Reducing mortality of bigeye tuna
during purse-seine fishing
A research agenda

insert picture

Johann Bell, Jonathan Peacey and Kevin Stokes
Betty and Gordon Moore Center for Science and Oceans, Conservation International
2011 Crystal Drive, Suite 500, Arlington, VA 22202, USA

Preferred citation
Bell J., Peacey J. and Stokes K. 2015. Reducing Mortality of Bigeye Tuna during Purse-seine
Fishing: A Research Agenda. Conservation International, Arlington, USA.
Cover photo
Bigeye, yellowfin and skipjack tuna mixed together inside a purse-seine net(Photo: ISSF/Fabian Forget)

Contents

Executive summary

Page

ii-iv

1.

The problem ………………………….……………………………..………………………….
1
1.1 Other considerations ………………………………………….………………….…… 5
1.2 A way forward ………………………………………….………………….…………….. 62.
Previous and existing initiatives ……………………………………………………
7
2.1 Methods with limited potential ………………………………………………….. 7
2.2 Ongoing initiatives ……………………………………………………………………. 8
2.3 Other potential methods …………………………………………………………… 12
3.
Additional research and development required ………………………….. 13
3.1 Determining the relative abundances of tunaassociated with FADs …………………………………………………………………. 13
3.2 Identifying cues (stimuli) that attract or repel
different tuna species ………………………………………………………………… 14
3.3 Developing methods for splitting double FADs ………………………. 17
3.4 Leading bigeye tuna away from FADs using bait ……………………. 18
4.
Possible approaches to support further research ……………………….. 18
5.
Conclusions ……………………………………………………………………………………… 19
Acknowledgements …………………………………………………………………………………… 19
References ………………………………………………………………………………………………….. 20i

Executive summary
Progressive expansion of purse-seine fishing fortuna in the Western and Central Pacific Ocean(WCPO), and continued longline fishing, hasresulted in the spawning biomass of bigeye tunain the WCPO falling below the limit reference
point agreed by the Western and Central PacificFisheries Commission (WCPFC). The problem
has arisen because much of the recent purseseine fishing in the WCPO has been for tunaassociated with drifting fish aggregating devices(FADs), and because most bigeye tuna caught
by purse seine are taken around FADs.
Conservation and management measures
introduced by WCPFC to reduce the mortality ofbigeye tuna – banning the setting of purseseine nets around FADs for several months
each year; closing the high seas pockets topurse-seine fishing; prohibiting the discarding of
small tuna at sea; placing observers on all
purse-seine vessels and imposing annual catch
limits for bigeye tuna in the longline fishery –have not had the desired effect. The total
number of FAD sets has not declined and large
catches of bigeye tuna continue to be taken bypurse-seine vessels in the WCPO, with catches
in 2013 and 2014 among the highest on record.
The high mortality of bigeye tuna due to purseseine fishing is now preventing entry of FADcaught tuna into some markets. The capture of
bigeye tuna around FADs is also affecting theaspirations of Pacific Island countries to derive
greater economic benefits from tuna resources
because it does not maximise yield-per-recruit.
Proposals by some members of the WCPFC tofurther reduce the number of months that FADs

can be used each year to help address the highfishing mortality of bigeye tuna was the subjectof intense debate at the 11th Regular Session of
the WCPFC in Apia, Samoa, in December 2014.
Increasing the duration of FAD closures met
with opposition because it would result in a
disproportionate burden for some of the smaller
States. WCPFC failed to reach consensus at theth
11 Regular Session on a plan of action to end
overfishing of bigeye tuna in the WCPO.
Other ways that have been proposed to reduce
the fishing mortality of bigeye tuna by purseseine vessels include temporal-spatial closures,
vessel catch limits and increases in fees forvessels using FADs. However, implementation ofsuch measures in the EEZs of Pacific Island
nations and on the high seas is likely to be
protracted because WCPFC is comprised of >30
members and operates mostly by consensus.
There is also the challenge of harmonising
management measures for bigeye tuna
between the two regional fisheries management
organizations (RFMOs) in the Pacific, the WCPFC
a n d t h e I n t e r - A m e r i c a n Tr o p i c a l Tu n a
C o m m i s s i o n ( I AT T C ) ( r e s p o n s i b l e f o r
management east of 150°W).
The various issues and constraints involved inmanaging the purse-seine fishery to reduce thefishing mortality of bigeye tuna would be largely
removed if practical methods could be
developed to maintain high catch rates ofskipjack tuna from FAD fishing while reducing
the bigeye tuna catch. This has beenrecognised for some time and possible solutions
have been investigated by several
organisations, such as the International
Sustainable Seafood Foundation (ISSF). It is alsoa requirement of a recent WCPFC Conservationiiand Management Measure (CMM 2014-01),
which states that “the Commission shallpromote and encourage research to identify
ways for vessels to avoid the capture of juvenile
bigeye and yellowfin tuna during FAD sets”. This
report outlines a research agenda to achieve
this goal. It describes:
1) Previous and existing initiatives to reduce thecatch of bigeye tuna around FADs;
2) Additional research to determine whether
practical methods for reducing the catch of
bigeye tuna around FADs can be developed;
and3) Ways of supporting and stimulating suchresearch.
Previous research has demonstrated that four
solutions proposed to reduce the fishingmortality of bigeye tuna during purse-seining
operations – reducing the depth of purse-seine
nets, inserting sorting grids into nets to allow
bigeye tuna to escape, setting on tunaaggregations associated with FADs at a different
time of day and targeting skipjack schools thatbreak away from FADs – all appear impractical.
This is because for each of these proposedsolutions there is considerable overlap in the
distribution and behaviour of skipjack andbigeye tuna.
The most promising areas of research for
reducing the fishing mortality of bigeye tuna by
purse-seine fishing fall into two main categories.
First, detecting the relative abundance of thedifferent species of tuna associated with FADs,
enabling vessel captains to avoid setting nets
around FADs in areas with large quantities ofbigeye tuna. Second, identifying stimuli that
could be used to attract or repel either bigeyeor skipjack tuna, creating opportunities toseparate the species prior to setting the net.
Research in the first category needs to focus ondeveloping improved acoustic, sonar and visualsystems that would enable fishing vessels toremotely estimate the quantities of tuna species
beneath a FAD. Provided there is variability inthe percentage of bigeye tuna associated with
FADs, improved detection methods represent
the most straightforward way of reducing thefishing mortality of bigeye tuna.
For the second category of research, possible
stimuli for attracting or repelling the different
species of tuna include light, sound, depth of
FAD material, shading, surface water spraying
and bubble curtains. Application of these stimuli
would depend on use of the double FAD
systems under investigation by ISSF. Double
FADs can be split apart before the purse-seine
net is set, making it theoretically possible to set
mainly on skipjack tuna if most bigeye tunaremain associated with only one of the FADs.
Use of dead bait should also be included in thepossible stimuli because it may have potential tolead bigeye tuna away from a FAD prior tosetting. If so, double FAD technology would not
be required.
Priority investments to develop the desired new
fishing technology are:1) Support for gear technologists and scientists,
including the costs involved in chartering
purse-seine vessels to conduct experiments;and2) I n c e n t i v e s f o r v e s s e l o w n e r s , a n dmanufacturers of acoustic and opticalequipment for discriminating species, toengage in the necessary research.

iii

One way of providing an appropriate incentive
is organising a competition for a substantial
prize for the first vessel or fleet to consistently
achieve large reductions in the catch of bigeye
tuna using new technology.
The potential benefits of investments indeveloping purse-seine fishing methods that
reduce the catch of bigeye tuna are far
reaching, and include:
1) Paving the way to rebuild bigeye tuna stocks
without eroding the value of the skipjack
fishery;2) R e l e a s i n g s o m e S t a t e s f r o m t h edisproportionate conservation burden they
are currently shouldering to reduce thefishing mortality of bigeye associated with
FADs;
3) Increasing yield-per-recruit for bigeye tunaby making a greater proportion of the stock

available for capture at a larger size by thelongline fishery;4) Improving consumer acceptance of FADcaught tuna products; and
5) Enabling managers to focus on reducing nontuna catch associated with FADs, e.g. sharks.
New fishing technology alone will notnecessarily solve all the problems, however. If
practical methods can be developed to largely
avoid capturing bigeye tuna during purseseining operations, management measures (e.g.,
vessel catch limits) or market-driven incentives,
will still be required to ensure that fleets use
them.
The research agenda described here should
also assist purse-seine vessels targeting
skipjack tuna around FADs in other oceans to
reduce the catch of bigeye tuna.

A purse-seine vessel (Photo: ISSF - David Itano)iv

the EPO (IATTC, 2015). However, someassumptions made about bigeye tuna in the
EPO assessment model are less conservative
than those made about bigeye in the WCPO
assessment model.

1. The problem
Progressive expansion of purse-seine fishingfor tunas across the Pacific since the early
1990s, coupled with continued longline fishing(Figure 1), has resulted in the spawning
biomass of bigeye tuna in the Western andCentral Pacific Ocean (WCPO) falling below thelimit reference point of 20% of unfishedthspawning biomass (SBF=0) agreed at the 9Regular Session of Western and Central PacificFisheries Commission (WCPFC)1. [See Harley etal. (2014) for the details of the stock
assessment underpinning the limit reference
point for the bigeye stock in the WCPO].
Regular stock assessments of bigeye tuna in
the Eastern Pacific Ocean (EPO) by the InterAmerican Tropical Tuna Commission (IATTC)
suggest that bigeye tuna spawning biomass iscurrently above the level required for
maximum sustainable yield, and that
overfishing of bigeye tuna is not occurring in

Catch (mt)

3,200,000

The problem in the WCPO has arisen becausea significant amount of the purse-seine fishingin recent years has targeted skipjack tunaassociated with drifting fish aggregating
devices (FADs) (Williams and Terawasi, 2015),
and because bigeye tuna are commonly
associated with these FADs (Leroy et al., 2013).
As a result, larger catches of bigeye are beingcaught by purse-seine vessels in the WCPO,
mainly around FADs (Williams and Terawasi,2015).
The need to reduce the fishing mortality ofbigeye tuna in the WCPO was recognised
several years ago (WCPFC, 2008; 2009), when
the spawning biomass was estimated to beabout 30% of SBF=0. A series of conservation

PURSE SEINE

2,800,000

OTHER

2,400,000

POLE-AND-LINE
LONGLINE

2,000,000
1,600,000
1,200,000
800,000
400,000

2014

2012

2010

2008

2006

2004

2002

2000

1998

1996

1994

1992

1990

1988

1986

1984

1982

1980

1978

1976

1974

1972

1970

1968

1966

1964

1962

1960

0

Figure 1. Total catch (mt) of all tuna species combined from the Western and Central Pacific Ocean byfishing method since 1960 (source: Williams and Terawasi, 2015).
1 www.wcpfc.int/system/files/WCPFC9-Summary-Report-final.pdf

1and management measures were subsequently
introduced to reduce the mortality of bigeyetuna. These measures progressively included
prohibiting the use of FADs for two, then three,and now up to four months per year; closing thehigh seas pockets to purse-seine fishing;banning the discarding of small tuna at sea; andplacing observers on all purse-seine vessels.
The measures also established annual bigeye
catch limits in the longline fishery. For the six
countries with the largest catches, there arecountry-specific limits based on historical catchlevels; the sum of the limits declines each year
to 55,687 tonnes in 2017. For members thatcaught less than 2,000 tonnes in 2004, themeasures impose annual country-specific limitsof 2,000 tonnes.
Regrettably, these measures have not had thedesired effect – the total number of sets madeby purse-seine vessels around FADs each year
has not declined and catches of bigeye tuna
have continued to increase (Williams andTerawasi, 2015). In 2013, the 82,151 mt of bigeye
tuna caught by purse-seine nets in the WCPO
was the highest on record, exceeding thelongline catch for the first time. The purse-seine
catch of bigeye tuna in 2014 was also among
the highest on record (Figure 2).

A drifting fish aggregating device (FAD) (Photo: Fadio/IRD-Ifremer/Marc Taquet)

2The effects of FAD fishing on bigeye tuna (andon other non-tuna species) associated with thedevices is now preventing entry of some of the
FAD-caught tuna into more lucrative markets
because an increasing number of retailers are
striving to promote ecologically sustainableproducts and refusing to purchase, or pay
premium prices for, tuna captured around FADs
(Anon., 2015).

(i.e., ~$1.50 per kg), whereas larger bigeye tunacaught by longline are valued at around $9 per
kg (Williams and Terawasi, 2015).
Proposals by some members of the WCPFC tofurther reduce the number of months that FADs
may be used each year to help address the highfishing mortality for bigeye tuna have met with
opposition by smaller states in the central andeastern WCPO, such as Tuvalu and Tokelau.
These states rely heavily on access fees paid byforeign purse-seine fleets for government
revenue. They are concerned that such vessels
will be reluctant to fish in their waters if further
restrictions are placed on the use of FADs. This
concern arises because fleets targeting skipjack
tuna in the eastern part of the WCPO rely on theuse of FADs to operate efficiently (Williams andTerawasi, 2015). The small states are particularly
concerned that they will bear a disproportionate
burden for the conservation of bigeye tuna if
further FAD restrictions are introduced.

Catching bigeye tuna around FADs by purse
seine also falls well short of the aspirations ofPacific Island countries to maximise theeconomic benefits from tuna resources. In
particular, purse-seine fishing does not
maximise yield-per-recruit; the average size ofbigeye tuna caught by purse-seine vessels is ~5
kg (~60 cm), whereas the average size of fishcaught by longline is >40 kg (~130 cm) (Williams
and Terawasi, 2015) (Figure 3). In addition, theaverage price for bigeye tuna landed by purseseine vessels is the same as for skipjack tuna200,000

PURSE SEINE
OTHER
POLE-AND-LINE

Catch (mt)150,000

LONGLINE

100,000

50,000

2014

2012

2010

2008

2006

2004

2002

2000

1998

1996

1994

1992

1990

1988

1986

1984

1982

1980

1978

1976

1974

1972

1970

1968

1966

1964

1962

1960

0

Figure 2. Catch (mt) of bigeye tuna from the Western and Central Pacific Ocean by fishing method since1960 (source: Williams and Terawasi, 2015).

3

This issue was the subject of intense debate at
the 11th Regular Session of the WCPFC in Apia,
Samoa, in December 2014. Unfortunately,
members of WCPFC failed to reach consensus
on a plan of action to end overfishing of bigeye
tuna in the WCPO.
The issue of bigeye tuna capture in purse-seine
fisheries remains a major concern, however,and other ways to reduce the fishing mortality ofbigeye tuna by purse-seine vessels werediscussed at a recent workshop organised by
the United States Western Pacific Regional
Fishery Management Council (WPRFMC 2015).
These measures are summarised briefly below.
Temporal-spatial closures. The IATTC has
reduced the fishing mortality of bigeye tuna in
the EPO by introducing a total closure of thetuna fishery for two months each year (either
from 29 July to 28 September or 18 November
to 18 January). For the WCPO, a relatively long

Purse seine

total closure of the purse-seine fishery would beneeded to reduce the fishing mortality of bigeye
tuna by 50% (WPRFMC, 2015). However, ashorter closure would be required to limit theopportunity cost of a lower skipjack tuna catch.
A shorter closure would need to beaccompanied by additional measures to reducethe catch of bigeye tuna by purse-seine fleets.
Decisions about the scope for spatial closures
to reduce the fishing mortality of bigeye tuna
would need to be made in the context of total
bigeye tuna catch, catch-per-unit-effort (CPUE)
for bigeye tuna, and the proportion of bigeye
tuna per set. Although CPUE and proportion per
set are much greater in the central and eastern
WCPO, the greatest catches of bigeye tuna aremade in the western WCPO because that iswhere most sets are made (Harley et al., 2015).
The reality is that spatial closures within either
part of the region are unlikely to be endorsed by
WCPFC – closures in the west would reduce

Longline~5 kg juveniles>40 kg adults
Figure 3. The striking difference in average size of bigeye tuna caught by purse-seine and longline
vessels. See Williams and Terawasi (2015) for size-frequency distributions of bigeye tuna caught bypurse-seine and longline vessels.

4

skipjack catches significantly, closures in the
EEZs of smaller Pacific Island countries in thecentral and eastern WCPO would cause adisproportionate burden, and distant water
fishing nations would object to closures of highseas areas in the east.
Catch limits for bigeye tuna. Recent analyses byHarley et al. (2015) have shown large
differences in the catch of bigeye tuna among
purse-seine vessels. Establishment of individual
catch limits for purse-seine vessels within the
WCPFC would directly address fishing mortalityof bigeye tuna and create a disincentive that
could change fishers’ behaviour. This measuredepends on development of efficient andreliable systems to estimate the proportion of
bigeye tuna in purse-seine catches, either onboard, during transhipping operations or atcanneries.
Reducing FAD use through pricing. This
management measure is based on the premisethat the cost of a FAD set can be expected toinfluence fishing behaviour – as costs increase
there should be a progressive decline in use of
FADs. The Vessel Day Scheme (VDS) operated
by the Parties to the Nauru Agreement Office(PNAO) provides mechanisms to introduce anadditional daily fee for FAD sets. The fee per
FAD set could then be adjusted to reduce FAD
use to the recommended level. Anotheradvantage of FAD pricing is that the additional
revenue generated could be allocated toaddressing the disproportionate burden
incurred by small states as a result of reduced
FAD use.
An important consideration, however, is that
introduction of FAD pricing by PNAO would
address only some of the bigeye tuna mortality(WPRFMC, 2015). The WCPFC would need toagree to compatible measures for the high seas,
reform of the longline fishery, and develop
measures for the non-PNA EEZs (e.g.,
Philippines and Indonesia). Registering andmonitoring the large number of existing FADs
would also be difficult.

1.1 Other considerations
The International Sustainable SeafoodFoundation (ISSF) together with the World
Wildlife Fund (WWF) and Pew Charitable Trusts,
are advocating mandatory provision of data on
industrial FAD fishing (e.g., design and numbersof FADs deployed, FAD usage patterns) tofacilitate evidence-­‐based decisions for
introducing the types of management measures
outlined above. The call for this information was
also supported by WPRFMC (2015) and PNAO
will require access to information from buoys
attached to all FADs deployed in its waters from2016.
However, because any management measuresfor bigeye tuna in the WCPO need to beapproved by WCPFC, and because WCPFC
operates by consensus, it is likely to take aconsiderable time to achieve the necessary
reductions in mortality of bigeye tuna in the
EEZs of Pacific Island nations and on the highseas based on the options outlined above.
There is also the challenge of harmonising
management measures for bigeye tuna
between the WCPO and EPO. Although bigeye
tuna tagging information indicates that mostbigeye tuna remain predominantly in theequatorial EPO (east of 120oW) and WCPO (westof 180o) there is considerable mixing between

5120oW and 180o across the convention boundary(Schaefer et al., 2015). Until additional researchrequired to define the stock structure of bigeye
tuna throughout the Pacific Ocean with higher
confidence has been completed, WCPFC andIATTC should continue to explore effective
companion stock assessments for bigeye tuna
(McKechnie et al., 2015).

identify ways for vessels to avoid the capture ofjuvenile bigeye and yellowfin tuna during FADsets”.

1.2 A way forward

2. The additional research needed to determine
whether practical methods for reducing thecatch of bigeye tuna around FADs can be
developed; andThe existing impediments to reducing thefishing mortality of bigeye tuna during purseseining operations, and optimising government
revenue from the purse-seine fishery for small
island states, would be removed if practicalmethods could be developed to fish around
FADs that resulted in substantial reductions in
the catch of bigeye tuna (WPRFMC, 2015).
Purse-seine fleets in the EPO havedemonstrated that it is possible to modify fishingprocedures to reduce impacts on other species
associated with skipjack and yellowfin tuna (i.e.,
dolphins). The development of those methods
resolved a controversy that threatened theviability of the fishery (Joseph, 1994). The new
fishing practices and policies were subsequently
adopted by IATTC, and enshrined in aninternational agreement between fishing states(Wright, 2000; Hedley, 2001).
The idea of developing purse-seine methods forfishing around FADs without catching bigeye
tuna is not new – it has been promoted by tuna
RFMOs, ISSF, and other NGOs for some time. It
is also a requirement of a recent WCPFC
Conservation and Management Measure (CMM
2
2014-01) , which states that “the Commission
shall promote and encourage research to
2 www.wcpfc.int/conservation-and-management-measures

This report describes:
1. The previous and existing initiatives toreduce the catch of bigeye tuna arounddrifting FADs;

3. Ways of stimulating and supporting suchresearch.
The emphasis of this report – a proposed
research agenda to reduce fishing mortality ofbigeye tuna during purse-seining operations –does not imply that new fishing technology
alone will solve the problems. If practicalmethods can be developed to largely avoid
capturing bigeye tuna during purse-seining
operations, management measures, such as
vessel catch limits, will still be required toensure that fleets use them. Market forces also
have a role to play in providing additional
incentives to apply the new technology (Anon.,2015; WPRFMC, 2015). If canneries stopped
buying bigeye tuna or payed a greatly reduced
price, purse-seine vessels would be quick todevelop FAD-fishing technology to reduce thepercentage of bigeye in catches and/or fish inareas with lower abundance of bigeye tuna.
Although the focus is on the WCPO, theresearch agenda outlined here should also
assist purse-seine vessels targeting skipjack
tuna around FADs in other oceans to reduce thecatch of bigeye tuna.

6

2. Previous and existing initiatives

2.1.2 Sorting grids

A number of attempts have already been madeto reduce catch of bigeye tuna during purseseine operations targeting skipjack tuna (Itano,
2005; Morgan, 2011; Hall and Roman, 2013;
Re s t r e p o, 2 0 1 4 ) . T h e s e i n i t i a t i v e s a r esummarised below.

The limited trials involving insertion of solid or
flexible panels of larger mesh within purse-seine
nets to facilitate escape of juvenile bigeye tuna(Hall and Roman, 2013) have not proven to beeffective in the EPO (Nelson, 2004) or the
WCPO (Nasegawa et al., 2010). Further researchon sorting grids is not generally considered to
be worthwhile because there is little consistency
in size differences between skipjack and bigeye
tuna species in FAD sets (Hall and Roman, 2013).
In fact, juvenile bigeye tuna are often larger than
the skipjack tuna targeted. Hall and Roman(2013) and WPRFMC (2015) outline some factorsthat may allow bigeye to be separated fromskipjack tuna within purse-seine nets but any
such measures have the disadvantage of
dealing with fish within nets under greater
stress, increasing the probability of mortality
even if the bigeye tuna are assisted to escape3
before brailing occurs.

2.1 Methods with limited potential
2.1.1 Reducing depth of purse-seine nets
Based on observations that bigeye tuna
sometimes occur at greater depths thanskipjack tuna below FADs, limiting the depth ofpurse-seine nets has been proposed as a wayof reducing the bigeye catch (Matsumoto et al.,2006; Leroy et al., 2009). However, this is not apractical solution because bigeye tuna are also
found close to the surface at night and so haveoverlapping depth distributions with skipjack
and yellowfin tuna around FADs prior to dawn
(Schaefer and Fuller, 2005; 2013), when mostsets occur. Thus, bigeye would still bevulnerable to shallower nets (Opnai, 2002;
Itano, 2005; Leroy et al., 2010; Delgado de
Molina et al., 2010). Also, it is probably
impractical to make purse-seine nets shallower
because such modifications may permit skipjack
tuna to escape.

2.1.3 Setting nets at a different time of day
Potential differences in the time of day that thethree main tuna species associate with FADs
were investigated by Forget et al. (2015) using
acoustic tagging. The investigations showedthat there is little scope for reducing bigeye
catch (without adversely affecting the catch of
skipjack tuna) by adjusting the time purse-seine
nets are set around FADs. The times of day thatbigeye tuna are most closely associated with
FADs are very similar to those for skipjack andyellowfin tuna (Figure 4).

3 Brailing is the process of transferring fish in the purse-seine net to the vessel once the large net is alongside the boat.

The brail net used for this purpose usually holds around 5 tonnes of fish. It is used to scoop fish from the purse-seine
net and is hoisted on board with the use of ropes and derrick.

7

Figure 4. Detection of three species of tuna, and other fish species, near FADs in the Western Indian
Ocean over the 24 hour cycle using acoustic tags (after Forget et al. 2015).

2.1.4 Targeting schools that breakaway from FADs

2.2.1 Assessing bigeye tuna around
FADs prior to setting

Targeting mono-specific skipjack schools (orsub-schools) when they move away from drifting
FADs does not appear to be a feasible option
for reducing fishing mortality of bigeye tuna. The
main problems are that schools of skipjack tuna
which separate from drifting FADs are only aportion of the total skipjack present at a FAD,
and those sub-schools are difficult to catch(Schaefer and Fuller, 2013).

Current acoustic technology cannot yet reliably
determine the relative abundance of tuna
species associated with FADs. Most FADs now
have attached echo-sounder buoys that provide
information on the relative abundance of all fish
to the vessel. However, they do not have theability to discriminate among tuna species, orsizes (Delgado de Molina et al., 2005). Nor dothey provide information consistent with thatavailable from echo sounders on vessels (Lopezet al. 2010). Buoys attached to FADs that canremotely report the species, size distributions
and relative abundances of species (Moreno etal., 2005; 2007; Miquel et al., 2006; Morón,
2008, Dagorn et al. 2009), and minimise theeffects of sea conditions and thermocline depth
on interpretation of acoustic data (Durand andDelcroix, 2000; Kessler, 2006), are needed.

2.2 Ongoing initiatives
ISSF (Restrepo et al., 2014) and the FisheryResearch Agency of Japan (Satoh et al., 2012)
are currently pursuing three other lines ofresearch to reduce the catch of bigeye tuna.
These initiatives are summarised below.

8

Provided that the acoustic technology
associated with FADs can be improved
substantially to allow captains and fishingmasters on purse-seine vessels to remotely
determine the relative abundance of bigeye andskipjack tuna around FADs, it would be possible
to significantly reduce the catch of bigeye tuna.
Such echo-sounder records from aggregations
associated with FADs could also potentially be
used to help assess the status of tuna stocks(WPRFMC, 2015).

2.2.2 Investigating whether FAD depth
affects bigeye catches
This research is based on the observation thatbigeye tuna sometimes aggregate below FADs
at a greater depth than skipjack tuna
(Matsumoto et al., 2006). ISSF and IATTC
currently have a field experiment underway toevaluate the tuna species composition from setson shallow and normal depth drifting FADs in
the equatorial EPO. (See Section 3.3.1 for moredetails about the potential for this observation to
help separate skipjack and bigeye tuna prior tosetting a purse-seine net.)

2.2.3 Assessing the potential of otherstimuli to segregate tuna species
This research consists of improving knowledge
on the schooling behaviour of bigeye, yellowfin
and skipjack tuna, the sensory abilities of eachspecies, and their responses to different stimuli.
The results of the second and third types ofresearch above could potentially be used during
fishing operations to separate bigeye tuna fromskipjack tuna using the ‘double FAD’ technology

Netting suspended below a raft to attract tuna
(Photo: ISSF-David Itano)9

Figure 5. Double FADs – using competition between different stimuli to segregate tuna species as thetwo FADs are separated (after ISSF).
already under consideration by ISSF for
separating bycatch species (e.g., sharks) fromtarget species during purse-seining operations.
Double FADs can be split apart before thepurse-seine net is set, making it theoreticallypossible to set mainly on skipjack and yellowfin
tuna if most bigeye tuna remain associated with
only one of the FADs (Figure 5).
In theory, a purse-seine fishing method thatlargely avoided capturing bigeye tuna could be
developed by identifying a stimulus that
repelled bigeye from a single FAD but did notaffect skipjack or yellowfin tuna. However, thismay be difficult considering that the stimulus

would have to be strong enough to overcome
the attracting power of the FAD, and that any
stimulus repelling bigeye tuna (presumably
related to predation threat) would alsopresumably deter the other species of tuna as
well.
The advantage of the double FAD concept isthat, having attracted tuna to the floating object,
it allows research to focus on second-orderstimuli that may attract a given tuna species to
one of the two FADs during separation, while
the other species remain associated with theother FAD. In practice, the probability of
attracting bigeye tuna alone to one of the FADs

10

during separation may be low because bigeye
and yellowfin tuna are closely related (both arein the genus Thunnus) and may have similar
responses to a given stimulus.
To reduce the capture of bigeye tuna using
double FAD technology, tuna could be attracted
in any one of four possible combinations. These
combinations, the advantages anddisadvantages of each combination, andsuggested priorities for research are given in
Table 1.

The disadvantages of combinations 2 and 3 inTable 1, which preclude yellowfin tuna frompurse-seine catches, are relatively minor
because skippers often prefer to catch yellowfin
tuna in free schools, which are generallycomprised of larger fish that fetch a higher pricethan FAD-caught yellowfin (Williams andTerawasi, 2015).

Table 1. The four combinations of tuna attraction that would allow bigeye tuna to be isolated using
double FAD technology, together with the advantages and disadvantages of each combination andsuggested priorities for research; BET = bigeye tuna, YFT = yellowfin tuna; SKJ = skipjack tuna.
Tuna species to
be attracted
BET YFT SJK

Ideal, if possible, because BET can be concentrated around one of
the two FADs and led away, enabling vessels to set on the remaining
FAD with SKJ and YFT. But might be difficult to achieve because YFT
could have a similar response to BET to a given stimulus due to theirclose genetic relationship.

z

X

X

Comments

Reasonable probability of occurring (see above). Disadvantage is that
YFT will also be led away and lost from the catch when the net is set
around the FAD with SJK only.

X

X

Might be most practical because SKJ is in a different genus to BET
and YFT and is the main target of the purse-seine fishery.
Disadvantage is that YFT would be lost from the purse-seine catch.

X

Unlikely to occur because YFT and SKJ are in different genera but
ideal because both SKJ and YFT would be caught.

Researchpriority

1

2

3

4

11

2.3 Other potential methods
Bait could be used to attempt to attract bigeye
tuna into deeper water prior to setting thepurse-seine net. This method does not dependon double FAD technology and is based on
observations by master fishermen at the
Secretariat of the Pacific Community (SPC) thatbigeye and yellowfin tuna (and sharks) are

attracted to dead bait, whereas skipjack tuna
are not. This method would involve deploying askiff from a purse-seine vessel to manipulate abag of chum bait near a FAD before the net isset, slowly lowering it to a depth of 300 m to
attract bigeye and yellowfin tuna and sharks,
and then moving the bag several hundred
metres away from the FAD before the net is set
around the skipjack tuna remaining there.

Yellowfin tuna (Photo: Fadio/IRD-Ifremer/Marc Taquet)123. Additional research anddevelopment required
The key challenges involved in evaluating anddeveloping methods for purse-seine fishingaround FADs to avoid the capture of bigeye
tuna involve completing and integrating the
research initiated by ISSF and partners. Up tofour areas of research may warrant
investigation:
1. Developing ‘smart’ FADs with a newgeneration of acoustics capable ofdetermining the relative abundances of tuna
species associated with FADs;
2. Identifying cue(s) that attract or repel thedifferent species of tuna to segregate bigeye
tuna using double FAD technology;
3. Developing methods that can be used to split
double FADs apart remotely and to produce
the most effective stimulus for attracting
particular tuna species to one of the two
FADs; and4. Determining whether baiting is a practical wayof leading bigeye tuna and sharks away from
FADs.
These tasks need to be tackled sequentially. If
the development of acoustic methods that allow
purse-seine skippers to identify FADs with few
bigeye work successfully, and good catches ofskipjack tuna (the target of the fishery) can still
be made on FADs with few bigeye tuna, there
will be no need to proceed further. However, if
only a small proportion of FADs have low
bigeye:skipjack biomass ratios, or if large
schools of skipjack tuna are usually also
associated with relatively high numbers ofbigeye tuna, there will be a need to embark on
the other areas of research listed above.

Details of the factors to be considered inaddressing each of these lines of research are
outlined below.

3.1 Determining relative abundancesof tuna associated with FADs
The potential to use acoustics to help bycatch
reduction is widely recognised (Simmonds andMacLennan, 2005; Lopez et al., 2010,2014).
Development of effective acoustic
systems to enable fishing vessels to determine
the number of bigeye around a FAD will dependon:• Demonstrating that there is variability in thepercentage of bigeye tuna in catches madearound FADs (Sancristobal et al., 2014); and• Progressing existing acoustic technology,
which can already discriminate among tuna
species based on known differences inacoustic target strength related to fish length,presence/absence of a swim bladder, andswim bladder volume (Arnaud and Josse,
2000; Schaefer and Fuller 2008; Imaizumi etal., 2012), to the point where it can be used toestimate the relative abundances of different
tuna species in large aggregations associatedwith FADs.

Ultimately, the second requirement will involve
verification of the pre-set estimates fromacoustic information with the species and size
composition data from individual sets around
FADs, as done in the equatorial EPO by Fuller
and Schaefer (2014).
Given the potential difficulties in discriminating
between individual bigeye tuna and yellowfin
tuna that have swim bladders of the same size,
even though the individuals are different

13lengths, the focus for new acoustic technology
could be on distinguishing skipjack tuna frombigeye and yellowfin tuna. Although this may
ultimately result in reduced catches of yellowfin
tuna around FADs, any effects on the economics
of purse-seine fishing are likely to be minor
because, as mentioned above, there areadvantages in catching yellowfin in free schools.
Experienced purse-seine skippers are adept atinterpreting echo-sounder data to identify tuna
species based on presence/absence of swim
bladders, but interpretation is complicated in
mixed species aggregations. Hall and Roman(2013) have pointed out that development ofbetter acoustic technology would benefit frommulti-frequency systems and software todiscriminate species composition and sizes.
They suggest that creating a library of images
with data on the captures obtained in each case,would assist progressive improvements in theinterpretation of acoustic data.
Other alternatives for identifying tuna species
associated with FADs which have been
considered are:• multi-beam sonar (Okamoto et al., 2010); and• underwater video systems (Itano et al., 2009).
Any alternatives used in practice will need tobalance:• effectiveness with cost of hardware,
maintenance, and processing; and• effectiveness and attractiveness of automatedsystems as opposed to systems using a
degree of human integration.

Combined acoustic and optical systems havealready been developed for fish stock
assessment purposes (Macaulay et al., 2012;
O’Driscoll et al., 2012) and are now being
4
deployed by industry in some fisheries . Use ofsuch approaches elsewhere holds promise fortuna applications.

3.2 Identifying cues (stimuli) that attract
or repel different tuna species
3.2.1 Potential stimuli for attracting tuna
Marine fish are known to respond to features of
their environment, or behaviour of theirconspecifics. Some of these features can be
reproduced and magnified to potentially attract
or repel particular species of tuna or stimulate
behaviours that selectively separate or
aggregate different species of tuna. The various
stimuli, and evidence for the potential of thesesimulated features to affect the behaviour oftuna, are summarised below.
Sound and vibrations. Sound and vibrations arewell known to influence fish behaviour (Popper
and Carlson, 1998; Gabriel et al., 2005; Yan etal., 2010). The inner ear in fish has evolved todetect acoustic signals and the lateral line todetect vibrations. The larvae of a broad range oftropical marine fish species use sound as a cue
for orientating towards coral reefs prior tosettling from the plankton (Montgomery et al.,2006). And sound has been used to condition
cultured marine fish released into the wild togather at feeding stations (Lindell et al., 2012),
where they can be recaptured.

4 www.sealord.com/docs/default-source/News/state-of-the-art-technology-to-boost-fisheries-research-media-

release-.pdf?sfvrsn=014

Tuna and non-tuna species in the purse-seine
fishery are also known to respond to sound.
Iverson (1967) trained captive yellowfin tuna toswim between two nets when a sound wasmade and to swim straight in the absence ofsound. Yellowfin tuna have also been recorded
making their own sounds (Allen and Demer,
2003). Silky sharks are attracted to sound andthe attraction increases as the frequency
spectrum decreases from 500-1000 Hz to 25-50
HZ, and pulsation increases from 1 to 20 pulses
per second (Myrberg et al., 1972).
In addition, there are many anecdotal reports
from fishermen that certain fishing boat designs
are better for trolling for tuna than others, withthe best results obtained from slower revving

diesel motors that emit low frequency sound.
Others report better catches in the vicinity oflarge cargo vessels as they pass by, apparently
due to the sounds from the vessel.
There is also considerable evidence that FADs
attract tuna and other species of large pelagic
fish partly as a result of the sounds made bysurface rafts and the vibrations made by anchor
ropes (Yan et al., 2010).
The challenge is to find sounds and vibrations
that are distinctly attractive to bigeye, or to
skipjack tuna. Experimentation in the open
ocean is not straightforward and practical rather
than definitive approaches will be necessary.

Brail used to transfer tuna from the purse-seine net for storage onboard (Photo: ISSF-David Itano)15

Light. Artificial light has been used to catch awide variety of fish, both as adults (Ben-Yami,
1976) and juveniles (Doherty, 1987). Light alreadyplays a role in tuna fisheries, where it is used tocatch the baitfish for pole-and-line operations,and where chemical light sticks are used tomake the dead baits on longlines more
attractive to tuna and swordfish by mimicking
the bioluminescence of prey organisms (Sokimi
and Beverly, 2010).
There is some evidence for phototaxis(movement of an organism toward or away froma source of light) in tuna. Preliminary
experiments have shown that bigeye tuna movehorizontally away from a flashing light, albeit
with varying responses that appear to be related
to the distance of the fish from the light and thetime of day (Hasegawa et al., 2010). Other
species of fish are also repelled by strobe
lighting (Patrick et al., 1985). Thus, because mostpurse-seining operations around FADs are done
before dawn (ISSF, 2014), there may be scopefor using artificial light to repel bigeye tuna from
FADs while retaining skipjack tuna. Experimentswith other species of fish in the Mediterranean(Machesan et al., 2005) have shown that light ofdifferent intensities and wavelengths affected
phototaxis in most species examined.
At least one other large pelagic fish species,
yellowtail amberjack (Seriola lalandi), is known
to be attracted to some forms of light.
Commercial fishermen in NSW, Australia, haveused mirrors to ‘bait’ pelagic fish traps for thisspecies. The traps proved to be so efficient thatthey were banned (Stewart et al., 2001).

Depth of FAD material. There is some evidence
from the EPO suggesting that the occurrence ofbigeye tuna around drifting FADs is related to
the length (depth) of material below the raft
(Lennert-Cody et al., 2007). However, Sato et al.
(2008) and Moon et al. (2008) found no strong
evidence for such a trend in the WCPO but
acknowledge that several other factors may
have influenced their findings. Although there
appears to be little scope for using theoccasional deeper occurrence of bigeye tunanear FADs to exclude them from catches byreducing the depth of purse-seine nets (Section
2.1.1), the fact that bigeye tuna can be found in
deeper water than skipjack tuna near FADs
(Josse and Bertrand, 2000; Schaefer and Fuller,
2005) merits equipping one of the FADs in a
double-FAD setup with deeper netting todetermine whether:1) Bigeye tuna are consistently attracted to thedeeper FAD; and
2) Slow separation of the two FADs (perhaps in
combination with surface sprays/bubble
curtains – see below) leads most bigeye
away from skipjack and yellowfin tuna.
For the reasons explained in Section 2.1.1,
separation of the two FADs may need to bedone during daylight hours, not prior to dawn.
Shading, spray and bubble curtains. Althoughshading from light can attract or repel somespecies of fish (Cocheret de la Moriniere et al.,
2004; Verweij et al., 2006), there is little or no
scope for using shading to separate species oftuna around FADs because most sets are madeprior to dawn.

16© Comstock Images

Spraying water on the surface is known toattract skipjack, yellowfin and bigeye tuna andhas been an essential part of the pole-and-line
fishing method. In conjunction with the releaseof live bait, spraying gives the illusion that thewater surface is alive with small fish and helps
5
keep tuna beside the fishing vessel .
Experimentation with water sprays around oneof the two FADs during separation of double
FADs would be worthwhile to see if bigeye tunaor skipjack tuna responded more strongly tothat stimulus.
There are mixed reports about the effects of air
bubbles on fish. Some tuna are apparently
attracted to air bubbles, although it is not clear
whether it is the bubbles themselves that attract
the fish, or the acoustics associated with thenoise produced by air bubbles (Gabriel et al.,2005). Other studies report that some speciesof fish are repelled by air bubbles (Patrick et al.,1985).
It is also worth noting that trolling lures for tunaand other large gamefish are often designed tocreate trails of air bubbles to attract these fish.
Overall, there is evidence to suggest that both
spraying and air bubbles may influence thebehaviour of tuna. Therefore, experiments
rigging FADs with solar-powered equipment toproduce surface spray and underwater curtainsof air bubbles could be considered to see ifthese stimuli have differential effects on bigeyeand skipjack tuna.

5 FAO Fishing methods (www.fao.org/fishery/fishtech/30/en)

3.2.2 Applying the stimulus to FADs
Assuming that the above lines ofexperimentation are successful in developing astimulus that either attracts of repels bigeyetuna, or skipjack tuna, practical methods willthen need to be developed to place theequipment required to produce this speciesspecific stimulus by remote control on one of
the two FADs within a double FAD.

3.2.3 Verifying the species compositionof tuna around the target FAD
Prior to setting the purse-seine net around theselected FAD, it would be advantageous (butnot always essential) to verify that the fish
associated with that FAD are mainly skipjack
tuna, and that very few if any bigeye tuna arepresent. The technology described under 3.1
could be used to do this where bigeye tuna are
relatively abundant, e.g. in the central andeastern WCPO.

3.3 Developing methods for
splitting double FADs
Once a technique has been developed for
producing the species-specific stimulus for
attracting/repelling bigeye tuna, or skipjack
tuna, a practical method will be needed forseparating the two parts of the double FAD.

17

3.4 Leading bigeye tuna awayfrom FADs using bait
The key experiments here will involve
identifying which baits, bait presentation
methods, and bait presentation times, are most
attractive to bigeye tuna. The experiments
should involve testing whether low-value nontuna species (e.g., rainbow runner) are effective
baits because this would help reduce costs.

4. Possible approaches tosupport further research
Given the investments already made by ISSF
and other agencies, it is logical to build on theinitiatives already underway to develop FAD
fishing methods to reduce mortality of bigeyetuna. Investments should also be made toharness the experience and expertise of purseseine captains and fishing masters. After all,many of the most effective methods for
reducing unwanted catch, e.g., dolphin-freepurse-seine fishing in the EPO, have beendeveloped by fishermen. [See examples inreviews by Glass (2000), Kennelly (2007),
Jordan et al. (2013) and papers collected by the6Consortium for Wildlife Bycatch Reduction ].
Priority investments are:1. Support for gear technologists and scientists
to do the research in the four areas describedin Section 3, including the costs involved inchartering purse-seine vessels to conduct
experiments; and2. I n c e n t i v e s f o r v e s s e l o w n e r s , a n dmanufacturers of acoustic equipment, todevelop FAD fishing methods that greatly
reduce the mortality of bigeye tuna.

Tuna in a purse-seine net (Photo: ISSF-Jeff Muir)
6 www.bycatch.org/publications

18

Pressure from tuna retailers, among othergroups, is expected to provide increasingly
strong incentives to develop FAD fishingmethods that take few if any bigeye tuna.
However, one way of providing an appropriate
incentive quickly, that has proved to be effective
in helping to find technical solutions to otherdifficult problems, is organising a competition
for a substantial prize. This type of incentive
leads to the formation of creative researchteams and usually leverages other sources of
support to help solve the problem. A pertinent
example is the ANSARI X Prize7.

5. Conclusions
The prospect of developing FAD fishingmethods that capture relatively few bigeye tunais real – continued investment in acoustics
research is expected to improve the ability ofvessels to identify the species composition,
relative abundance and size of tuna associatedwith FADs, opening the way for more selective
fishing.
In the event that use of more sophisticatedacoustics demonstrates that it is not as
profitable to fish for skipjack tuna exclusively
around FADs with few bigeye, there are anumber of stimuli known to attract or repel fish,
including tuna, which could be manipulated toseparate bigeye from skipjack tuna during FAD
fishing operations. The double FAD technology
already under investigation by ISSF warrants
further research. Although somewhat
speculative, luring bigeye tuna away from FADs
with dead bait also merits research because, if
successful, it would negate the need for using
double FADs.
7 www.ansari.xprize.org/

The potential benefits of investments indeveloping purse-seine fishing methods that
reduce the catch of bigeye tuna are far
reaching, and include:
1. Paving the way to restore bigeye tuna stocks
without eroding the value of the skipjack
fishery;
2. Releasing small island states from thedisproportionate conservation burden they
are currently shouldering to reduce thefishing mortality of bigeye tuna associatedwith FADs;
3. Increasing yield-per-recruit for bigeye tuna bymaking a greater proportion of the stock
available for capture at a larger size by thelongline fishery; adding to the volume andvalue of the catch and creating opportunities
to charge higher fishery access fees forlongline vessels;
4. Improving consumer acceptance of FADcaught tuna products; and5. Enabling managers to focus on reducing
other bycatch associated with FADs, e.g.,
sharks.

Acknowledgements
The authors thank John Hampton, Shelton
Harley, David Itano, Simon Nicol, Victor
Restrepo, Kurt Schaefer, Peter Sharples, William
Sokimi and Sebastian Troeng for usefulcomments on the draft text and/or helpful
discussions about the proposed areas ofresearch. We also thank Nicole Han for thegraphic design and layout of this report.

19

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Back Cover Photo
Purse-seine fishing for skipjack tuna (Photo: SPC)

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