Shark Depredation and Unwanted Bycatch in Pelagic Longline

USA Atlantic, Gulf of Mexico and Caribbean Pelagic Longline Swordfish and Tuna Fisheries
if encompassing more areas, was derived for the aggregate of shark
species captured in PLL operations from 1992-2000 (Beerkircher
et al., 2002). As referenced, silky sharks are second only to blue
sharks in the total number of catches in this fishery. However, in
assessments of the SAB and FEC exclusively, silky sharks were the
predominant species taken (Beerkircher et al., 2002). Dusky and
night sharks also exceeded blue sharks for CPUE and overall catch
numbers. Quarterly deviations in CPUE also exist for several species
with silky shark catch rates the highest in the 1st and 4th quarters of
the year (Table A9.5). Comparable to the aforementioned declines
in blue shark and alternate species CPUE in recent years, these
authors also report a 1992-2000 decline when weighed against 1981
to 1983 estimates taken from Berkeley and Campos (1988) (Table
A9.6). These downward trends may reflect shifts in abundance but
could also be indicative of spatial and gear factors that have served to
reduce catchability (Beerkircher et al., 2002).
Based on the catch disposition figures derived by Hoey and Moore
(1999), rankings of the highest weight of shark species caught by area
have been tabulated (Table A9.7). Greater resolution of these rankings
reveals that apart from the MAB, NEC and NED, a shark species or
group never exceeded 100,000 pounds in a given area during the
collective years covered in the report (1990-1997). In over half of the
areas, the most captured shark by weight didn’t exceed ~ 20,000 lbs.
Because in rare instances, disproportionately high numbers of sharks
were caught when fishermen shifted to directed shark fishing (Hoey and
Moore, 1999), the degree of shark bycatch (excluding shortfin makos)
was likely even lower than the figures allude. However, the magnitude
of overall catch weight, data only representative of when observers
were present, is secondary. What is most germane is that fishing in
certain regions will lead to increased shark encounters. Moreover,
despite constituting the primary aggregate of bycatch species in the
PLL fishery (e.g. Beerkircher et al., 2002), shark catches aside from
those of blue sharks, do not appear exorbitantly high in U.S. domestic
operations; at least when compared to the catches by foreign fleets.
A9.2.3. Shark depredation
In addition to being captured as bycatch, sharks are also responsible
for removing baits and gear (bite-offs) and inflicting damage upon
other catch already hooked and thus less able to avoid predation.
Depredation thus represents a major nuisance to fishermen. To the
detriment of industry, POP data indicates that of all damage between
1990 and 1997 (4% of total observed catch), 68% occurred on catches
of swordfish, yellowfin and bigeye tuna collectively (Hoey and Moore,
1999). Although damaged catch has been reported in assessments
of catch disposition by several authors (e.g. Hoey and Moore, 1999;
Beerkircher et al., 2004; Diaz, 2006; Kerstetter and Graves, 2006B), the
available data are not usually resolvable to a level where possible to
definitively identify the depredating species (Lawrence Beerkircher,
personal communication). The point of interaction is rarely observed
to enable positive identification. In addition to sharks, cetaceans
(Hoey and Moore, 1999) are also responsible for damaging catch and
pilfering baits and gear. In fact, depredation by marine mammals
could equal or even exceed that done by sharks in certain areas such
as the MAB (Lawrence Beerkircher, personal communication). To
facilitate the ability to distinguish between the depredating species,
NMFS is implementing a new data collection protocol requiring more
comprehensive reporting details in early 2007 (Lawrence Beerkircher,
personal communication). However, even if able to distinguish
between damage done by sharks and other animals, interspecific
identification of the depredating animal will likely remain impossible.
If such data are deemed as important, accounting for auxiliary
factors such as the CPUE, abundance, distribution, and behavioral
tendencies of potentially depredating species may strengthen the
ability to identify the source species.
Importantly, depredations on target or alternative catch may not
always be accounted for. In cases where soak times are protracted,
animals that die or are weakened from stressors experienced
while hooked are much easier targets for scavenging species such
as blue sharks (Ward et al., 2004). Thus, reducing soak-times is a
straightforward method that could possibly mitigate both shark
catch rates and depredation.
A9.3. Potential Strategies to Reduce Shark
Interactions and Mortality
A9.3.1. That related to other taxa or fisheries
A comprehensive Atlantic PLL Take Reduction Plan (APLTRP)
was recently submitted to NOAA (2006). Although not directly
addressed, it is conceivable that some of the measures intended by
the Take Reduction Team (TRT) to reduce the number of serious
injury interactions with pilot whales and Risso’s dolphins could also
be successfully applied to sharks. The applicable proposals described
in the APLTRP include proper handling protocols; mandatory
mainline, gear and bait requirements to be discussed in greater detail
shortly; and additional time-area closures.
There are also existing regulations applying to the Atlantic and GOM
bottom longline fishery that may be effective tools in increasing
post-release survival for sharks taken by PLL (APLTRT, 2006). These
include the rapid release of non-retained species with an immediate
retrieval of gear remaining deployed; the use of line-cutters and
dip-nets for release purposes; and the maintenance of seawater flow
across the gills (as must be maintained in sawfish caught by bottom
longline). However, these should be viewed as techniques that could
potentially reduce the detrimental effects from, rather then the rates
of, interactions.
A9.3.2. That related to pelagic operations
POP data, as previously reported upon by Hoey and Moore (1999),
have revealed sets yielding disproportionately high shark catches.
In some instances, PLL fishermen have reportedly altered operative
strategies in order to land increased numbers of sharks. Alternatively,
a particular set of conditions may translate into higher catch rates.
Independent of whether or not intended however, these particular
sets can be analyzed for environmental and operative factors that
may have contributed to the higher catch rates. Accounting for such
variables could prove beneficial to industry if deriving measures to
help circumvent high numbers of shark interactions, or to enhance
survivorship of discards.
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