

2 

2 

2 

2 

2 

28/04/2018 

AFBI/MI 

2 

Charter 
7 

Shore 
Competit
ion 

Shore 
competit
ion 

Angling 
Boat 
Competit
ion 

Charter 

170 

Shore 
Angling 

Shore 
Angling 

Sea 
Angling 

7 

9 

4 

Pot 
Fishing 
Trip 

Commerci
al 
Observati
on 

Cod 
specific 
Shore 
Competit
ion 

Shore 
Angling 

0 

4 

8 

3 

17 

Western Irish 
Sea spawning 
grounds 

Demersal 
trawl 

0 

0 

- 

- 

Wexford 

Rod and 
line 

Ballyhack, 
Wexford 

Rod and 
line 

1 

1 

- 

Kilmore Quay 

Rod and 
line 

Pot 
bycatch 

1 

0 

1 

- 

Down coast 

3 

3 

0 

- 

East Antrim 
coast 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

30/08/2018 

MI 

30/08/2018 

AFBI 

29/04/2018 

06/05/2018 

08/05/2018 

08/05/2018 

02/06/2018 

04/06/2018 

11/06/2018 

MI 

MI 

MI 

MI 

MI 

MI 

MI 

2 

2 

2 

2 

2 

2 

2 

2 

2 

Charter 
9 

Charter 
10 

Charter 

85 

3 

13 

Charter 

203 

5 

16 

Shore 
Angling 

Shore 
Angling 

Sea 
Angling 

Sea 
Angling 

Shore 
Angling 

Shore 
Angling 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

1 

1 

32 

19 

2 

4 

2 

4 

2 

2 

4 

2 

4 

2 

0 

1 

0 

0 

0 

0 

0 

- 

- 

- 

- 

- 

- 

- 

Western Irish 
Sea 

Demersal 
trawl 

Western Irish 
Sea 

Mid-
water 
trawl 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

 
 
 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

16/06/2018 

MI 

17/06/2018 

MI/AFBI 

23/06/2018 

24/06/2018 

01/07/2018 

01/07/2018 

07/07/2018 

11/07/2018 

MI 

MI 

MI 

MI 

MI 

MI 

13/07/2018 

AFBI 

2 

2 

2 

2 

2 

2 

2 

2 

2 

Sea 
Angling 

Sea 
Angling 

5 

5 

Sea 
Angling 

Sea 
Angling 

25 

22 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Sea 
Angling 

Sea 
Angling 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Sea 
Angling 

Sea 
Angling 

Sea 
Angling 

Sea 
Angling 

5 

3 

8 

4 

5 

5 

3 

4 

0 

4 

30 

23 

3 

2 

1 

1 

0 

0 

1 

0 

0 

1 

1 

- 

- 

- 

- 

- 

- 

- 

- 

- 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Irish 
Sea/Southern 
Irish Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

13/07/2018 

AFBI 

21/07/2018 

03/08/2018 

04/08/2018 

MI 

MI 

MI 

05/08/2018 

AFBI 

12/08/2018 

AFBI 

21/08/2018 

28/08/2018 

MI 

MI 

08/09/2018 

AFBI 

2 

2 

2 

2 

2 

2 

2 

2 

2 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Sea 
Angling 

Sea 
Angling 

1 

3 

9 

Sea 
Angling 

Sea 
Angling 

10 

0 

3 

1 

2 

Sea 
Angling 

Sea 
Angling 

Sea 
Angling 

Sea 
Angling 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

21 

17 

5 

1 

1 

2 

1 

1 

1 

2 

0 

0 

1 

1 

1 

1 

0 

0 

0 

- 

- 

- 

- 

- 

- 

- 

- 

- 

North Coast 
Ireland 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

Southern Irish 
Sea/Celtic Sea 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

 
 
 
Date 

Organisation 

Phase 

Trip 

Type 

Fish 
Tagged 

Below 
MCRS 

Days 
at 
sea 

Hauls 

Location 

Capture 
method 

07/10/2018 

AFBI 

16/10/2018 

AFBI 

01/11/2018 

AFBI 

23/11/2018 

AFBI 

09/12/2018 

AFBI 

17/12/2018 

AFBI 

18/12/2018 

AFBI 

24/12/2018 

AFBI 

2 

2 

2 

2 

2 

2 

2 

2 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

Voluntar
y Tagger 

Voluntary 
Tagger 

4 

1 

12 

1 

3 

1 

1 

6 

4 

1 

4 

1 

3 

1 

1 

6 

0 

0 

0 

0 

0 

0 

0 

0 

- 

- 

- 

- 

- 

- 

- 

- 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

North Coast 
Ireland 

Rod and 
line 

 
 
 
In person 

GETTING IN TOUCH WITH THE EU 

All over the European Union there are hundreds of Europe Direct information centres. 
You can find the address of the centre nearest you at: 

https://europa.eu/european-union/contact_en  

On the phone or by email 

Europe Direct is a service that answers your questions about the European Union. You 
can contact this service: 

– by freephone: 00 800 6 7 8 9 10 11 (certain operators may charge for these calls), 

– at the following standard number: +32 22999696, or 

– by email via: https://europa.eu/european-union/contact_en  

FINDING INFORMATION ABOUT THE EU 

Online 

Information about the European Union in all the official languages of the EU is 
available on the Europa website at: https://europa.eu/european-union/index_en  

EU publications 

You can download or order free and priced EU publications from: 

https://publications.europa.eu/en/publications  

Multiple copies of free publications may be obtained by contacting Europe Direct or 
your local information centre (see https://europa.eu/european-union/contact_en). 

EU law and related documents 

For access to legal information from the EU, including all EU law since 1952 in all the 
official language versions, go to EUR-Lex at: http://eur-lex.europa.eu 

Open data from the EU 

The EU Open Data Portal (http://data.europa.eu/euodp/en) provides access to 
datasets from the EU. Data can be downloaded and reused for free, for both 
commercial and non-commercial purposes. 

 
 
 
 
 
 
 
 
H
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doi: 10.2926/869813 
ISBN 978-92-95225-48-0