Shark Depredation and Unwanted Bycatch in Pelagic Longline

Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries
sharks placed on this line will have fallen off before the end of the
haul, while most sharks remaining on the line when it is retrieved
are dead. In the Italy Mediterranean industrial longline swordfish
fishery, some fishermen reported that, in the past when shark capture
rates were higher, they would routinely kill all caught sharks in order
to reduce local shark abundance, which they believed would result
in an increase in their swordfish catch by reducing shark predation
on the swordfish (Piovano, this volume). Reducing such practices
that result in the mortality of discarded sharks would contribute to
preventing the unsustainable exploitation of sharks.
A few of the interviewed fishers report killing all caught sharks in
order to prevent the inconvenience of recapturing them, reduce
shark depredation, and in one case, to reduce shark populations
to reduce the likelihood of being killed by a shark if he ever has to
abandon ship (Brothers, this volume; Gilman, this volume; Thomson,
this volume). While this attitude was not held by the majority of the
fishers interviewed, it is useful to identify as a potential obstacle to
introducing new techniques to improve the post-release survival
prospects of caught sharks that are discarded.
While circle hooks may result in a higher shark CPUE than J and Japan
tuna style hooks (Section 6.2.3), because fish caught on circle hooks
are more likely to be hooked in the mouth versus swallowing the hook,
removal of circle hooked sharks is easier and may result in reduced
injury to the shark than sharks caught on other hook types.
None of the interviewed pelagic longline fishermen use dehookers
to discard live sharks. Only two fishers, one from the Chile artisanal
mahi mahi and shark longline fishery and one from the Peru artisanal
mahi mahi and shark longline fishery, report that they use a dehooker
to recover hooks from sharks when these are onboard and already
dead (Alfaro-Shigueto et al., this volume; Mangel and Alfaro-
Shigueto, this volume). The Peruvian fisher possessed the dehooker
initially for the purpose of releasing sea turtles. A benefit of using a
dehooker might include reduced time to retrieve terminal tackle. The
U.S.-based company Aquatic Release Conservation produces a 0.8 m
long “Big Game” and a 0.4 m long “Bite Block” dehooker, which are
designed to remove hooks from sharks brought onboard. For sharks
that are too large to bring onboard, this company also produces a
2.4 m long and 3.7 m long dehooker on a pole. The effect of dehookers
on the post-release survival prospects of sharks compared to
crew cutting a hook from the shark or cutting the branch line and
leaving a hook and trailing line attached to the shark has not been
assessed. Almost all of interviewed fishers from all fisheries included
in the study, except South Africa where all the fishers thought use
of a dehooker would reduce shark injury, believe that it would be
inefficient to use dehookers to remove sharks from gear, either because
this would increase the safety risk, or because it is simply not feasible
to use this device to remove sharks. Some caught sharks will twist
and spin when hauled to the vessel, which could cause the dehooker
to be lost overboard and be a hazard for crew to handle before being
lost overboard. Because sharks may be on the sea surface when being
hauled, some fishers were concerned that the incidence of having
branch lines break if a shark pulls the line would increase with use of a
dehooker because to use a dehooker requires bringing the shark close
to the vessel.
Gear soak time also likely affects the proportion of caught sharks
that are alive when hauled to the vessel. Changes to fishing gear and
methods that reduce soak time could contribute to reducing fishing
mortality of discarded sharks.
6.2.5. Reduce retention of sharks on branch lines
Retention of sharks on branch lines with wire leaders or other durable
material is substantially higher than in gear with no wire leader where
nylon monofilament is connected directly to the hook (Chapter 4) (e.g.,
Williams, 1997). For instance, an owner-operator of a vessel in the Japan
nearshore pelagic longline fishery who seasonally will target sharks
stated that he does not use a wire leader (nylon monofilament is used
through to the hook) when he is targeting tunas and billfish because
he believes that this gear design maximizes the catch of these target
species. He uses a 1 m wire trace at the hook during seasons when he
targets sharks (Clarke, this volume). Also, fishers in the Chile and Peru
artisanal longline mahi mahi and shark fisheries will add a wire leader
to increase shark catch rates (Alfaro-Shigueto et al., this volume; Mangel
and Alfaro-Shigueto, this volume).
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