Disentangling the causes of protected‐species bycatch in gillnet fisheries

Abstract

Gillnet fisheries are widely thought to pose a conservation threat to many populations of marine mammals, seabirds and turtles. Gillnet fisheries also support a significant proportion of small-scale fishing communities worldwide. Despite a large number of studies on protected species bycatch in recent decades, relatively few have examined the underlying causes of bycatch, and fewer still have looked at the issue from a multi-taxon perspective. We used three bibliographic databases and one search engine to identify studies by year of publication and taxon. The majority of studies on the mechanisms of gillnet bycatch are not This article is protected by copyright. All rights reserved. 2 accessible through mainstream published literature sources. Many are reported in technical papers, government reports and University theses. We reviewed over 600 published and unpublished studies of bycatch where causal or correlative factors were considered, and identified therein 28 environmental, operational, technical and behavioral factors that might plausibly be associated with higher or lower bycatch rates of the three taxa. Of the factors considered, 11 were found to have been associated with potential bycatch reduction in two out of the three taxa, while three factors (water depth, mesh size and net height) were associated with trends in bycatch rate for all three taxa. These findings provide a basis to guide further experimental work to test hypotheses about which factors most influence bycatch rates, and to explore ways of managing fishing activities and/or improving gear design to minimize the incidental capture of species of conservation concern, whilst ensuring the viability of the fisheries concerned.

Use of time-depth recorders to determine fishing depth of commercial setnets and inform management in Cook Inlet, Alaska

“…This study provides the first concrete data on how setnets actually fish in Cook Inlet, and the specific technical methods we report here likely have much broader potential application to managing bycatch worldwide (e.g., [23,24]). The exceedingly shallow net depths we document in Cook Inlet were unexpected given the potential maximum depth of the nets.…”

Need to address gaps in global fisheries observation

“…Gillnets are simple and relatively cheap to operate and, thus, commonly used in coastal waters around the world, particularly in developing countries (Northridge et al. ).…”

“…Gillnets (anchored or drifting) often generate high bycatch rates, particularly for vulnerable megafauna (e.g., marine mammals) (Lewison et al 2004). Gillnets are simple and relatively cheap to operate and, thus, commonly used in coastal waters around the world, particularly in developing countries (Northridge et al 2017).…”

Assessing the importance of net colour as a seabird bycatch mitigation measure in gillnet fishing

“…The size of mesh and setting depth varies depending upon the size and habitat of the target species, with net meshes typically ranging from 15 mm to more than 250 mm, and net length ranging up to several kilometres (Žydelis et al, 2013). Recent reports suggest that gillnets with mesh sizes greater than 60 mm have greater seabird bycatch rates as a typical trend (Dagys & Žydelis, 2002;Northridge, Coram, Kingston, & Crawford, 2017).…”

“…The characteristics of modern monofilament nylon gillnets have made them almost transparent to a range of species (Žydelis et al, 2013), thereby increasing interaction and bycatch (Waugh et al, 2011). Recent reports suggest that gillnets with mesh sizes greater than 60 mm have greater seabird bycatch rates as a typical trend (Dagys & Žydelis, 2002;Northridge, Coram, Kingston, & Crawford, 2017). The size of mesh and setting depth varies depending upon the size and habitat of the target species, with net meshes typically ranging from 15 mm to more than 250 mm, and net length ranging up to several kilometres (Žydelis et al, 2013).…”