Shark Depredation and Unwanted Bycatch in Pelagic Longline

Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries
deeper setting fisheries. Gear designs are extremely variable between
fisheries, as well as from seaport to seaport, by season and fishing
ground within a fishery, between vessels within a fishery, and for
some parameters, within the gear of an individual vessel.
Incentives to reduce shark interactions in pelagic longline fisheries
vary along a continuum, based on whether sharks represent an
economic disadvantage or advantage. On one extreme, there
are pelagic longline fisheries with a regulatory framework
limiting shark catches or placing restrictions on shark handling,
or lack of markets for shark products, resulting in negligible
retention of sharks. In these fisheries, the costs from shark
interactions exceed benefits from revenue from sharks, due to:
(i) Depredation;
(ii) Damage and loss of gear;
(iii) Reduced catch of marketable species due to baited hooks being
occupied or removed by sharks;
(iv) Risk of crew injury from handling caught sharks and being hit
by weights when branch lines containing sharks break during
gear retrieval; and
(v) Reduced fishing efficiency due to the time required to remove
sharks from gear for discarding and to repair and replace gear.
Fishers identified the time required to repair damaged and lost gear
from shark interactions, and to remove sharks to be discarded from
gear, as a substantial problem. Lost revenue from shark damage to
target species can amount to several thousand U.S. dollars in a single
set in some fisheries. In some of these fisheries, there is large interest
in minimizing shark interactions. On the other extreme, there are
pelagic longline fisheries where revenue from sharks exceeds costs
from shark interactions, a large proportion of caught sharks are
retained (> 99% in some fisheries), and sharks are either always
an important target species, are targeted seasonally or at certain
fishing grounds proximate to ports where there is demand for shark
products, or are an important incidental catch species.
Several nations, including half of the countries covered in this
study (Australia, Italy, South Africa, and U.S.A.), have legally
binding prohibitions in effect on the removal of shark fins
(including the tail) and discarding of the remainder of the shark at
sea in their pelagic longline fisheries. Some of the longline fisheries
included in this study are subject to prohibitions on the use of wire
leaders (Australia, South Africa), have shark retention trip limits
(Australia, South Africa), and have size limits for certain shark
species (Peru). Fishers in the Italian sector of the Mediterranean
industrial longline swordfish fishery are unaware of the finning
restriction, and thus the legislation does not affect their practices.
However, no shark finning is reported to occur in the fishery
due to the lack of a local market for the fins. In the fisheries in
Australia, South Africa, and Hawaii, U.S.A., finning restrictions have
substantially reduced shark retention, most captured sharks are now
discarded alive, while revenue from sharks has been substantially
reduced. Japanese longline fishermen in the distant water fleet have
adapted to finning regulations applicable in some areas by landing
sharks in recently developed local markets rather than by attempting
to avoid shark interactions. In waters without finning regulations,
including Japanese waters and the North Pacific, sharks are either
finned or landed whole, and in either case the ability to sell shark
products has contributed to a lack of interest in reducing shark
bycatch. A trip limit on the retention of shark carcasses in Australia
has likely had no effect on industry practices, as vessels typically will
not reach the per-trip limit for shark species that are worth retaining.
A shark per-trip retention limit in South Africa may have a large
adverse economic effect; however, there may be low compliance.
A shark size limit in effect in the Peru longline fishery is not strongly
enforced, and few fishers are aware of the rule. Of the few fishers
who reported that they were aware of the regulations, all report
that they still retain sharks that are under the minimum size limit.
A prohibition on the use of a wire trace in the Australia and South
Africa longline fisheries has resulted in an increased economic cost
from shark interactions from an increase in lost terminal tackle. In
fisheries where most sharks are released alive, it is not known how
the injury to sharks from retaining hook and trailing monofilament
line affects their survival prospects compared to being caught on
lines with wire trace. In fisheries where a large proportion of caught
sharks are killed either for retention or discarding, prohibiting wire
leaders will likely reduce shark fishing mortality. Prohibiting wire
leaders may exacerbate seabird bycatch problems: Fishers will be less
likely to attach weights close to hooks on branch lines lacking a wire
leader due to safety concerns, thus, reducing the baited hook sink
rate, and increasing seabird catch rates.
In fisheries where there is an incentive to avoid shark interactions,
predominant shark avoidance practices are to:
(i) Avoid fishing in areas known to have high shark interactions;
and
(ii) Change fishing grounds when shark catch rates, depredation
and gear loss are high but the target species catch rate is low.
Some fishers indicate that using fish instead of squid for bait reduces
shark interactions. Experimental trials have shown that using fish
instead of squid as bait results in a significant and large decrease in
shark catch rates. However, results of trials of different hook types
are less conclusive, with some studies showing small but significant
increases in shark catch rates with a circle hook, and other studies
indicating no difference between circle and other hooks. Longline
fishers identified numerous fishing methods and gear characteristics
that they conventionally employ to maximize catch rates of non-shark
target species, which may contribute to reducing shark catch rates.
For instance, the depth of baited hooks; timing of gear setting, soak
and hauling; location of fishing grounds in relation to topographic
and oceanographic features (e.g., position with regard to oceanic
fronts); type and size of bait and hook; leader material; non-use of
lightsticks; and other fishing methods and gear designs selected by
fishermen to maximize their non-shark target species catch rates may
be effective shark avoidance strategies. Research is needed to improve
the understanding of the shark avoidance efficacy of these practices.
Beyond these strategies, the state of knowledge to reduce unwanted
bycatch and depredation by sharks in pelagic longline fisheries is
poor. A prioritized next step is to test promising, new strategies