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Research
Cite this article: Mangel JC, Wang J, Alfaro-
Shigueto J, Pingo S, Jimenez A, Carvalho F,
Swimmer Y, Godley BJ. 2018 Illuminating
gillnets to save seabirds and the potential for
multi-taxa bycatch mitigation. R. Soc. open sci.
5: 180254.
http://dx.doi.org/10.1098/rsos.180254
Received: 8 February 2018
Accepted: 4 June 2018
Subject Category:
Biology (whole organism)
Subject Areas:
ecology
Keywords:
seabirds, bycatch, gillnets, vision,
small-scale sheries
Author for correspondence:
Jerey C. Mangel
e-mail: jerey_mangel@yahoo.com
Electronic supplementary material is available
online at https://dx.doi.org/10.6084/m9.
gshare.c.4143956.
Illuminating gillnets to save
seabirds and the potential
for multi-taxa bycatch
mitigation
Jerey C. Mangel1,2, John Wang3, Joanna Alfaro-
Shigueto1,2,4, Sergio Pingo1, Astrid Jimenez1, Felipe
Carvalho3, Yonat Swimmer3and Brendan J. Godley2
1ProDelphinus, Jose Galvez 780-E, Miraores,Lima 18, Peru
2Centre for Ecology and Conservation, University of Exeter, Penryn, Cornwall TR10
9FE, UK
3NOAA, National Marine Fisheries Service, Pacic Islands Fisheries Science Center,
Honolulu, HI 96818, USA
4Facultad de Biologia Marina, Universidad Cientica del Sur, Panamericana Sur Km 19,
Villa, Lima, Peru
JCM, 0000-0002-9371-8606
Bycatch in net fisheries is recognized as a major source
of mortality for many marine species, including seabirds.
Few mitigation solutions, however, have been identified. We
assessed the effectiveness of illuminating fishing nets with
green light emitting diodes (LEDs) to reduce the incidental
capture of seabirds. Experiments were conducted in the
demersal, set gillnet fishery of Constante, Peru and compared
114 pairs of control and illuminated nets. We observed captures
of a total of 45 guanay cormorants (Phalacrocorax bougainvillii),
with 39 caught in control nets and six caught in illuminated
nets. Seabird bycatch in terms of catch-per-unit-effort was
significantly (p<0.05) higher in control nets than in illuminated
nets, representing an 85.1% decline in the cormorant bycatch
rate. This study, showing that net illumination reduces seabird
bycatch and previous studies showing reductions in sea turtle
bycatch without reducing target catch, indicates that net
illumination can be an effective multi-taxa bycatch mitigation
technique. This finding has broad implications for bycatch
mitigation in net fisheries given LED technology’s relatively
low cost, the global ubiquity of net fisheries and the current
paucity of bycatch mitigation solutions.
1. Introduction
Gillnet bycatch is a major source of mortality in many
species of seabirds, sea turtles, and marine mammals, many
2018 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
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of which have declining populations [1–3]. Zydelis et al.[4], for example, estimated that seabird bycatch
in gillnet fisheries likely exceeds 400 000 birds annually. Despite this threat, few mitigation solutions for
seabird bycatch have been identified, let alone implemented on a large scale.
One approach to bycatch mitigation solutions is to use sensory cues to evoke behavioural changes in
animals that reduce their vulnerability to fishing gear [5,6]. Such approaches have been discussed with
regard to incidental captures of sea turtles [5] and elasmobranchs [7]. In addition, a recent assessment of
seabird bycatch from a sensory biology perspective highlighted the importance of visual cues to seabirds
and their potential use in reducing bycatch [6]. While solutions to seabird bycatch in net fisheries have
been elusive, Melvin et al.[8] showed that the entanglement of mures and auklets in salmon drift gillnet
fisheries decreased when high-visibility net mesh were used. Similarly, Trippel et al.[9] found that nylon
barium sulfate gillnets reduced bycatch of both porpoises and seabirds and posited that the decline in
seabird interactions was a result of increased net visibility.
A simple strategy of placing light emitting diodes (LEDs) on nets to create a visual alert has been
shown to reduce the bycatch of sea turtles while not impacting target catch [10–12]. Such a visual cue may
also be useful in mitigating seabird bycatch. The purpose of this study was to assess the effectiveness
of this net illumination strategy to reduce the bycatch of seabirds in a demersal set gillnet fishery in
coastal Peru.
2. Material and methods
Fishing trials were conducted from January 2011 to July 2013 in Sechura Bay, northern Peru. Volunteer
fishers and their small-scale demersal set gillnet fishing vessels from the port of Constante accompanied
by an onboard observer were used in the experiments. The primary target species were flounders
(Paralichtys spp.), guitarfish (Rhinobatos planiceps) and rays (superorder Batoidea). Each fishing trial
consisted of a pair of bottom set gillnets each approximately 600 m in length, composed of 24 cm stretched
diagonal mesh and made of multifilament nylon. The gillnet pairs consisted of two net types: a non-
illuminated control net set at a minimum of 200 m from an illuminated net that had green LEDs placed
every 10 m along the float line. Nets were deployed in the late afternoon and retrieved the following
morning. For each gillnet, onboard observers monitored the fishing operation, gear characteristics as well
as counts of target catch and bycatch. Additional details of the experimental design are provided in Ortiz
et al.[11], which focused on net illumination effects on target catch and sea turtle bycatch in the same
fishing sets described here. We calculated seabird catch-per-unit-effort (CPUE) for each net as the number
of individual seabirds captured/([net length/1000 m] ×[net soak time/24 h]). As in Wang et al.[12],
we compared seabird CPUE between the control and illuminated nets using a Wilcoxon matched-pairs
signed-rank test (GraphPad PRISM).
3. Results
We deployed 114 paired nets and observed the bycatch of 45 guanay cormorants (Phalacrocorax
bougainvillii), with 39 caught in control nets and six caught in illuminated nets (table 1). Four Peruvian
boobies (Sula variegata) were also caught in the illuminated net but were not included in the analysis
due to the small sample size. Cormorant bycatch CPUE was significantly (p<0.05) higher in control nets
(0.97 ±0.41 s.e.) than in illuminated nets (0.15 ±0.06 s.e.), representing an 85.1% decline in the cormorant
catch rate (figure 1). All 45 cormorants were recovered dead. As detailed in Ortiz et al.[11], the predicted
mean CPUE of the target catch of guitarfish, rays and flounders in these same illuminated sets was
unchanged compared to the control nets.
4. Discussion
In the previous study of this active small-scale fishery, we showed that net illumination reduced green
turtle (Chelonia mydas) bycatch by 63.9% and had no impact on catch rates of the three target species of
flounder, guitarfish, rays and flounders [11]. For this study, we show that net illumination also reduced
the bycatch of cormorants by 85.1% in these same fishing sets (table 1 and figure 1).
The potential application of net illumination at reducing bycatch of multiple taxa (i.e. seabirds and
sea turtles) in net fisheries is promising for several reasons. First, in many cases, bycatch of these taxa co-
occurs [4]. Second, identification of one bycatch mitigation technology that addresses multiple taxa could
help encourage and streamline adoption and reduce implementation costs (i.e. as opposed to having to
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1.5
1.0
CPUE
0.5
0
control net illuminated net
treatment
Figure 1. Comparison of the CPUE (no. caught per 1000 m ×24 h) of guanay cormorants between control and illuminated nets showing
an 85.1% decline in illuminated nets. Error bars represent s.e. Pictured are an entangled guanay cormorant and an LED attached to a
gillnet.
Table 1. Summary of total shing eort and guanay cormorant bycatch by net type (control and illuminated) for paired gillnet sets
in Sechura Bay, Peru.
net type sets total eort (km ×24 h) guanay cormorant bycatch
control 114 48.96 39
.........................................................................................................................................................................................................................
illuminated 114 47.71 6
.........................................................................................................................................................................................................................
implement several taxa-specific mitigation measures). Third, the global ubiquity of net fisheries means
that there is potential for large conservation gains if net illumination proves similarly successful in other
regions or fisheries. This is particularly true regarding small-scale fisheries, for which the identification
of relatively low-cost, easy to implement solutions that do not impact target catch may be especially
important towards promoting implementation.
While several studies show the effectiveness of net illumination in reducing sea turtle interactions
with gillnets [10–12], this is the first to show that net illumination also reduces seabird bycatch. As
with sea turtles, the specific mechanism by which net illumination reduced seabird interactions is
unclear. In this study, seabird bycatch was composed primarily of guanay cormorants which forage
by pursuit underwater [13]. Net illumination could simply have increased the visual signature of the
gillnets and allowed the seabirds to better avoid becoming entangled [6,8,9]. Tests of net illumination
in fisheries with other seabird assemblages (e.g. penguins and procellariformes), or that use different
foraging strategies could help explain the specific mechanism of this bycatch reduction while also
determining if net illumination could be broadly effective at reducing seabird bycatch [14]. Seabird
species at high risk globally from bycatch in net fisheries include ducks, loons and auks, all of which
dive while foraging [4,6]. Additional tests of net illumination in fishing areas with plunge diving species
would be particularly useful given the observed bycatch in this study of four Peruvian boobies in the
illuminated nets.
Development of net illumination as a bycatch reduction technology should continue to focus on other
bycatch taxa such as elasmobranches and marine mammals. In addition, differences in the behavioural
response of fish species may lead to beneficial changes in target catch rates. If so, it may be possible
to match specific wavelengths of net illumination to the specific conservation needs of a particular
fishery. Other research directions could include assessments of LED intensities, wavelengths and spacing
along the net, as well as comparisons with other bycatch reduction techniques, including those that use
other sensory strategies (e.g. high-visibility net panels, acoustic alarms) [6,15]. Tests under true fishery
conditions could be particularly important in assessing net illumination’s effectiveness and encouraging
its uptake by fishers.
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Here, we have described a technology that has been shown to reduce bycatch of two marine taxa
of global conservation concern [11], and to do so with no impact on the fishery’s target catch rate [11].
These findings alone are encouraging. In addition, these results are with fisheries that are massive in scale
and extent, and employs a technology that could potentially become cost-effective even in small-scale
fisheries [11], increasing the urgency for further testing leading to broader implementation. Ultimately,
such technologies could help increase opportunities for fisheries to be sustainable for both target and
bycatch species.
Ethics. All necessary permits and permissions were obtained to carry out this research.
Data accessibility. The dataset supporting this article has been uploaded as electronic supplementary material.
Authors’ contributions. J.C.M., J.W., B.J.G. and J.A.S. designed the study. S.P. and A.J. performed the study. J.C.M., J.W. and
F.C. performed the statistical analysis. All authors interpreted data and contributed to writing the manuscript and
gave final approval for publication.
Competing interests. We declare we have no competing interests.
Funding. This work was supported by the DEFRA Darwin Initiative, the NOAA Fisheries Service and the National Fish
and Wildlife Foundation.
Acknowledgements. We thank the entire ProDelphinus team that participated in data collection. We also thank the
fishermen and their families at Constante, Piura, Peru, for their support on every fishing trip.
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