Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
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Industry Attitudes <strong>and</strong> Practices<br />
Fig. 6.1. Some of the tools used by longl<strong>in</strong>e fishermen to immobilize <strong>and</strong> retrieve term<strong>in</strong>al tackle from sharks. Hawaii longl<strong>in</strong>e fisherman demonstrat<strong>in</strong>g how<br />
he clips a loop of rope onto a branch l<strong>in</strong>e below the weighted swivel at the top of the wire leader to assist with remov<strong>in</strong>g ahook from caught sharks (left);<br />
r<strong>in</strong>g <strong>and</strong> electrical cord of a “shocker” device used by a Japanese nearh shore longl<strong>in</strong>e fishermen to immobilize sharks (middle); Chilean fisherman’s improvised<br />
dehooker (right). (Photos by E. Gilman, J. Mangel, T. Miyamoto)<br />
with a 9/0 J hook with squid. Research <strong>in</strong> an experimental Japanese<br />
North Pacific longl<strong>in</strong>e fishery found no difference <strong>in</strong> the capture rate<br />
of blue sharks between a circle <strong>and</strong> Japan tuna hook (Yokota et al.,<br />
2006a,b). Thus, results from controlled experiments <strong>in</strong> the Azores<br />
<strong>and</strong> U.S. North Atlantic longl<strong>in</strong>e fisheries <strong>in</strong>dicate that fish<strong>in</strong>g<br />
with fish <strong>in</strong>stead of squid for bait causes a significant decrease <strong>in</strong><br />
shark CPUE, while us<strong>in</strong>g a wider circle hook <strong>in</strong>stead of a narrower<br />
J hook may cause a significant but small <strong>in</strong>crease <strong>in</strong> shark CPUE.<br />
An assessment of observer data from the Hawaii longl<strong>in</strong>e swordfish<br />
fishery is consistent with results from the controlled experiments<br />
(Gilman et al., 2006a). <strong>Shark</strong> comb<strong>in</strong>ed species CPUE was significantly<br />
lower by 36% after regulations came <strong>in</strong>to effect, which required the<br />
fishery to switch from us<strong>in</strong>g a 9/0 J hook with squid bait to us<strong>in</strong>g a<br />
10o offset 18/0 circle hook with fish bait (Gilman et al., 2006a).<br />
Avoid<strong>in</strong>g certa<strong>in</strong> material for branch l<strong>in</strong>es could also reduce shark<br />
depredation <strong>and</strong> catch rates. For <strong>in</strong>stance, the use of rope/steel<br />
(“Yankee”) gangions resulted <strong>in</strong> lower juvenile s<strong>and</strong>bar shark<br />
catch rates than when us<strong>in</strong>g monofilament gangions (Branstetter<br />
<strong>and</strong> Musick, 1993). In another study, percent-capture of blue shark<br />
with the use of monofilament gangions (66%) exceeded that when<br />
employ<strong>in</strong>g multifilament gangions (34%) (Stone <strong>and</strong> Dixon, 2001).<br />
Shortf<strong>in</strong> mako shark catches adhered to the same pattern (60% <strong>and</strong><br />
40%) for ‘mono’ <strong>and</strong> ‘multi’ l<strong>in</strong>e, respectively. Stone <strong>and</strong> Dixon (2001)<br />
surmise that the relative aversion to the multifilament gangion could<br />
have been a function of strong visual acuity, a trait shared by pelagic<br />
predators that often hunt nocturnally.<br />
6.2.4. Reduce <strong>in</strong>jury to discarded sharks<br />
Most sharks caught <strong>in</strong> pelagic longl<strong>in</strong>e fisheries are alive when hauled<br />
to the vessel (Chapter 3) (Williams, 1997; Brothers, this volume;<br />
Gilman, this volume; Mangel <strong>and</strong> Alfaro-Shigueto, this volume;<br />
Thomson, this volume), suggest<strong>in</strong>g that improved h<strong>and</strong>l<strong>in</strong>g <strong>and</strong><br />
release methods to improve shark post-release survival prospects<br />
holds promise to reduce fish<strong>in</strong>g mortality of discarded sharks. When<br />
a shark will be discarded, the majority of the time <strong>in</strong> the fisheries<br />
<strong>in</strong>cluded <strong>in</strong> this study, fishers will cut branch l<strong>in</strong>es to discard hooked<br />
sharks, will cut the hook out of the shark’s mouth or will pull the<br />
hook out by force <strong>in</strong> order to retrieve the term<strong>in</strong>al tackle before<br />
discard<strong>in</strong>g the shark. A m<strong>in</strong>ority of <strong>in</strong>terviewed fishermen report<br />
occasionally l<strong>and</strong><strong>in</strong>g <strong>and</strong> kill<strong>in</strong>g sharks <strong>in</strong> order to recover fish<strong>in</strong>g<br />
gear before discard<strong>in</strong>g a shark. Fig. 6.1 shows some of the tools used<br />
by longl<strong>in</strong>e fishermen to assist with immobiliz<strong>in</strong>g <strong>and</strong> retriev<strong>in</strong>g<br />
term<strong>in</strong>al tackle from sharks. The survival of sharks that are not<br />
f<strong>in</strong>ned, that are deeply hooked (where the shark has swallowed the<br />
hook) <strong>and</strong> have hooks removed by fishers pull<strong>in</strong>g the hook out likely<br />
depends on where they were hooked <strong>and</strong> how much damage is done<br />
by pull<strong>in</strong>g out the hook. In these cases of deeply hooked sharks,<br />
as is believed to be the case for sea turtles (Gilman et al., 2006a),<br />
prospects for shark post release survival might be improved by<br />
hav<strong>in</strong>g fishers cut the l<strong>in</strong>e as close to the shark as safely possible.<br />
A large proportion of sharks caught <strong>in</strong> longl<strong>in</strong>e gear that is released<br />
after removal of the hook from the mouth are expected to survive.<br />
Research conducted by the U.S. National Mar<strong>in</strong>e Fisheries Service<br />
Pacific Isl<strong>and</strong>s Fisheries Science Center us<strong>in</strong>g pop-up satellite<br />
archival tags found that 97.5% of pelagic sharks released after capture<br />
on longl<strong>in</strong>e gear survived (1 of 40 captured sharks died), while a study<br />
by the U.S. National Mar<strong>in</strong>e Fisheries Service Southwest Fisheries<br />
Science Center found that 94% of 17 tagged shortf<strong>in</strong> mako sharks<br />
survived beyond two months after release (U.S. National Mar<strong>in</strong>e<br />
Fisheries Service, 2005). Some vessels <strong>in</strong> the Australia longl<strong>in</strong>e<br />
fishery are known to use firearms to safely <strong>and</strong> efficiently kill caught<br />
sharks to retrieve term<strong>in</strong>al tackle, while some vessels <strong>in</strong> the Australia<br />
<strong>and</strong> Hawaii fisheries will kill caught sharks <strong>in</strong> an effort to avoid the<br />
<strong>in</strong>convenience of their recapture. However <strong>in</strong> the Hawaii fisheries,<br />
observer data show that a very small proportion of caught sharks<br />
that are alive when hauled to the vessel are killed before discard<strong>in</strong>g<br />
(Gilman, this volume). This is also the case <strong>in</strong> the Peru longl<strong>in</strong>e mahi<br />
mahi <strong>and</strong> shark fishery (Pro Delph<strong>in</strong>us, unpublished data; Mangel <strong>and</strong><br />
Alfaro-Shigueto, this volume). Some vessels <strong>in</strong> the Hawaii longl<strong>in</strong>e<br />
fleet report that, when they are busy process<strong>in</strong>g commercially<br />
valuable species on deck, they will place a tarred rope with a knot<br />
at the end off the stern <strong>and</strong> will clip branch l<strong>in</strong>es conta<strong>in</strong><strong>in</strong>g sharks<br />
onto this rope so they can remove the sharks from the gear after they<br />
have completed the haul <strong>and</strong> processed the catch. The majority of<br />
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