DOI: 10.5061/dryad.4tmpg4fkt
title: Centurial life history parameters in Baltic cod (Gadus morhua)

description: The status of Eastern Baltic cod (EBC) Gadus morhua has remained poor
 despite low fishing mortality for over a decade, including a fishing ban
 since 2019. Although the decline in productivity can be explained by lower
 individual growth and survival rates, other aspects of life history
 changes such as maturation patterns for EBC has so far not been
 sufficiently explored. According to current stock assessments, the median
 size at maturity (L50) has halved from 40 to around 20 cm in total length
 since the 1990s, while the overall size distribution has become
 increasingly truncated. It has previously been suggested that changes in
 L50 can be attributed to both fishing-induced evolution and phenotypic
 plasticity induced by growth rates. However, since L50 is currently
 occurring around 20 cm, the maturation process must have been initiated at
 much smaller sizes, i.e. long before the fish could be caught in the
 dominant trawl fishery at around 35 cm. In this study, we aimed to further
 investigate what drivers may have led to reduced productivity in EBC by
 determining variations in size at sexual maturity in longer time series
 than has been done before (1930s to 1980s) and include prey productivity
 and quality. We found that L50 declined already in the 1930s and
 thereafter remained stable at around 40 cm up to the 1990s. On a centurial
 perspective, L50 has been positively correlated to growth potential (L95),
 length diversity, total stock biomass, total catch, and yield per recruit,
 while Fulton's condition factor  was not related to L50. Our
 results suggest that the links between life history parameters and
 external drivers are complex, but the present unprecedented early onset of
 maturity and hence decline in L50 since the 1990s signals a decline in
 growth potential, which also has hampered the productivity of EBC.
description: Archived reports from Swedish historical trawl surveys, which
 were conducted for various reasons, contain records on Baltic cod between
 1919 and 1983. The archived material is permanently located at the
 Regional State Archives in Gothenburg (https://riksarkivet.se/-goteborg), where it can be requested. The survey data are available from locations in the Baltic proper (i.e. ICES SD25-SD29). The archived material contains three sources of information: Source 1: Catch records, including information on the date, location, fishing gear, time of fishing, number of fish per length group (usually 1 cm) per species. Source 2: Records on individual cod, giving information on the date, fishing method, fishing location, species, fish length, and sometimes including details on sex and maturity stage. Source 3: Notes written on the envelopes used for storing biological material such as otoliths and scales saved for age reading purposes, providing information on length, location, and date of catch, as well as occasional information on weight, sex, and maturity stage. These sources are noted in the database. For the latter period between 1991 and 2022 given in the input files, ICES DATRAS database (www.ices.dk) as well as published analytical assessment information (ICES 2023).
description: # Centurial life history parameters in Baltic cod (Gadusmorhua)
 **Authors:** Henrik Svedäng Baltic Sea Centre, Stockholm University,
 SE-106 91 Stockholm, Sweden
 [henrik.svedang@su.se](mailto:henrik.svedang@su.se) Sara Hornborg RISE
 Research Institutes of Sweden, Department Agriculture and Food, Gothenburg
 Anders Grimvall Swedish Institute for the Marine Environment
 (SIME), Gothenburg University \**Files found at Dryad.
 [https://doi:10.5061/dryad.4tmpg4fkt](https://doi:10.5061/dryad.4tmpg4fkt) ** *Survey_data.txt* *L50_CF.txt* *Inputdata_figures3_6.csv* *lipid_content.csv* *Tables_supplementary.docx* **File descriptions** *Survey_data.txt* Archived reports from Swedish historical trawl surveys, which were conducted for various reasons, contain records on Baltic cod between 1919 and 1983. The archived material is permanently located at the Regional State Archives in Gothenburg ([https://riksarkivet.se/-goteborg](https://riksarkivet.se/-goteborg)), where it can be requested. The survey data are available from locations in the Baltic proper (i.e. ICES SD25-SD29). File file consists of extracted data from the archives on Eastern Baltic cod (EBC) used in the above mentioned article. The data from the archives are from three different sources: Source 1: Catch records, including information on the date, location, fishing gear, time of fishing, number of fish per length group (usually 1 cm) per species. Source 2: Records on individual cod, giving information on the date, fishing method, fishing location, species, fish length, and sometimes including details on sex and maturity stage. Source 3: Notes written on the envelopes used for storing biological material such as otoliths and scales saved for age reading purposes, providing information on length, location, and date of catch, as well as occasional information on weight, sex, and maturity stage. The following variables are included: *Data* *source* - 1-3 (see above) *ID_No* - identification number *ID_Name* - identification name *Area* - study area name *Station* - specific station *ICES_SD* - designated ICES subdivision number *Year* - calendar year *Date* - date *Month* - month *Quarter* - quarter of the year *Research_Vessel*/*Fishery* - vessel name *Haul_No* - haul number *Nr_Individual* - number given to an individual fish *Total_length_cm* - total length in cm *Weight_g* - total weight in g *Fulton_CF* - estimated Fulton’s condition factor *Sex* - Fe, Ma, unknown Maturity - maturity scale, i.e. eight-degree Maier’s scale *Im*/*mature* - estimated immature-1: scale 1-2, mature-2: scale 3-7 *Maturation* - given as text: immature/ mature *Species* - only cod included Cells with no available data are marked as "NA". For the latter period between 1991 and 2022, ICES DATRAS database ([www.ices.dk](http://www.ices.dk)) as well as published analytical assessment information (Eero 2021a,b; ICES 2023) have been used. *L50_CF.txt* Estimated L50 at maturity and Fulton’s condition factor by ICES SD (subdivision). The following variables are included: *Year* - calendar year    *SD* - ICES subdivision *SEX* - all *L50* - the estimated maturity ogive by ICES subdivision *CF*- estimated Fulton’s condition factor by ICES subdivision *CF_no_obs* - number of estimated observations in Fulton’s condition factor  Cells with no available data are marked as "NA". *Inputdata_figures3_6*.*csv*  Input data for figures 3 and 6 in the above-mentioned article. The following variables were included: *Year* - calendar year *L50* - yearly arithmetic mean value of *L*50 as calculated based on the available annual estimates on *L*50 in SD25-29 (see also Table S3; Tables_supplementary.docx). *cf_sd2529*- yearly estimates of Fulton’s condition factor for the size interval 41-60 cm in EBC in SD25-SD29 in the year *i* by using records on weight (*W*) and length (*L*) from the historical surveys between 1937 and 1983, if the number of records &gt;5 (Table S5; see Tables_supplementary.docx) *LDI* - length diversity index, *LDI*, in year *i *was defined by the Shannon-Wiener diversity of the annual length-frequency distribution (Probst et al. 2013, and the references therein) for the annual survey catch where *L*min was set to 20 cm. *Landings_ICES23* - time series between 1946 and 1965 on landings and total catch from 1966 and onwards, including both landings and discards in ICES (2023) *Discard_23* - discards from 1966 and onwards, including both landings and discards               *SSB_ICES23* - spawning stock biomass of EBC in tonnes (ICES, 2023)) *Recruits_ICES23* - annual recruits of EBC in numbers (ICES, 2023) *TSB&gt;35* - total stock biomass of EBC above 35 cm in total length in tonnes (ICES, 2023)   *F_46_ICES23* - Fishing mortality for ages 4-6* *for the period 1991-2020 (ICES 2023) *CBH_SSB_23* - stock spawning biomasses of central Baltic herring in tonnes ICES 2023) *SPR_SSB_23* - stock spawning biomasses of sprat in tonnes (ICES 2023) *meanL95* - length of the 95th percentile *Fat_content* - fat content in herring (see *lipid_content.csv*) Cells with no available data are marked as "n/a".   *lipid_content.csv* The herring samples were regularly collected in October-November at Utlängan at the southeast corner of the County of Blekinge (Bignert et al. 2017). Fat estimates were made on gillnet caught female herring in sizes 17-19 cm. The following variables were included: *stationID* – identification of station   *station* - locality of sampling *year* - calendar year *month* - month *day* - day of the month *Age* - age of the sampled fish in years *Fat_percent* – estimated fat content as percentage of body weight *Length* - fish length in cm *Weight* - fish weight in g Cells with no available data are marked as "n/a". *Tables_supplementary.docx* The following supplementary tables are included in the article. Table S1a: The number of years of investigation, the total number of hauls, and the number of caught fish per ICES subdivision (SD). Table S1b: The number of hauls per ICES subdivision (SD) and year. Table S2. The number of immature (Im) and maturing (Mat) per year, sex and ICES subdivision. Table S3. The length maturity ogive modelling results (*L*50, and the length percentile 2.5 and 97.5) for fish caught between April and August by ICES subdivision (SD) 24-31 and year. R-square, number of observations and the length interval for each modelling are show. Table S4. Summary of linear regression of log(W) ~ log(a) + b*log(L) for all individual cod between 20 and 100 cm. Table S5. The mean Fulton’s condition factor (*C.F.*) and number of observations within the size interval 41-60 cm by ICES subdivision (SD) 24-31 and year. ### Code/software Logistic regression was fitted to the material, using the r-script “FSA” developed by Ogle *et al*. (2018) for estimating *L*50 for sexes combined ### References: Bignert, A., Danielsson, S., Ek, C., Faxneld, S., Nyberg, E., 2017. Comments Concerning the National Swedish Contaminant Monitoring Programme in Marine Biota, 2017 (2016 years data). Naturhistoriska riksmuseet, rapport 10:2017. Eero M (2021a) Indicator for Eastern Baltic cod length structure. Technical University  of Denmark Dataset [https://doi.org/10.11583/DTU.16887601.v1](https://doi.org/10.11583/DTU.16887601.v1)         Eero M (2021b) Eastern Baltic cod size at maturity and body condition. Technical University of Denmark Dataset [https://doi.org/10.11583/DTU.16895254.v1](https://doi.org/10.11583/DTU.16895254.v1)ICES (2023) Baltic Fisheries Assessment Working Group (WGBFAS), 2023, ICES Headquarters, Copenhagen, Denmark. [www.ices.dk](http://www.ices.dk) Ogle D, Wheeler P, Dinno A (2018) Simple Fisheries Stock Assessment Methods. Package “FSA” [https://github.com/droglenc/FSA](https://github.com/droglenc/FSA) Probst WN, Stelzenmüller V, Kraus G (2013) A simulation-approach to assess the size structure of commercially exploited fish populations within the European Marine Strategy Framework Directive. Ecol Indic 24:621-632 [https://doi.org/10.1016/j.ecolind.2012.08.026](https://doi.org/10.1016/j.ecolind.2012.08.026)