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Technical mitigation to reduce marine mammal bycatch and entanglement in commercial fishing gear: lessons learnt and future directions

June 2019
Reviews in Fish Biology and Fisheries 29(3)

DOI:10.1007/s11160-019-09550-6

Authors:

Sheryl Hamilton

University of Tasmania

G. Barry Baker

University of Tasmania

Fisheries bycatch is one of the biggest threats to marine mammal populations. A literature review was undertaken to provide a comprehensive assessment and synopsis of gear modifications and technical devices to reduce marine mammal bycatch in commercial trawl, purse seine, longline, gillnet and pot/trap fisheries. Successfully implemented mitigation measures include acoustic deterrent devices (pingers) which reduced the bycatch of some small cetacean species in gillnets, appropriately designed exclusion devices which reduced pinniped bycatch in some trawl fisheries, and various pot/trap guard designs that reduced marine mammal entrapment. However, substantial development and research of mitigation options is required to address the bycatch of a range of species in many fisheries. No reliably effective technical solutions to reduce small cetacean bycatch in trawl nets are available, although loud pingers have shown potential. There are currently no technical options that effectively reduce marine mammal interactions in longline fisheries, although development of catch and hook protection devices is promising. Solutions are also needed for species, particularly pinnipeds and small cetaceans, that are not deterred by pingers and continue to be caught in static gillnets. Large whale entanglements in static gear, particularly buoy lines for pots/traps, needs urgent attention although there is encouraging research on rope-less pot/trap systems and identification of rope colours that are more detectable to whale species. Future mitigation development and deployment requires rigorous scientific testing to determine if significant bycatch reduction has been achieved, as well as consideration of potentially conflicting mitigation outcomes if multiple species are impacted by a fishery.

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REVIEWS

Technical mitigation to reduce marine mammal bycatch

and entanglement in commercial ﬁshing gear: lessons learnt

and future directions

Sheryl Hamilton .G. Barry Baker

Received: 17 September 2018 / Accepted: 12 January 2019

ÓSpringer Nature Switzerland AG 2019

Abstract Fisheries bycatch is one of the biggest threats

to marine mammal populations. A literature review was

undertaken to provide a comprehensive assessment and

synopsis of gear modiﬁcations and technical devices to

reduce marine mammal bycatch in commercial trawl,

purse seine, longline, gillnet and pot/trap ﬁsheries.

Successfully implemented mitigation measures include

acoustic deterrent devices (pingers) which reduced the

bycatch of some small cetacean species in gillnets,

appropriately designed exclusion devices which reduced

pinniped bycatch in some trawl ﬁsheries, and various

pot/trap guard designs that reduced marine mammal

entrapment. However, substantial development and

research of mitigation options is required to address the

bycatch of a range of species in many ﬁsheries. No

reliably effective technical solutions to reduce small

cetacean bycatch in trawl nets are available, although

loud pingers have shown potential. There are currently no

technical options that effectively reduce marine mammal

interactions in longline ﬁsheries, although development

of catch and hook protection devices is promising.

Solutions are also needed for species, particularly

pinnipeds and small cetaceans, that are not deterred by

pingers and continue to be caught in static gillnets. Large

whale entanglements in static gear, particularly buoy

lines for pots/traps, needs urgent attention although there

is encouraging research on rope-less pot/trap systems and

identiﬁcation of rope colours that are more detectable to

whale species. Future mitigation development and

deployment requires rigorous scientiﬁc testing to deter-

mine if signiﬁcant bycatch reduction has been achieved,

as well as consideration of potentially conﬂicting miti-

gation outcomes if multiple species are impacted by a

ﬁshery.

Keywords By-catch Cetacean Gillnet Longline 

Pinniped Trawl

Introduction

Marine mammals are incidentally killed in a range of

ﬁsheries throughout the world (Lewison et al. 2014;

Read et al. 2006). This bycatch in active ﬁshing gear is

Electronic supplementary material The online version of

this article (https://doi.org/10.1007/s11160-019-09550-6) con-

tains supplementary material, which is available to authorized

users.

S. Hamilton (&)G. B. Baker

Institute for Marine and Antarctic Studies, University of

Tasmania, Hobart, TAS 7001, Australia

e-mail: sheryl.hamilton@latitude42.com.au

S. Hamilton

Centre for Marine Socioecology, University of Tasmania,

Hobart, TAS 7001, Australia

S. Hamilton G. B. Baker

Latitude 42 Environmental Consultants Pty Ltd., 114

Watsons Rd, Kettering, TAS 7155, Australia

123

Rev Fish Biol Fisheries

https://doi.org/10.1007/s11160-019-09550-6(0123456789().,-volV)(0123456789().,-volV)

one of the biggest threats to marine mammal popula-

tions, particularly cetaceans (whales, dolphins and

porpoises) and pinnipeds (e.g. seals and sea lions)

(Jaiteh et al. 2013;Read2008; Reeves et al. 2013). As

these species are long-lived with high adult survival

and low breeding productivity, populations are often

slow to recover from declines, even under conducive

environmental conditions. Therefore, anthropogenic

activities that increase mortality levels, such as

ﬁsheries bycatch, can have signiﬁcant, long-term

population impacts (Gilman 2011; Lewison et al.

2004; Reeves et al. 2003).

Cetaceans and pinnipeds interact with ﬁsheries as

they may: (1) feed on the same target species or

associated non-target species of a ﬁshery, (2) be

attracted to ﬁshing operation discards, and/or (3)

passively encounter ﬁshing gear in the water column

(Fertl and Leatherwood 1997; Hamer et al. 2012).

These interactions may result in the bycatch of

individuals caught in active ﬁshing components (e.g.

nets, hooks, traps), or entangled in supporting gear and

lines. Bycatch in trawl, purse seine, longline, gillnet

and pot/trap ﬁsheries has been identiﬁed as a major

threat to many species (Hall 1998; Hamer et al. 2012;

Hamer and Goldsworthy 2006; Hamer et al. 2008;

Knowlton et al. 2012; Reeves et al. 2013; Werner et al.

2015). Other gear types, such as those used in troll and

squid jigging ﬁsheries, are considered to be more

selective in targeting species and, therefore, have less

bycatch risk (Wakeﬁeld et al. 2017).

Over the past decade, there has been heightened

awareness and attention on the development of

solutions to reduce ﬁsheries bycatch. For example,

the Food and Agriculture Organization of the United

Nations (FAO), as part of an ongoing commitment to

bycatch management work, convened a workshop to

consider means to reduce marine mammal mortality in

ﬁsheries and aquaculture operations (FAO 2018).

Also, a number of bycatch mitigation reviews have

focussed on particular aspects of mitigation or gear

type, or on certain species or species groups (Dawson

et al. 2013; Geijer and Read 2013; Hamer et al. 2012;

How et al. 2015; Laverick et al. 2017; Leaper and

Calderan 2018; Werner et al. 2006,2015). However,

there is no readily accessible synthesis of best practice

mitigation methods for marine mammals and, further-

more, the high level of bycatch that continues to occur

in ﬁsheries around the world (Gray and Kennelly

2018; Reeves et al. 2013) necessitates an update and

expansion from previously published assessments.

This paper presents the ﬁrst comprehensive global

review of technical mitigation measures designed to

reduce marine mammal bycatch in commercial ﬁshing

gear, including assessments of mitigation testing,

effectiveness and, where relevant, operational deploy-

ment, and a synthesis of best practice mitigation and

areas requiring greater attention.

Methods and scope

Although there has been considerable progress in

some ﬁsheries regarding the development, testing and

implementation of mitigation measures to reduce

marine mammal bycatch in commercial ﬁshing gear,

much of this information is not easily accessible. A

literature review was undertaken using a range of

sources including peer-reviewed journals, unpub-

lished reports, magazine articles, conference papers,

websites, and information from government and non-

government organisations. An electronic literature

search was conducted up to and including August

2018 using Web of Science and Google Scholar.

Search terms were bycatch,by-catch and/or mitigat*

combined with: ﬁsher*, trawl,purse seine,longline,

gillnet,pot,trap,line,cetacean,whale,dolphin,

porpoise,pinniped,seal,sea lion in any ﬁeld. Refer-

ences from other published papers and the authors’

personal bibliographic resources were used to identify

relevant papers. Key researchers were contacted via

email or ResearchGate (https://www.researchgate.net/)

to access relevant non-published reports.

Studies on the development and implementation of

technical mitigation measures (i.e. gear modiﬁcations

and mitigation devices) for marine mammal bycatch in

commercial trawl, purse seine, longline, gillnet and

pot/trap ﬁshing gear were reviewed. Fisheries not

considered to be high risk to marine mammal species,

such as trolling and jigging (Arnould et al. 2003), and

mitigation of mortalities from lost, discarded or

abandoned gear (i.e. ghost ﬁshing) were not included.

Reviewed studies predominantly addressed cetacean

and/or pinniped bycatch as most mitigation research

has focussed on these taxa.

Technical measures are presented on a ﬁshing gear

basis (trawl, purse seine, longline, gillnet and pot/trap)

with the exception of pingers and a range of weakened

gear, which are applicable to different ﬁshing gears

123

Rev Fish Biol Fisheries

and are therefore more effectively dealt with in a

collated section. For each measure, the scientiﬁc

evidence for mitigation effectiveness, caveats or

uncertainties in the methods or results, research

requirements and, where possible, recommendations

for effective operational implementation were

identiﬁed.

Although outside the scope of this review, it was

apparent that effective bycatch mitigation strategies

often comprise a suite of management measures in

conjunction with technical mitigation. These include

traditional input and output controls, operational

adjustments through ‘codes of practice’ protocols

(e.g. ‘move-on’ provisions, handling and release

protocols) and implementation of appropriately des-

ignated spatial and/or temporal closures (Hamer and

Goldsworthy 2006; Hamer et al. 2008,2011; Read

2013; Reyes et al. 2012; Rojas-Bracho and Reeves

2013; Slooten 2013; Tixier et al. 2014; Werner et al.

2015). Instigation of multi-jurisdictional agreements,

regulations and/or legislation to facilitate mitigation

implementation are also likely to be important (Geijer

and Read 2013; Leaper and Calderan 2018).

Results of reviewed technical mitigation measures

A synopsis of the technical mitigation assessment is

provided below, with details on mitigation and ﬁshery-

speciﬁc studies provided in Supplementary Material,

Tables S1–S5. A summary of the assessment and

effectiveness of each technical measure identiﬁed is

provided in Table 1. Where appropriate, a subjective

evaluation of the economic viability, practicality,

impact on target catch and the ease of compliance

monitoring for each technical measure is provided in

Table 2. However, although this provides a general

overview, due to ﬁshery-speciﬁc characteristics (e.g.

size of target species, operational elements), the

evaluation responses are not deﬁnitive, and results

may differ across ﬁsheries. For example, a range of

ﬁshery-speciﬁc factors would affect the economic

feasibility of mitigation implementation such as

operational speciﬁcations, target species value and

how much the mitigation reduces target species

damage or depredation by bycatch species.

Mitigation relevant to multiple types of ﬁshing

gear

Pingers (Acoustic deterrent devices)

Pingers, small electronic devices with relatively low

acoustic outputs (\160 dB), were developed to

reduce high levels of small cetacean bycatch in

gillnets (Dawson et al. 2013; Kraus et al. 1997;

Reeves et al. 2013). Pingers also include louder

devices ([132 dB) to deter marine mammals from

trawl nets or to reduce pinniped or odontocete

interactions and depredation around aquaculture,

longline or pot/trap operations (Dawson et al. 2013;

Hamer et al. 2012; Mackay and Knuckey 2013). The

effectiveness of pingers in reducing bycatch differs

between trawl, longline, gillnet and pot/trap gear

(Table 1), and between species and ﬁsheries. Further-

more, the economic viability of deploying pingers

varies between gear types. It is likely to be more

economically viable to deploy pingers on gear

contained within a relatively small range (e.g. gillnets,

trawls, pot/trap lines) than using pingers to deter

marine mammals from longlines, which can extend

over tens of kilometres (Table 2).

For trawl ﬁsheries (Table S1), while there are likely

to be inter- and intra-speciﬁc differences in responses

to pingers with different signals, the effectiveness of

pingers in reducing cetacean bycatch is unclear.

Correctly deployed, loud pingers (e.g. Dolphin Dis-

suasive Devices

,‘DDD’) may reduce common

dolphin (Delphinus delphis) bycatch in seabass (Di-

centrarchus labrax) pair trawl ﬁsheries (Northridge

et al. 2011), although decreases in reported bycatch

may be partly due to reduced ﬁshing effort (de Boer

et al. 2012) and results from other trials (with different

pinger models) were inconclusive (Morizur et al.

2008). Furthermore, controlled experiments in the

absence of the loud operational conditions of trawls

indicated pingers may not provide a consistently

effective deterrent for common dolphins (Berrow et al.

2009). Pingers may also have less effect on foraging

compared to travelling groups of cetaceans (van

Marlen 2007). Neither DDDs nor quieter pingers were

effective in reducing bottlenose dolphin (Tursiops

truncatus) interactions in Australia’s Pilbara demersal

ﬁsh trawl ﬁshery (Santana-Garcon et al. 2018;

Stephenson and Wells 2006). While one study

suggested pingers may increase rates of bottlenose

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Rev Fish Biol Fisheries

Table 1 Summary of whether a technical measure developed to reduce pinniped and/or cetacean bycatch in commercial trawl, purse seine, longline, gillnet and pot/trap

operations has been assessed (A) and if there is evidence that it is effective (E) in reducing bycatch

Technical measure Trawl Purse seine Longline

Pinniped Cetacean Pinniped Cetacean Pinniped Cetacean

AEAEAEAEAEAE

Acoustic deterrent devices (pingers) No ? Yes ? – – – – No ? Yes No

Acoustic scarers; e.g. alarm or predator calls, explosions – – – – ? No Yes No No ? Yes No

Acoustically reﬂective nets – – – – – – – – – – – –

Auto-trawl systems No ? No ? – – – – – – – –

Back-down manoeuvre with Medina panels – – – – No ? Yes Yes – – – –

‘‘Buoyless’’ nets – – – – – – – – – – – –

Catch protection devices—demersal longline – – – – – – – – No ? Yes ?

Catch protection devices (triggered)—pelagic longline – – – – – – – – No ? Yes ?

‘‘Dolphin gate’’ with additional weights – – – – No ? Yes ? – – – –

Exclusion device: hard grid and top-opening escape Yes Yes Yes No – – – – – – – –

Exclusion device: soft/ﬂexible grid and top-opening escape Yes ? Yes No – – – – – – – –

Exclusion device: hard grid and bottom-opening escape Yes ? Yes ? – – – – – – – –

Exclusion device: soft/ﬂexible grid and bottom-opening escape No ? Yes No – – – – – – – –

Mesh enlargement – – – – No No ? No – – – –

Net binding No ? No ? – – – – – – – –

Net colour No ? No ? – – – – – – – –

Passive acoustic deterrents – – – – – – – – No ? Yes ?

Pot/trap excluder devices – – – – – – – – – – – –

Reduced strength rope – – – – – – – – – – – –

Reduced strength nets – – – – – – – – – – – –

Rope colour changes – – – – – – – – – – – –

Rope or mesh barriers No ? Yes ? – – – – – – – –

Rope-less pot/trap systems – – – – – – – – – – – –

‘‘Seal socks’’ – – – – – – – – – – – –

Sinking groundlines – – – – – – – – – – – –

Stiff ropes – – – – – – – – – – – –

Visually detectable nets – – – – – – – – – – – –

Weak hooks – – – – – – – – No ? Yes ?

Weak links – – – – – – – – – – – –

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Rev Fish Biol Fisheries

Table 1 continued

Technical measure Gillnet Pot/trap

Pinniped Cetacean Pinniped Cetacean

A EAEAEAE

Acoustic deterrent devices (pingers) No ? Yes Yes No ? Yes ?

Acoustic scarers; e.g. alarm or predator calls, explosions – – ––––––

Acoustically reﬂective nets No ? Yes No – – – –

Auto-trawl systems – – ––––––

Back-down manoeuvre with Medina panels – – ––––––

‘‘Buoyless’’ nets No ? No ? – – – –

Catch protection devices—demersal longline – – ––––––

Catch protection devices (triggered)—pelagic longline – – ––––––

‘‘Dolphin gate’’ with additional weights – – ––––––

Exclusion device: hard grid and top-opening escape – – ––––––

Exclusion device: soft/ﬂexible grid and top-opening escape – – ––––––

Exclusion device: hard grid and bottom-opening escape – – ––––––

Exclusion device: soft/ﬂexible grid and bottom-opening escape – – ––––––

Mesh enlargement – – ––––––

Net binding – – ––––––

Net colour – – ––––––

Passive acoustic deterrents – – ––––––

Pot/trap excluder devices – – – – Yes Yes Yes Yes

Reduced strength rope – – – – No ? No ?

Reduced strength nets Yes ? No ? – – – –

Rope colour changes – – ––––Yes?

Rope or mesh barriers – – ––––––

Rope-less pot/trap systems – – ––––Yes?

‘‘Seal socks’’ – – – – Yes Yes No ?

Sinking groundlines – – ––––YesNo

Stiff ropes – – ––––YesNo

Visually detectable nets No ? No ? – – – –

Weak hooks – – ––––––

Weak links No ? No ? No ? Yes ?

‘‘?’’ for assessed category = unclear whether there has been any assessment of the measure

‘‘?’’ for effective category = lack of knowledge of the measure’s effectiveness, results have been inconclusive and/or more trials are needed

‘‘–’’ = measure is not applicable for relevant ﬁshing gear

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Rev Fish Biol Fisheries

Table 2 Subjective evaluation of the economic viability (EV), practicality (P), impact on target catch (ITC) and compliance requirement (CR) for mitigation measures shown to

be, or have the potential to be, effective in reducing pinniped and/or cetacean bycatch in trawl, purse seine, longline, gillnet and pot and trap ﬁshing gear. Note that, although this

evaluation provides a general overview, due to ﬁshery-speciﬁc characteristics (e.g. size of target species, operational elements), responses may differ across ﬁsheries

Technical measure Trawl Purse seine Longline

EV P ITC CR EV P ITC CR EV P ITC CR

Acoustic Deterrent Devices (Pingers) Yes Maybe No OBS – – – – No No Unk OBS

Acoustic scarers; e.g. alarm or predator calls, explosions – – – – – – – – Maybe Maybe Unk OBS

Acoustically reﬂective nets – – – – – – – – – – – –

Auto-trawl systems Yes Yes No OBS – – – – – – – –

Back-down manoeuvre with Medina panels – – – – Yes Yes No OBS – – – –

‘‘Buoyless’’ nets – – – – – – – – – – – –

Catch protection devices—demersal longline – – – – – – – – Yes Maybe No OBS

Catch protection devices (triggered)—pelagic longline – – – – – – – – Yes Maybe No OBS

‘‘Dolphin gate’’ with additional weights – – – – Yes Maybe No OBS – – – –

Exclusion device: hard grid and top-opening escape Yes Maybe Maybe OBS – – – – – – – –

Exclusion device: soft/ﬂexible grid and top-opening escape Yes Maybe Maybe OBS – – – – – – – –

Exclusion device: hard grid and bottom-opening escape Yes Maybe Maybe OBS – – – – – – – –

Exclusion device: soft/ﬂexible grid and bottom-opening escape Yes Maybe Maybe OBS – – – – – – – –

Net binding Yes Yes No OBS – – – – – – – –

Net colour Maybe Yes No DCK

–– –– – – ––

Passive acoustic deterrents – – – – – – – – Yes Maybe No OBS

Pot/trap excluder devices – – – – – – – – – – – –

Reduced strength rope – – – – – – – – – – – –

Reduced strength nets – – – – – – – – – – – –

Rope colour changes – – – – – – – – – – – –

Rope or mesh barriers Yes Maybe Maybe OBS – – – – – – – –

Rope-less pot/trap systems – – – – – – – – – – – –

‘‘Seal socks’’ – – – – – – – – – – – –

Visually detectable nets – – – – – – – – – – – –

Weak hooks – – – – – – – – Maybe Maybe No DCK

Weak links – – – – – – – – – – – –

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Rev Fish Biol Fisheries

Table 2 continued

Technical measure Gillnet Pot/trap

EV P ITC CR EV P ITC CR

Acoustic deterrent devices (pingers) Yes Yes No OBS Yes Yes No OBS

Acoustic scarers; e.g. alarm or predator calls, explosions – – – – – – – –

Acoustically reﬂective nets Maybe Yes No DCK* – – – –

Auto-trawl systems – – – – – –

Back-down manoeuvre with Medina panels – – – – – – – –

‘‘Buoyless’’ nets Yes Maybe Maybe OBS – – – –

Catch protection devices—demersal longline – – – – – – – –

Catch protection devices (triggered)—pelagic longline – – – – – – – –

‘‘Dolphin gate’’ with additional weights – – – – – – – –

Exclusion device: hard grid and top-opening escape – – – – – – – –

Exclusion device: soft/ﬂexible grid and top-opening escape – – – – – – – –

Exclusion device: hard grid and bottom-opening escape – – – – – – – –

Exclusion device: soft/ﬂexible grid and bottom-opening escape – – – – – – – –

Net binding – – – – – – – –

Net colour – – – – – – – –

Passive acoustic deterrents – – – – – – – –

Pot/trap excluder devices – – – – Yes Yes Maybe OBS

Reduced strength rope – – – – Yes Yes No DCK

Reduced strength nets Yes Yes Unk DCK – – – –

Rope colour changes – – – – Yes Yes No DCK

Rope or mesh barriers – – – – – – – –

Rope-less pot/trap systems – – – – Yes Maybe No DCK

‘‘Seal socks’’ – – – – Yes Yes Unk OBS

Visually detectable nets Yes Yes Maybe DCK – – – –

Weak hooks – – – – – – – –

Weak links Yes Yes Unk OBS Yes Yes No OBS

Economically viable (EV): based on the cost for initial outlay plus any ongoing maintenance = yes,no,maybe

Practicality (P): i.e. has no great impact on ﬁshing operation and operational efﬁciency = yes,no,maybe

Impact target catch (ITC): i.e. could cause a reduction in the amount or quality of catch = yes,no,maybe,unk (= unknown)

Compliance requirement (CR): either requiring at-sea observations (OBS) or whether dockside inspections would be adequate (DCK)

‘‘—’’ = measure is not applicable for relevant ﬁshing gear or assessed to be ineffective for pinnipeds and cetaceans in that type of gear (see Table 1)

*Only if all nets on board were of appropriate material. A mix of netting material would require ‘OBS’

Only if all trawl nets on board were of appropriate colour

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Rev Fish Biol Fisheries

and Risso’s (Grampus griseus) dolphin bycatch in

mid-Atlantic bottom trawl ﬁsheries, there is low

conﬁdence in this ﬁnding due to small sample sizes

and limited information on the type and quantity of

deployed pingers (Lyssikatos 2015).

In longline ﬁsheries (Table S3), while there has

been a high degree of variability in device design and

deployment, there is no clear evidence that pingers

effectively deter marine mammals (Hamer et al. 2012;

Tixier et al. 2015; Werner et al. 2015). This may be

largely due to the difﬁculty in protecting longlines

which are set over large distances (Rabearisoa et al.

2012).

In gillnet ﬁsheries (Table S4), although pingers

have effectively reduced the bycatch of some small

cetacean species, the results are not universal and

mitigation effectiveness is likely to be species- and

ﬁshery-speciﬁc. A number of studies have shown that

pingers reduced harbour porpoise (Phocoena pho-

coena) bycatch (Dawson et al. 2013; Kraus et al. 1997;

Larsen and Eigaard 2014; Larsen et al. 2013; Palka

et al. 2008; Reeves et al. 2013). However, results for

bottlenose dolphins have been less clear with some

research reporting signiﬁcantly reduced interactions

(Crosby et al. 2013; Gazo et al. 2008; Leeney et al.

2007; Mangel et al. 2013), while others showed no

deterrent effect (Cox et al. 2003; Erbe et al. 2016).

Pingers have been ineffective, or the results have been

inconclusive, in deterring Hector’s dolphin (Cepha-

lorhynchus hectori), tucuxi (Sotalia ﬂuviatilis), and

other small coastal species such as the Australian

snubﬁn (Orcaella heinsohni) and humpback dolphin

(Sousa chinensis) (Berg Soto et al. 2013; Dawson and

Lusseau 2005; Dawson and Slooten 2005). Pingers

may also attract some species, particularly pinnipeds,

to depredate captured ﬁsh (Bordino et al. 2002;

Mackay and Knuckey 2013). Although initial testing

showed California sea lion (Zalophus californianus)

and northern elephant seal (Mirounga angustirostris)

bycatch reduced with pinger use (Barlow and

Cameron 2003), monitoring of pinger deployment

over 14 years subsequently showed sets with pingers

had almost twice the amount of California sea lion

bycatch although this increase was most likely due to

increased sea lion abundance and was not considered

to be caused by pinger use (Carretta and Barlow 2011).

There is no indication pingers would reduce bycatch

risk for other species of seal, sea lion or dugong

(Dugong dugon) in gillnets (Bordino et al. 2002;

Gearin et al. 2000; Hodgson et al. 2007; Northridge

et al. 2011). As pingers might deter some cetaceans

while attracting some pinnipeds, addressing a bycatch

issue is likely to be challenging if more than one

species is at risk and they have conﬂicting responses to

pingers (Mackay and Knuckey 2013).

For pot/trap ﬁsheries (Table S5), pinger effective-

ness in deterring large whales from high-risk entan-

glement areas, particularly pot or trap ﬁshery

operations, appears to be variable depending on

species, migration direction and social category. In

Canadian inshore trap ﬁsheries, acoustic devices

appeared to reduce the collision frequency between

humpback whales (Megaptera novaeangliae) and cod

traps (Lien et al. 1992). However, in Australia, while

southward migrating humpback whales exhibited

aversion behaviour to acoustic stimuli (Dunlop et al.

2013), northward migrating whales showed no

detectable response to pingers (Harcourt et al. 2014;

Pirotta et al. 2016). There were indications that pingers

could potentially deter grey whales (Eschrichtius

robustus) from high risk coastal areas, although results

were inconclusive due to low statistical power

(Lagerquist et al. 2012).

Ensuring pingers are functioning correctly and with

the required number in the correct net location is

important for maintaining effectiveness in gillnet

ﬁsheries (Orphanides and Palka 2013). However, the

ﬁnancial cost of implementing pingers may limit their

applicability in many developing countries and/or

smaller ﬁsheries (Dawson et al. 1998,2013; Read

2008), and more cost-effective, durable pingers are

needed (Crosby et al. 2013). Pingers are also unlikely

to be effective in deterring dolphins if they are not

fully functional (e.g. fully charged batteries) or in

suboptimal locations on trawl gear (Deepwater Group

2018; Northridge et al. 2011), and they should be

positioned to ensure they do not impact operational

equipment, such as net monitoring systems (Morizur

et al. 2007).

Evidence of harbour porpoise habituation to

pingers, which would reduce their effectiveness in

mitigating gillnet bycatch, was provided by some

experimental studies (Carlstrom et al. 2009; Cox et al.

2001; Dawson et al. 2013; Gearin et al. 2000; Read

2013), but not others (Hardy et al. 2012). However,

long-term studies monitoring operational gillnets

showed no sign of harbour porpoise, common dolphin

or beaked whale habituation to pingers (Carretta and

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Barlow 2011; Dawson et al. 2013; Palka et al. 2008).

Inshore, resident porpoise populations may be more

likely to develop habituation to pingers than more

migratory species (Dawson et al. 1998,2013). The

effectiveness of pingers in deterring coastal, inshore or

river ﬁnless porpoises (Neophocaena spp.) from

gillnets decreased after a few months, and developing

regimes which include periods with no pinger use

(Amano et al. 2017), as well as randomising pinger

frequency, time interval and strength, may help to

maintain effectiveness. Developing ‘responsive pin-

gers’ for gillnets, which only emit sounds in response

to cetacean echolocations, may reduce the likelihood

of pinger habituation for some species (Leeney et al.

2007; Waples et al. 2013). Bottlenose dolphins may

become more sensitised to pingers, which could

increase the mitigation effect on this species over

time (Cox et al. 2003). With respect to trawl gear,

some captive pinniped species became habituated to

pingers on a simulated net and continued to depredate

netted ﬁsh, while some dolphin species charged the

netting despite pinger presence (Bowles and Anderson

2012). An interactive pinger for pelagic trawls,

designed to emit signals in response to the presence

of dolphin echolocations, may delay habituation and

reduce noise pollution in the marine environment, with

initial tests showing evasive behavioural responses

from bottlenose dolphins, although not from common

dolphins (van Marlen 2007). In longline operations,

there is evidence that false killer whales (Pseudorca

crassidens) and killer whales (Orcinus orca) became

habituated to acoustic devices (Mooney et al. 2009;

Tixier et al. 2015).

The increasing level of anthropogenic sound in the

marine environment may negatively impact the

behaviour, physiology and auditory systems of some

marine species (Kastelein et al. 2015), with indications

that some gillnet pingers may affect target and non-

target ﬁsh (Goetz et al. 2015; Kastelein et al. 2007).

Pinger deployment could impact small cetacean

species that are neophobic and with small, restricted

ranges by excluding them from crucial habitat, with

the displacement effect potentially more pronounced

in coastal locations where topographical features limit

access to key bodies of water (Dawson et al. 2013). In

longline operations, there is concern that frequent

exposure to higher amplitude devices may affect the

echolocation ability of killer whales (Tixier et al.

2015).

Weakened gear

Different types of weakened gear, designed to release

caught animals, have been proposed and/or trialled in

different ﬁsheries (Table 1) including:

a. ‘‘Weak’’ hooks in longline ﬁsheries (Table S3):

These may reduce the bycatch risk for some

species (e.g. false killer whales) without loss of

target catch (Bayse and Kerstetter 2010; Bigelow

et al. 2012; McLellan et al. 2015; Werner et al.

2015), although there is currently insufﬁcient

evidence to support this. Low rates of cetacean

interactions during experimental trials has ham-

pered the ability to assess bycatch reduction

(Bigelow et al. 2012). Weak hooks would not

reduce interactions or prevent depredation (Hamer

et al. 2015; Werner et al. 2015).

b. Reduced-strength nets or ropes: Thin twine gill-

nets may signiﬁcantly reduce seal and harbour

porpoise bycatch compared to thick twine nets

(Northridge et al. 2003) (Table S4). Similarly, as

strong polypropylene ropes used in modern pot/-

trap ﬁsheries have increased the mortality risk of

entangled cetaceans, use of ropes with reduced

breaking strengths could substantially decrease

mortalities of whales entangled in ﬁxed gear

(Knowlton et al. 2016) (Table S5).

c. Weak links between the vertical line from a

pot/trap to a buoy: These do not appear to have

reduced the incidence or severity of whale entan-

glements in USA lobster ﬁsheries (Knowlton et al.

2012,2016; Pace et al. 2014; Salvador et al. 2008;

Van der Hoop et al. 2013) (Table S5). Also, when

buoys separate from vertical pot or trap lines,

released whales may retain sections of gear

(Laverick et al. 2017; Moore 2009). Some USA

ﬁsheries require weak links in gillnets to allow

entangled whales to break free (NOAA 2018),

although no research was identiﬁed that tested the

efﬁcacy of this measure (Table S4).

Trawl

Marine mammals are frequently caught in pelagic or

midwater trawls as these often target the same pelagic

species eaten by marine mammals, have relatively

high tow speeds with large nets, and usually operate

within marine mammal diving ranges for extended

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periods (Fertl and Leatherwood 1997; Hall et al. 2000)

(Table S1). However, in US ﬁsheries, marine mam-

mals are caught more often in demersal rather than

midwater trawls (Carretta et al. 2017; Jannot et al.

2011; Waring et al. 2016). The technical mitigation

measures identiﬁed and assessed for trawls, in addition

to pingers (see ‘‘Pingers (Acoustic Deterrent

Devices)’’ section), are net colour, net binding,

exclusion devices,rope or mesh barriers and auto-

trawl systems (Table 1, Table S1).

Net colour

In an Australian ﬁshery, more bottlenose dolphins

were caught in a grey trawl net compared to a standard

green net, although management variations between

the two trial vessels, resulting in different net speeds

through the water during winching, could also have

contributed to bycatch differences (Stephenson and

Wells 2006). Changing net colour has not been tested

as a means of reducing marine mammal bycatch risk.

However, this may not be a feasible mitigation option

as, particularly for some small cetacean and fur seal

species that are known to deliberately enter nets to

depredate the captured ﬁsh (Fertl and Leatherwood

1997; Hamer and Goldsworthy 2006; Lyle et al. 2016;

Wakeﬁeld et al. 2017), bycatch risk may not be linked

to their lack of awareness of a trawl net’s presence.

Visual detection of nets may also be limited if

visibility is poor or variable at ﬁshing depths.

Furthermore, as well as vision, many cetacean species

may primarily rely on echolocation to forage and

pinnipeds may use tactile senses (Martin and Crawford

2015).

Net binding

An organic material, such as sisal string, is used to

bind the net until it has sunk below the water surface.

Once the trawl doors are paid away, the water force

separating the doors breaks the bindings so the net can

form its standard operational position. Net binding,

used to mitigate seabird bycatch during net shots

(Sullivan et al. 2004), has also been used in some

Australian ﬁsheries to reduce fur seal (Arctocephalus

spp.) interactions during setting (Australian Fisheries

Management Authority, personal communication),

although there is a lack of operational information or

testing to determine whether this effectively reduces

seal bycatch. As marine mammal interactions often

occur during the haul (Hamer and Goldsworthy 2006),

net binding, if it is shown to be effective, may need to

be used in combination with other mitigation.

Exclusion devices with separation grids

It is widely accepted that appropriately designed

exclusion devices successfully prevent mortalities of a

range of non-target marine species in nets without

signiﬁcantly impacting target catch (Dotson et al.

2010; Grifﬁths et al. 2006; Hamilton and Baker 2015a;

Wakeﬁeld et al. 2017; Zeeberg et al. 2006), although

there are differing outcomes for pinnipeds and

cetaceans. The grid design and escape hole conﬁgu-

ration of exclusion devices need to ensure target

species ﬂow smoothly into the codend without com-

promising catch quality and quantity (Table 2), while

ensuring all size classes of the non-target marine

mammal species are prevented from passing into the

codend and can escape (Hamilton and Baker 2015a).

In ﬁsheries with large target species, designing grids

that have no impact on target catch is likely to be more

challenging.

Top-opening, hard-grid exclusion devices (Fig-

ure S1) have effectively reduced pinniped bycatch in a

number of trawl ﬁsheries (CCAMLR 2017; Hamilton

and Baker 2015a;Lyleetal.2016; Tilzey et al. 2006).

Operational constraints may inﬂuence exclusion

device design, which could limit bycatch reduction.

For example, on-board net drum storage may neces-

sitate top-opening devices to have ﬂexible grids, as an

upwardly angled grid is counter to net drum rotation.

However, soft-grids deformed under a seal’s weight

causing partial entanglements, provided no passive

assistance in directing seals out an opening, and

ﬂexible grid distortion may also restrict the ﬂow of

target species into the codend resulting in reduced

catches (Bord Iascaigh Mhara and University of St

Andrews 2010; Lyle et al. 2016) (Table 2).

There has been limited success in demonstrating

exclusion devices effectively reduce cetacean bycatch.

Dolphins may deliberately enter trawl nets to depre-

date captured ﬁsh but do not appear to manoeuvre as

easily as pinnipeds within the conﬁnes of a net (Jaiteh

et al. 2013; Lyle et al. 2016). They appear to become

distressed when far into nets and unable to ﬁnd, or

negotiate escapes, particularly those with bottom-

opening exits (Jaiteh et al. 2014; Wakeﬁeld et al. 2017;

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Zeeberg et al. 2006). While there are reports of

bottlenose dolphins seeming to favour an exit out the

bottom of a net (Zollett and Rosenberg 2005), they

have also been reported to preferentially attempt

escape via the net mouth rather than exclusion devices

and, therefore, may be more likely to die if progressing

too far into a net (Wakeﬁeld et al. 2017). Exclusion

devices showed potential in reducing common dolphin

bycatch in UK midwater pair trawls (Northridge et al.

2011), although only a small number successfully

exited via the escape hole with most appearing to

detect the grid some distance beforehand and attempt-

ing to, unsuccessfully, escape in that area (Bord

Iascaigh Mhara and University of St Andrews 2010).

While ensuring no impact on target catch, net drag or

operational functioning, it was thought that position-

ing exclusion devices as far forward as practical, with

multiple, obvious escape routes, may be critical for

small cetacean survival (van Marlen 2007).

Unobservable and unreported cryptic mortality

may occur with exclusion devices due to injuries

incurred during interactions with devices or because

dead animals may fall out escape openings, although

scientiﬁc evidence has shown that cryptic mortalities

from direct interactions with top-opening, hard-grid

exclusion devices are unlikely (Hamilton and Baker

2015a,b). A forward-facing hood, held in place with a

‘kite’ (i.e. material strip) and ﬂoats over a top-opening

escape (Figure S1), directs water ﬂow into the net

across the grid and is likely to minimise potential loss

of dead or incapacitated animals and target catch,

while keeping the escape hole open and assisting live

animals to escape (Hamilton and Baker 2015a). Target

species, dead seals and dead dolphins have been

observed falling out of devices with bottom-opening

escapes, or top-opening escapes without a cover or

hood (Hamilton and Baker 2015a; Jaiteh et al. 2014;

Lyle et al. 2016; Stephenson and Wells 2006),

although unaccounted mortality was considered neg-

ligible even with bottom-opening devices (Wakeﬁeld

et al. 2014,2017). While Lyle et al. (2016) stated that

passive ejection of dead animals had been reported for

top-opening devices citing Robertson (2015) and

Wakeﬁeld et al. (2014), there is no evidence to support

this. Robertson (2015) stated there were no data to

show dead sea lions were either retained or passively

ejected from openings, but made no differentiation

between top-opening and bottom-opening devices and

did not acknowledge that a hood or cover helps

prevent passive loss of animals (see Hamilton and

Baker 2015b). Wakeﬁeld et al. (2014) reported one

incident where a dead dolphin fell out a device with a

top-opening escape hole, although this occurred when

the net rotated 180°during the haul so the hole (with

no cover) was orientated downward.

Rope or mesh barriers

Restricting dolphin access into trawl nets may be the

key to preventing mortality, although there has been

limited success in deterring them from entering nets

(Wakeﬁeld et al. 2017). A small number of dolphins

escaped through a top-opening hole, covered with

parallel ‘bungee’ cords, located ahead of a mesh

barrier, though most barriers trialled in pair trawls

(e.g. various designs in van Marlen 2007) had reduced

target catch rates (Bord Iascaigh Mhara and University

of St Andrews 2010; van Marlen 2007) (Table 2).

Auto-trawl systems

Intuitively, ensuring the net entrance does not collapse

during any trawl phase should reduce entrapment risk

and maintain the effective operation of installed

exclusion devices, though the efﬁcacy of auto-trawl

systems as a bycatch mitigation measure requires

veriﬁcation. Use of otter-board sensors and eliminat-

ing sharp turns while trawling are thought to have

reduced dolphin mortalities in trawl nets (Wakeﬁeld

et al. 2017). Recent research assessing bottlenose

dolphin interactions with trawls gives further support

to improving and monitoring trawl gear stability as

potentially the most effective mitigation strategy for

reducing dolphin bycatch, while the use of acoustic

deterrent devices was ineffective (Santana-Garcon

et al. 2018).

Purse seine

In purse seine operations, bycatch mitigation has

concentrated on reducing dolphin mortality, mainly

related to eliminating the practice of setting around

dolphin pods associated with target tuna species in the

eastern Paciﬁc Ocean (EPO) (Gilman 2011)

(Table S2). Less commonly, sets on tuna schools

associated with live whales have also occurred.

Reducing cetacean bycatch has been mainly through

the cessation of sets on dolphin-associated or whale-

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associated tuna schools (Hall and Roman 2013). There

was a general lack of information on mitigation

development for other purse seine ﬁsheries although a

‘dolphin gate’ (detachable cork-line section) and

weights to help sink the cork-line were trialled in an

Australian small pelagic ﬁshery but require further

development and testing to determine if effective

(Hamer et al. 2008). In the EPO tuna ﬁsheries, a shift

to sets around ‘Fish Aggregating Devices (FADs)’ (i.e.

artiﬁcial ﬂoating elements with relocation aids) raised

new environmental concerns regarding overﬁshing

and marine species’ entanglement in FAD components

(Hall and Roman 2013). Mitigation development

currently focuses on improving FAD design to reduce

shark and turtle entanglement (Restrepo and Dagorn

2011; Restrepo et al. 2014,2016), while this appears

less of an issue for marine mammals. In terms of

technical mitigation, to reduce bycatch and increase

the likelihood of dolphin escape, the use of enlarged

mesh sizes was unsuccessful as target species and

dolphins are often similar size, and acoustic methods

to frighten dolphins out of nets (e.g. playback of

alarms calls or killer whale sounds) were also

ineffective (Gabriel et al. 2005). The primary mitiga-

tion that has substantially reduced dolphin mortality in

tuna purse seine ﬁsheries, without causing loss of

entrapped tuna (Restrepo et al. 2016), has been the

‘back-down’ manoeuvre with the addition of ‘Medina’

panels as described by Hall and Roman (2013)

(Table 1). Speedboats equipped with towing bridles

can also help keep the net open and assist dolphin

escape as well as the use of a raft inside the net to

facilitate manual rescue (National Research Council

1992). While ‘cryptic’ impacts, including the post-

escape mortality of injured dolphins and potential

effects of chasing and encirclement on reproductive

success, are a potential issue (Anderson 2014; Archer

et al. 2004; Cramer et al. 2008; Gerrodette and Forcada

2005; Wade et al. 2007), no studies were identiﬁed that

monitored the post-release survival of marine

mammals.

Longline

While mitigation has primarily focussed on reducing

the economic impact of marine mammal depredation

of target catch in longline operations, depredation

behaviour also puts them at risk of becoming hooked

or entangled (Bigelow et al. 2012) (Table S3).

Mitigation measures that have been unsuccessful in

reducing interactions include the use of explosives,

chemical deterrents, ﬂare guns or predator sounds

(Werner et al. 2006). An assessment of mitigation

measures including spatial ﬁsheries management,

altered ﬁshing strategies, and acoustic and physical

techniques, concluded that terminal gear modiﬁca-

tions had the greatest mitigation potential (Werner

et al. 2015). In this updated review, along with pingers

and weak hooks (see ‘‘Pingers (Acoustic Deterrent

Devices)’’ and ‘‘Weakened gear’’ sections), mitigation

with potential to reduce interactions with longlines are

passive acoustic deterrents and catch protection

devices (Table 1, Table S3).

Passive acoustic deterrents

Echolocation Disruption Devices and passive acoustic

measures may affect a cetacean’s ability to echolocate

hooked ﬁsh (Hamer et al. 2012; O’Connell et al. 2015).

While there were indications that spherical beads

attached near longline hooks could reduce interactions

between sperm whales (Physeter microcephalus) and

longlines, it was inconclusive whether they were

effective (O’Connell et al. 2015).

Catch protection devices: demersal longline

In demersal longline ﬁsheries, odontocetes are more

likely to access hooked ﬁsh during the haul compared

to line soaking that may be at depths beyond their

normal foraging range (Gilman et al. 2006; Guinet

et al. 2015; Hamer et al. 2012; Soffker et al. 2015;

Tixier et al. 2014). However, there may be exceptions

to this such as recent evidence of interactions between

southern elephant seals (Mirounga leonina) and a sub-

Antarctic demersal longline ﬁshery during the line

soak period at depths [1 km (van den Hoff et al.

2017). ‘‘Net sleeves’’, which cover hooked ﬁsh with

the downward pressure of hauling, protect ﬁsh from

depredation and reduce bycatch risk (Hamer et al.

2012; Moreno et al. 2008). The ‘‘cachalotera’’, a type

of net sleeve, substantially reduced depredation by

killer and sperm whales in the Chilean industrial

Patagonian toothﬁsh (Dissostichus eleginoides) long-

line ﬂeet (Moreno et al. 2008) (Figure S2), although

some killer whales have learnt how to depredate

around cachaloteras (Arangio 2012). A similar system

to reduce sperm whale depredation and seabird

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bycatch on Spanish vessels consists of ‘‘umbrella’’

devices ﬁxed on branchlines, which open to extend

over hooked ﬁsh, combined with stones for faster line

sinking (Figure S3). While ‘‘umbrella and stones’’ net

sleeve trials were promising, evidence for their

efﬁcacy in reducing interactions was inconclusive

(Goetz et al. 2011). While there was no reduction in

target catch rates with ‘‘cachaloteras’’ (Moreno et al.

2008), ‘‘umbrella and stones’’ net sleeves signiﬁcantly

reduced toothﬁsh catch, which may be due to different

attachment designs as ‘‘cachaloteras’’ slide up and

down the branchline whereas the ‘‘umbrellas’’ are

ﬁxed (Goetz et al. 2011). In some operations, gear and

vessel conﬁgurations (e.g. if hooks are close together

and gear is coiled for storage) may make net sleeves

impractical to use (O’Connell et al. 2015).

Triggered catch protection devices: pelagic longline

Compared to demersal longlines, pelagic longlines

may be at risk of depredation during setting, soak time

and hauling as marine mammals often occur across the

same depths as target ﬁsh and, therefore, net sleeves

that slide over the hook only during the haul have

limited use (Hamer et al. 2015; Rabearisoa et al.

2015). Therefore, devices developed for pelagic

longlines have mechanical triggers to release the net

sleeve structure with the pressure when a ﬁsh is

hooked. These include:

a. ‘‘Chain’’ devices and cone-like ‘‘cage’’ devices

(Figure S4): Trials on Australian pelagic longlines

showed all odontocete interactions occurred on

branchlines without devices and there was negli-

gible impact on target catch, although results were

inconclusive due to small sample sizes (Hamer

et al. 2015);

b. Eight strand ‘‘spider’’ devices and conical ‘‘sock’’

devices [see photographs in Rabearisoa et al.

(2012)]: Trials on commercial tuna longliners off

the Seychelles showed limited success in reducing

odontocete depredation, although interaction rates

were low during trials (Rabearisoa et al. 2012);

c. ‘‘DEPRED’’ device (Figure S5): Initial results

were encouraging although, as trials used small

delphinid interactions with a small pelagic ﬁsh

ﬁshery as a proxy for odontocete interactions with

tuna and billﬁsh ﬁsheries, further development

and testing is required (Rabearisoa et al. 2015).

During trials, some devices falsely triggered when a

ﬁsh was not present, did not deploy when a ﬁsh was

hooked, or became entangled in the longline gear

(Hamer et al. 2015; Rabearisoa et al. 2012). While

most devices provide a simple physical barrier to

protect hooked ﬁsh, there has been preliminary testing

of devices with metal wire incorporated in streamers to

affect an odontocete’s ability to echolocate hooked

ﬁsh (McPherson et al. 2008).

Gillnet

There have been a number of reviews, with a range of

focuses and objectives, relating to marine mammal

bycatch and mitigation measures for gillnets (Dawson

et al. 2013; Leaper and Calderan 2018; Mackay and

Knuckey 2013; Northridge et al. 2017; Read

2008,2013; Reeves et al. 2013; Uhlmann and

Broadhurst 2015; Waugh et al. 2011). As it may be

difﬁcult for many marine mammal species to avoid

gillnets, well-designed spatial and/or temporal ﬁshery

closures are likely to be important and effective

mitigation options (Dawson and Slooten 2005; Hall

and Mainprize 2005; Hamer et al. 2011,2013; Read

2013; Rojas-Bracho and Reeves 2013; Slooten 2013).

In this updated review, in addition to pingers, reduced

strength nets and weak links (see ‘‘Pingers (Acoustic

Deterrent Devices)’’ and ‘‘Weakened gear’’ sections),

the potential mitigation options identiﬁed are acous-

tically reﬂective nets,visually detectable nets and

‘‘buoyless’’ nets (Table 1, Table S4).

Acoustically reﬂective nets

As nylon may be difﬁcult for echolocating marine

mammals to detect, nets that utilise different materials,

or incorporate reﬂective components (e.g. metal

compounds) into the net ﬁlament, have been trialled

(Bordino et al. 2013; Larsen et al. 2007; Mooney et al.

2004; Trippel et al. 2003). Some studies showed a

reduction in harbour porpoise bycatch with metal

oxide nets (Larsen et al. 2007; Trippel et al.

2003,2008) while others reported no reduction in

harbour porpoise, franciscana or seal bycatch (Bor-

dino et al. 2013; Mooney et al. 2004; Northridge et al.

2003). It was suggested that observed bycatch reduc-

tion may be due to net stiffness rather than acoustic

reﬂectivity (Cox and Read 2004; Larsen et al. 2007).

However, while increasing net stiffness could be a

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Rev Fish Biol Fisheries

low-cost mitigation option (Northridge et al. 2017),

there was no signiﬁcant difference in franciscana

bycatch between barium sulphate nets, nets with

increased nylon twine stiffness and standard nets

(Bordino et al. 2013). Furthermore, increased net

stiffness decreased target catch rates in some studies

(Larsen et al. 2007). Increasing a net’s acoustic

reﬂectivity would also be ineffective if a small

cetacean encountered the net when it was not echolo-

cating (Dawson 1991). There was also no evidence

that passive acoustic additions (e.g. metal beaded

chains) reduced cetacean bycatch (Hembree and

Harwood 1987).

Visually detectable nets

Increasing the visual detectability of nets using

illumination or visible panel inserts have not yet been

tested as a mitigation option for marine mammals.

Light-emitting diodes signiﬁcantly reduced bycatch of

other taxa and could potentially reduce the bycatch of

small cetaceans (Mangel et al. 2018). Conversely,

installing contrasting patterned panels to increase net

detectability could possibly alert pinnipeds to gillnet

presence which may increase catch depredation

(Martin and Crawford 2015). To date, changing gillnet

colour has not been tested as a measure to reduce

marine mammal bycatch, although orange-coloured

gillnets may be more apparent to some penguin

species (Hanamseth et al. 2018).

‘‘Buoyless’’ nets

Nets with reduced numbers of buoys per metre

signiﬁcantly reduced sea turtle bycatch probably due

to a decreased vertical proﬁle of the nets. While this

gear modiﬁcation could potentially reduce marine

mammal bycatch (Peckham et al. 2016), this is yet to

be veriﬁed.

Pot/Trap

Management of large whale entanglement has pre-

dominantly focussed on strategies to respond and

release entangled whales or establish seasonal closures

(Robbins et al. 2015; Van der Hoop et al. 2013), with

less research on technical solutions to prevent inter-

actions or entanglements (Table S5). Developing

better species-speciﬁc knowledge of the interaction

and mechanism of entanglement, particularly the parts

of gear that whales mainly encounter, will aid in

implementing effective mitigation (Johnson et al.

2005; Northridge et al. 2010). A number of potential

techniques have been proposed but have not been

considered a priority for development including ropes

that glow underwater and lipid soluble ropes which

dissolve if embedded in whale blubber (Werner et al.

2006). This review provides an updated assessment of

entanglement mitigation based on previous reviews

(Laverick et al. 2017; Leaper and Calderan 2018), as

well as mitigation to reduce pinniped and small

cetacean entrapment in pots and traps. In addition to

pingers, reduced strength rope, weak links and line

cutters (see ‘‘Pingers (Acoustic Deterrent Devices)’’

and ‘‘Weakened gear’’ sections), the mitigation iden-

tiﬁed are pot/trap excluder devices,‘‘seal socks’’,

sinking groundlines,rope-less pot/trap systems,rope

colour changes and stiff ropes (Table 1, Table S5).

Pot/trap excluder devices

Technical alterations or additions reduce the entrance

size and/or shape of pots and traps to prevent marine

mammals entering thereby reducing bycatch risk as

well as catch depredation. The shape as well as the size

of pot entrances is likely to be important to ensure

target catch quantity and size range are not affected

(Konigson et al. 2015). ‘Bungee’ cord guards reduced

bottlenose dolphin interactions with crab pots (Noke

and Odell 2002; Werner et al. 2006); wire guards and

stronger netting reduced seal damage and bycatch risk

in salmon trap-nets (Hemmingsson et al. 2008;

Suuronen et al. 2006); and smaller crab fyke trap

openings reduced sea otter (Enhydra lutris) bycatch

without reducing target catch (Hatﬁeld et al. 2011).

‘Spike’ excluder devices are mandatory in bycatch

risk areas to prevent Australian sea lion pups and

juveniles entering lobster pots, although testing of

alternative industry-designed 150 mm diameter cir-

cular openings (which are more practical and safe to

use) has shown they may also effectively exclude most

pups (Campbell et al. 2008; Mackay and Goldsworthy

2017).

‘‘Seal socks’’

A cylindrical net attached to shallow water (\2m

deep) fyke nets allowed trapped seals access to the

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surface to breathe and reduced ringed seal (Phoca

hispida botnica) bycatch, although was less effective

for Baltic grey seals (Halichoerus grypus baltica)

(Oksanen et al. 2015).

Sinking groundlines

The implementation of measures in USA ﬁxed-gear

ﬁsheries, including negatively buoyant or sinking

groundlines which aim to lie closer to the ocean

bottom, has not reduced serious injuries and mortality

of northern right whales (Eubalaena glacialis)to

sustainable levels (Brillant and Trippel 2010; Knowl-

ton et al. 2012).

Rope-less pot/trap systems

To reduce cetacean entanglement risk, rope-less

systems remotely release buoys linked to pots or

traps, thereby reducing surface markers with vertical

lines in the water column. There are no published

studies that show rope-less systems mitigate bycatch

or are practical and cost-effective for implementation

in operational ﬁsheries (Laverick et al. 2017),

although trials have been undertaken on rope-less

system prototypes using timed-release (Partan and

Ball 2016) or acoustic-release mechanisms (How et al.

2015; Salvador et al. 2006; Turner et al. 1999).

Acoustic releases have been used for some years in an

Australian lobster ﬁshery (Liggins 2016), although

research on acoustic release technology in this ﬁshery

(Hodge 2015) is yet to be published. Acoustic-release

systems may be preferable to pre-speciﬁed time-

release mechanisms which may release ropes before

ﬁshers are in the vicinity to haul gear (How et al. 2015;

Laverick et al. 2017).

Rope colour changes

Preliminary studies showed northern right whales

visually detected red and orange ‘simulated’ ropes at

greater distances than black and green ropes (Kraus

et al. 2014; Kraus and Hagbloom 2016), which

suggested that changing to red and/or orange com-

mercial ﬁshing ropes may improve their ability to

avoid entanglements. However, an over-representa-

tion of yellow and orange ropes in humpback whale

entanglements in Australia may indicate this species

actively target these ropes or, in contrast to right

whales, yellow and orange are less visually

detectable to humpback whales (How et al. 2015).

Minke whales appeared to detect black and white

ropes more easily than other colours (Kot et al. 2012).

Stiff ropes

Although increasing rope stiffness could reduce

entanglement risk as whales may be able to glide off

stiff ropes more easily, there are no published studies

on whether ropes with greater stiffness or tension

reduce entanglements (Consortium for Wildlife

Bycatch Reduction 2014). However, experimental

testing using a model of a right whale ﬂipper indicated

that stiff ropes may increase injuries at the point of

contact (Baldwin et al. 2012).

Conclusions and recommendations

Trawl: conclusions and research needs

Fishery-speciﬁc variables and issues need to be

considered when designing exclusion devices includ-

ing the size, biology and behaviour of non-target and

target species; size, operation and storage of gear;

towing speed; and trawl hydrodynamics in relation to

net size/grid and escape hole ratios. Exclusion device

grid construction (material, grid angle, bar spacing and

size); escape hole size, shape and location (e.g. top or

bottom); and the addition of a cover or hood, are all

important components that will impact bycatch reduc-

tion efﬁcacy (Baker et al. 2014). Appropriately

designed exclusion devices have effectively reduced

pinniped bycatch in trawl nets. In particular, devices

with hard separation grids angled to top-opening

escape holes, with a cover or hood held open by a kite

and ﬂoats, effectively allow pinnipeds to escape and

post-escape mortality is likely to be low (Hamilton and

Baker 2015a). Loss of dead animals out top-opening

holes with covers is considered unlikely, although this

requires further veriﬁcation, ideally by direct assess-

ment of pinniped interactions with exclusion devices

in operational ﬁsheries. While there has been limited

success with bottom-opening devices, air-breathing

marine mammals are probably less likely to escape

downwards (Allen et al. 2014) and, furthermore,

bottom-opening devices, particularly without covers,

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may be more likely to have unreported bycatch from

dead animals dropping out.

Exclusion devices are not fully effective in miti-

gating cetacean bycatch in trawl ﬁsheries. Research is

required on options for reducing cetacean bycatch

including further information on the escape behaviour

of dolphin species that interact with nets to inform the

optimal location for exclusion devices (probably

further forward in nets) and ensure escape options

are clear, while retaining target catch (van Marlen

2007). It is inconclusive whether rope or mesh barriers

prevent entry of small cetaceans past the fore section

of trawls, thereby, reducing bycatch. Furthermore,

barriers may reduce target catch to unacceptable levels

(Bord Iascaigh Mhara and University of St Andrews

2010).

Net binding may be effective in reducing bycatch

risk during the shot, although would only be feasible in

operations where the net is removed from the water

and brought onto the trawl deck after each trawl. The

efﬁcacy of net binding in reducing marine mammal

bycatch requires testing, including research to estab-

lish the optimal technical speciﬁcations to ensure the

net remains bound until it reaches depths beyond the

diving range of bycatch species. Net binding would

only potentially reduce interactions during net shoot-

ing and is likely to be ineffective for mitigating

bycatch of deep-diving species.

Loud pingers show promise in reducing small

cetacean interactions with trawl gear, particularly for

common dolphins (Northridge et al. 2011), although

may not be effective for bottlenose dolphins (Santana-

Garcon et al. 2018). However, development of more

robust and operationally manageable devices is

required as well as more ﬁshery-speciﬁc testing to

determine the optimal conﬁguration and spacing of

pingers in trawl operations and veriﬁcation that

pingers signiﬁcantly deter dolphins (Bord Iascaigh

Mhara and University of St Andrews 2010; Northridge

et al. 2011; van Marlen 2007). Investigating the

likelihood of cetacean habituation to pingers as well as

the impact of the widespread use of loud pingers on the

behaviour, distribution and ecology of cetaceans and

other marine species is also needed (Northridge et al.

2011).

Maintaining the shape and structure of trawl nets

may be an integral bycatch mitigation strategy,

particularly for cetaceans (Santana-Garcon et al.

2018). Auto-trawl systems potentially mitigate

bycatch by ensuring the net entrance is always open

thereby reducing entrapment risk, although this needs

investigation and validation. However, as these sys-

tems are routinely used by some trawlers to improve

ﬁshing efﬁciency, evaluation of their mitigation

potential in an experimental framework may be

difﬁcult.

Purse seine: conclusions and research needs

Management measures, particularly the ‘back-down’

manoeuvre coupled with ‘Medina’ safety panels and

additional guidance from small boats, increase the safe

escape and have signiﬁcantly reduced the observed

bycatch of small cetaceans in tuna purse seine

ﬁsheries. However, information is needed on the

post-encirclement and post-release survival and health

of bycatch species through remote monitoring pro-

grams to inform best practice techniques for releasing

encircled animals (Restrepo et al. 2014). Although

potentially less relevant to marine mammal species,

continued research on the development and efﬁcacy of

non-entangling FADs is also important.

Longline: conclusions and research needs

There is a lack of technical mitigation shown to be

fully effective in reducing marine mammal bycatch in

longline ﬁsheries. However, there are indications that

catch protection devices, with speciﬁc designs for both

pelagic and demersal operations, reduce hooking risk

for odontocetes. Results have been variable on the

impact of different net sleeve devices on target catch

rates, and more research is required, particularly to

reduce interactions with killer whales that have learnt

to get around standard designs in demersal longline

operations (Arangio 2012; Goetz et al. 2011; Moreno

et al. 2008). In pelagic longline operations, further

research is required to reﬁne triggered catch protection

device designs, particularly increasing device relia-

bility, and verifying mitigation efﬁcacy in operational

ﬁsheries in the longer term (Hamer et al. 2015;

Rabearisoa et al. 2012,2015).

While the use of weak hooks may reduce bycatch in

pelagic longlines, this requires further operational

testing, including operational feasibility. There is also

a lack of information on the post-release health and

survival of marine mammals that are injured, retain or

ingest hooks, or remain entangled in gear (Bayse and

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Rev Fish Biol Fisheries

Kerstetter 2010; Hamer et al. 2012,2015; Hucke-

Gaete et al. 2004; Kock et al. 2006; Werner et al.

2015).

Gillnet: conclusions and research needs

Pingers effectively reduce the gillnet bycatch of some

(e.g. harbour porpoises), although not all, small

cetacean species, and may be most effective in

reducing bycatch of neophobic species with large

home ranges (Dawson et al. 2013). Pinger research

should include evidence that target species size and

catch are not impacted (Barlow and Cameron 2003;

Carlstrom et al. 2002; Gearin et al. 2000; Kraus et al.

1997; Larsen and Eigaard 2014; Waples et al. 2013).

While the evidence is that harbour porpoises do not

become habituated to pingers (Dawson et al. 2013;

Palka et al. 2008), further investigations regarding

habituation for other cetacean species are needed.

More research to understand small cetacean behaviour

in response to ‘reactive pingers’ is also required,

particularly if they may reduce the likelihood of

habituation and potential impacts from marine noise

pollution (Leeney et al. 2007). As pingers rely on

changing animal behaviour to avoid nets, they should

only be implemented after rigorous ﬁshery-speciﬁc

research on the impacts on all likely bycatch species

(Hodgson et al. 2007) and other vulnerable species

within the ecosystem. The long-term effects of pinger

exposure on small cetaceans, particularly exclusion

from key habitat areas, is not well known. Care should

be taken when deploying pingers to mitigate bycatch

in areas with ecologically important small cetacean

habitat, and intensive pinger use in coastal areas

should be carefully monitored (Carlstrom et al.

2002,2009; Kyhn et al. 2015). Operational testing

should include research on the optimal positioning and

spacing of pingers and, following implementation,

ongoing monitoring is required to maintain pinger

effectiveness. As commercially available pingers may

be prohibitively expensive in some ﬁsheries, more

cost-effective solutions are required. The development

of more durable pingers with battery change capabil-

ities may help to reduce implementation costs (Crosby

et al. 2013).

There have been conﬂicting results on the effec-

tiveness of acoustically reﬂective metal oxide nets in

reducing small cetacean bycatch, and further research

is needed to better understand the mechanism of why

some metal oxide nets showed bycatch reduction

(Northridge et al. 2017).

Increased research focus is needed on post-release

impacts following direct interactions with gillnets. For

example, pinnipeds and cetaceans released following

entrapments in deep-set gillnets (and trawls) may

incur gas embolism that could lead to post-release

mortality (Fahlman et al. 2017; Moore et al. 2009).

Pot/trap: conclusions and research needs

Fishery-speciﬁc trap guards or ‘excluder devices’ have

been effective in reducing the entrapment risk of

marine mammals while maintaining target catch rates

(Campbell et al. 2008; Konigson et al. 2015; Noke and

Odell 2002). The use of ‘‘seal socks’’ may be a

potential mitigation option in shallow-water fyke net

ﬁsheries, although may not be effective for all

pinniped species (Oksanen et al. 2015).

Results on the effectiveness of pingers in deterring

large baleen whales from potentially high-risk areas

have been variable (Dunlop et al. 2013; Lien et al.

1992; Pirotta et al. 2016) and species-speciﬁc inves-

tigations of different pingers are required to determine

if some designs may be more consistently effective.

However, identifying a lack spatial deterrent beha-

viour relative to a pinger in experimental trials may

not necessarily mean that pingers would be ineffective

in alerting marine mammals and reducing operational

interactions (McPherson 2017).

‘Rope-less’ buoy systems are a promising mitiga-

tion development, although further design reﬁnement

and efﬁcacy research is required. Acoustic-release

systems may be preferable to timed-release systems

but are likely to have higher establishment costs, and

research is needed on reliable deployment systems and

a device with enough rope for ﬁsheries operating in

deep water (How et al. 2015; Partan and Ball 2016;

Salvador et al. 2006).

While ropes with reduced breaking strength could

substantially decrease whale mortality in ﬁxed gear,

research is required on the practicalities and success of

using reduced-strength ropes in operational ﬁsheries

(Knowlton et al. 2016), and the post-release health and

survival of animals that remain entangled in lines or

sections of gear (Werner et al. 2015). The effective-

ness of weak links in buoy lines needs investigation

due to concerns regarding a lack of reduction in whale

entanglements following weak link implementation in

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Rev Fish Biol Fisheries

USA lobster ﬁsheries (Knowlton et al. 2012; Pace

et al. 2014; Van der Hoop et al. 2013). It is noteworthy

that weak links are not recommended in some

Australian ﬁsheries as disentangling ‘anchored’

whales from gear has been more successful than

locating and disentangling free-swimming whales

(How et al. 2015).

Whale responses to different rope colours appears

to be species-speciﬁc (How et al. 2015; Kot et al. 2012;

Kraus et al. 2014; Kraus and Hagbloom 2016). Further

species- and ﬁsheries-speciﬁc research is needed to

test and understand whale detectability of colours in a

range of conditions (Kraus et al. 2014).

Final summary and conclusions

Effective technical mitigation measures are a crucial

element of any robust, integrated bycatch management

program, which usually includes other management

directives such as temporal and spatial ﬁshing restric-

tions and appropriate operational ‘codes of practice’.

For some gear types and taxa, there are currently

limited technical options with strong evidence they

effectively reduce bycatch, and substantial develop-

ment and research of best practice mitigation options

is needed to address marine mammal bycatch in many

ﬁsheries. For mitigation to be considered effective, a

signiﬁcant reduction in bycatch mortality needs to be

demonstrated, together with maintenance of target

catch quality and quantity. Fishing industry engage-

ment to ensure design, development and effective

implementation of practical solutions is also essential.

Therefore, knowledge of the biological and beha-

vioural characteristics of target and bycatch species,

temporal and spatial overlap of bycatch species with

ﬁshing activities and operational factors is needed

(Baker et al. 2014). Determining mitigation efﬁcacy

should include species- and ﬁsheries-speciﬁc testing

with adequate scientiﬁc rigour, and a quantitative

target to enable efﬁcacy assessment.

The reviewed studies varied greatly in the level and

rigour of scientiﬁc testing to verify mitigation effec-

tiveness in reducing bycatch. Some measures have

undergone controlled studies in a range of conditions

[e.g. pingers to reduce harbour porpoise bycatch,

Dawson et al. (2013)], while others have not been

tested, testing has been inadequate, or experimental

design has been inappropriate (e.g. Dawson and

Lusseau (2005)). However, testing can be difﬁcult as

a technical measure may be implemented as part of a

suite of management actions, confounding attempts to

test its speciﬁc effectiveness in reducing bycatch

(Laverick et al. 2017). Ideally, if efﬁcacy is to be

efﬁciently demonstrated, mitigation needs to be tested

against a control of no-deterrent, although such trials

are often difﬁcult to implement for ethical reasons.

Additionally, the logistics of undertaking controlled

studies in operational ﬁsheries, including low or

sporadic marine mammal interaction rates during

trials, may limit the scientiﬁc robustness of testing

(Dawson et al. 1998; Hamer and Goldsworthy 2006).

Obtaining adequate data from comparable controlled

experiments may be particularly challenging in trawl

ﬁsheries with small numbers of vessels towing a single

net. Furthermore, due to the range of variables during

ﬁshing (e.g. location, weather, season, ecosystem

components), controlled experiments of the same

mitigation for the same bycatch species may produce

conﬂicting results in different operations. Technical

measures experimentally shown to be effective also

require post-implementation monitoring in opera-

tional ﬁsheries, and mitigation may not produce the

same bycatch decrease in an operational ﬁshery as

shown in controlled trials (Orphanides and Palka

2013). Ensuring the ongoing effectiveness of imple-

mented mitigation requires ﬁsheries to maintain

adequate observer coverage (either direct observa-

tions, or electronic monitoring and review), continue

correct deployment of the appropriate measure,

undergo frequent expert review of procedures, and

continue reﬁnement of measures and strategies as

required (Cox et al. 2007; Hall 1998). It is fundamental

that ﬁshing effort changes are factored into follow-up

assessments of mitigation efﬁcacy. For all ﬁshing gear,

obtaining estimates of post-release mortality from

direct ﬁsheries interactions is an area of research that

requires urgent research attention, although monitor-

ing released individuals this is likely to require a large

investment.

Despite these challenges, it is crucial that resources

are prioritised towards continued development, scien-

tiﬁc testing and subsequent implementation and mon-

itoring of proven, effective technical mitigation

measures to ensure the ecological sustainability of

commercial ﬁsheries. As marine mammal mortality

from ﬁshing gear interactions is likely to increase due

to human population growth, increasing

123

Rev Fish Biol Fisheries

industrialisation of ﬁsheries, increasing population

sizes of some marine mammal species, and ﬁsheries

expanding into new areas (Read et al. 2006), improv-

ing and implementing effective mitigation is essential.

From a global perspective, improving the environ-

mental sustainability of commercial ﬁsheries requires

wider dissemination of successful technologies and

knowledge of mitigation techniques and comprehen-

sive engagement of ﬁshers in the development of

appropriate bycatch solutions (Hall and Mainprize

2005). Developed countries have a level of obligation

to assist developing countries in addressing bycatch

issues particularly as many marine mammal species

have global distributions. At the least, this should

entail the publication of research on mitigation design,

development, scientiﬁc testing of efﬁcacy (or lack of

efﬁcacy) and monitoring of operational deployment. It

is hoped that this review contributes to this process by

having a ‘one-stop-shop’ on the current status of

mitigation techniques developed and assessed for

marine mammal bycatch in commercial trawl, purse

seine, longline, gillnet and pot/trap ﬁsheries.

Acknowledgements We thank Mary-Anne Lea and two

anonymous reviewers for their helpful comments on earlier

drafts of this manuscript. We thank the University of Tasmania

for support for S.H.

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The trawl supremacy: hegemony of destructive bottom trawl fisheries and some of the management solutions

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Oct 2024

Download pdf (free): http://www.oceancare.org/trawlsupremacy --- Trawling is a type of fishing characterized by the active towing of nets by a moving boat. Trawl nets vary greatly in size and shape, and they target a wide variety of species, including bottom-dwelling fish, crustaceans and molluscs, pelagic and semi-pelagic schooling fish, and deep-water fauna. In this report, we provide a general overview on towed gear, but we focus more specifically on bottom trawling: the towing of nets along the seabed. Bottom trawling has become a cornerstone of global food supplies, accounting for more than one quarter of global fishery landings. In 2016, this equated to over 30 million tonnes of seafood. In several European and African countries, half of fishery landings come from bottom trawling. Bottom trawling, however, has long been known to be detrimental to marine life. It was regarded as a destructive fishing method since the early 14th century, and was often vocally opposed by communities of fishers who saw it as a threat to marine resources and their own livelihoods. The introduction of steam and diesel engines (in the 1830s and 1930s, respectively) marked the modern era of trawling. Engine-powered trawling increased rapidly during the 1960s, and by the 1980s large fleets of trawlers were combing the global oceans. Today’s bottom trawlers can operate virtually anywhere, from shallow inland channels and rivers to deep offshore waters. Countless scientific studies, encompassing decades of fishery research, have documented the harmful nature of bottom trawling, with substantial cumulative evidence of damage to marine species and ecosystems. Bottom trawling reduces the biomass, diversity and complexity of benthic communities, and the action of trawl gear on the seabed causes dramatic mechanical and chemical alterations, compromising the seabed’s functionality and productivity. In addition to the target species, most types of trawl gear take unwanted species, such as threatened elasmobranchs, sea turtles, seabirds and marine mammals. Apart from these biological impacts, recent studies indicate that bottom trawling has a considerable carbon footprint, with high direct and indirect greenhouse gas emissions contributing to climate disruption. Information on the harmful effects of bottom trawling has resulted in public and institutional awareness of environmental damage, and in restrictions that have sometimes included complete bans. Trawling is often prohibited in the most coastal and shallow waters. However, regulations and enforcement levels vary greatly across areas, and environmental protection measures are often ineffective—to the point that the intensity of bottom trawling can be higher inside than outside some Marine Protected Areas. In this report, we review the evidence of how bottom trawling affects marine life and human life. We also summarize some of the primary management approaches that could help mitigate the harmful effects of trawling—consistent with international commitments to protect the marine environment. We conclude that the amount of seafood produced by bottom trawling can no longer justify or excuse the pervasive damage caused to marine ecosystems and communities of small-scale fishers, and we advocate the use of less destructive fishing gear, combined with the creation of areas protected from harmful fishing practices, and more sustainable strategies to “feed the world”.

Effects of fishing restrictions on the recovery of the endangered Saimaa ringed seal (Pusa hispida saimensis) population

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Dec 2024
PLOS ONE

Over the past three decades, incidental bycatch has been the single most frequent verified cause of death of the endangered Saimaa ringed seal (Pusa hispida saimensis). Spatial and temporal fishing closures have been enforced to mitigate bycatch, which is mainly caused by the gillnets of recreational fishers. In this study, we employed an array of statistical machine learning methods to recognize patterns of death and to evaluate the impacts of annual fishing closures (15th April–30th June) on the recovery of the Saimaa ringed seal population during 1991–2021. We additionally used the potential biological removal (PBR) procedure to assess bycatch sustainability. The study shows that gillnet restriction areas are reflected in the timing of juvenile bycatch mortality of the Saimaa ringed seal. In the 1990s, peak mortality occurred at the beginning of June, but as the restrictions expanded regionally in the 2000s, the peak shifted to the beginning of July. Longer temporal coverage of annual closures would have improved juvenile survival. The study also shows that estimated bycatch mortality is higher than observed: the estimated bycatch averaged approximately two unobserved bycatches per one observed bycatch. Despite the continuing bycatch mortality, a larger number of juveniles nowadays survive to the age of 15 months due to fishing closures, and the population (some 420 individuals) has increased an average 4% per year between 2017 and 2021. However, human-caused mortality limits (PBR) were exceeded by observed bycatch only, which could lead to population depletion in the long run.

Effects of fishery bycatch-mitigation measures on vulnerable marine fauna and target catch

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Sep 2024

Reducing fisheries bycatches of vulnerable species is critical to marine biodiversity conservation and sustainable fisheries development. Although various preventive technical measures have been implemented, their overall effects are poorly understood. Here, we used a meta-analysis approach to quantify the effects of 42 technical measures on the target catch and the bycatch of seabirds, elasmobranchs, marine mammals and sea turtles. We showed that these measures generally reduced the bycatch while having no statistically significant effect on the target catch. Sensory-based measures generally outperformed physical-based ones in reducing the bycatch. Mitigation measures that worked well for several fishing gears or taxa, although useful, were very rare. Most of the adoptions by regional fisheries management organizations (59%) were supported by our findings, although many others are yet to be robustly evaluated. Our study encourages the innovation and adoption of technical measures and provides crucial insights for policy-making and further research in sustainable bycatch management.

A Study on the Catch Losses and Mesh Selectivity Related to the Attachment of Marine Mammal Bycatch Reduction Devices on Midwater Trawl Gear

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Sep 2024

The National Institute of Fisheries Science in Korea is developing marine mammal bycatch reduction devices (BRDs) for high-risk fishing gear, such as trawls. We experimented with two BRD types (guiding nets) attached in front of codend at 30° and 45° angles, and catch losses and mesh size selectivity were investigated. Experimental fishing operations were conducted along the East Coast of South Korea where whales and dolphins are commonly sighted. The catch was classified according to fishing location, BRD type, codend, and covernet, with measurements recorded for body length, maximum girth, and weight. The average selectivity for each haul was analyzed using the ‘selfisher’ package. The catch loss rates with the BRD attached at tilt angles of 30° and 45° were 11% and 29% for common flying squid, 6% and 28% for sailfin sandfish, and 5% and 8% for pearlside. While the mesh selectivity rates for common flying squid and pearlside remained at 0.2–0.5 across all lengths and tilt angles, the mesh selectivity curve for sailfin sandfish was estimated. There were significant differences in catch loss between 30° and 45° angles, with the 30° angle being more effective in catch loss. We observed a masking effect in the codend.

Assessing the vulnerability of Elasmobranch species in the Bay of Bengal: Insights from Lakkha gill net fishery of Bangladesh

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The elasmobranch population is declining in the Bay of Bengal of Bangladesh due to large-mesh gill net fishing, locally known as the Lakkha net, which primarily targets Indian threadfin (Leptomelanosoma indicum). This study was the first attempt to identify megafaunal bycatch in Lakkha fishing and assess its vulnerability using Productivity Susceptibility Analysis. A total of 40 elas-mobranch bycatch species were identified, with sharks comprising 13 species from three families, while 27 rays belonged to six families, with the majority belonging to the Myliobatiformes order (60 %). Productivity and susceptibility scores were assigned to all identified species, with values ranging from 1.27 to 2.73 and 1.50 to 2.63, respectively. The target Lakkha fish exhibited the highest susceptibility score, followed by several pelagic sharks and eagle rays. Vulnerability assessment revealed that 31.7 % (n = 13) of species were highly vulnerable, while 43.9 % (n = 18) were classified as moderate, and 24.4 % (n = 10) were considered to have low vulnerability. All the high-risk megafauna species (n = 13) are classified as threatened by the global IUCN Red List. Sensitivity analysis highlighted susceptibility as a major contributor to species' vulnerability. Alterations in susceptibility scores led to significant changes in the vulnerability status of many species. The overall data quality assessment indicated moderate data quality across species, with variability observed between productivity (76 % of species received a poor data quality score) and susceptibility attributes. However, vulnerability of these species can be reduced through adequate gear modification, shorter net deployment periods, adoption of safe discharge techniques , identification of critical habitats, and establishment of marine protected areas within this region. This study provides valuable insights into the species composition and vulnerability of elasmobranchs in the Lakkha gill net fishery, emphasizing the need for conservation measures to mitigate bycatch impacts on threatened species.

An Endangered Population of Harbour Porpoise Phocoena phocoena Hidden in Plain Sight: Biology, Ecology and Conservation of the Iberian Porpoise

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Sep 2024

The Iberian harbour porpoise (Phocoena phocoena) reaches a larger body size than most other harbour porpoise populations and is genetically distinct, albeit closely related to the population in Northwest Africa. Currently comprising an estimated 3000–4000 individuals, genetic evidence and strandings data suggest that the population has declined in recent times, and it is considered to be at risk of extinction. It is distributed all around the Atlantic coast of the Iberian Peninsula, with the highest densities off Galicia in Northwest Spain and Northern and Central Portugal, a highly productive upwelling area characterised by cold-water upwelling. There are occasional reports from the Mediterranean and Macaronesia and some evidence of emigration into the Celtic Sea. It feeds mostly on fish, with pelagic fish being more important than in the diet of porpoises from northern Europe, perhaps due to excursions beyond the narrow continental shelf. The population faces a number of anthropogenic threats. Historically, porpoises were used for human consumption while current threats include polychlorinated biphenyls (PCBs), with some individuals having concentrations in their blubber above the threshold for impairment of reproduction, and nematode infections, probably also prey depletion, underwater noise and fatal attacks by bottlenose dolphins. The most serious current threat is fishery bycatch mortality. Stranding data suggest that the bycatch mortality increased in the last decade. Although based on information from a small number of documented mortalities (reflecting limited observer coverage especially for small-scale fishing as well as a low number of reported strandings), annual bycatch mortality estimates are in the order of a few hundred animals, which is clearly unsustainable. There is, however, an apparent incompatibility between the high bycatch estimates and the rather similar abundance estimates obtained from large-scale abundance surveys in 2005, 2016 and 2022. Consistent with population status assessments by Spain and Portugal, OMMEG (Convention for the Protection of the Marine Environment of the North-East Atlantic) concluded that bycatch mortality in Iberian porpoise “is critically exceeding the agreed threshold” of zero. There are several national initiatives in Spain and Portugal including the development of species conservation plans. Continuous reduction of bycatch mortality, preferably until such mortality is eliminated, is a priority to ensure that this population does not disappear in the near future.

Assessing the Vulnerability of Globally Threatened Elasmobranch Species in the Bay of Bengal: Insights from Lakkha Gill Net Fishery of Bangladesh

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Sep 2024

Voices from the Ocean: Human-Wildlife Coexistence Through the Lens of Fishermen’s Knowledge and Lived Experience

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Jul 2024

Climate change-related shifts in marine resource availability and species behavior are increasing rates of human-wildlife conflict (HWC). Although this trend poses significant risks to both human livelihoods and conservation efforts, strategies to resolve HWC focus largely on ecological outcomes, overlooking key impacts and contributions of human resource users. Here, we draw on the case study of whale entanglement in the Dungeness crab (Metacarcinus magister) fishery in California, U.S.A. to demonstrate the promise of integrating – and the consequences of neglecting – the voice and expertise of fishing communities. Semi-structured interviews with 27 commercial fishermen across nin