Shark Depredation and Unwanted Bycatch in Pelagic Longline

USA Atlantic, Gulf of Mexico and Caribbean Pelagic Longline Swordfish and Tuna Fisheries
and/or deeper lodging of J-style hooks. Otherwise, studies have
uniformly found no effect between the hook types on blue shark catch
rates (Yokota et al., 2006; Kerstetter et al., 2006; Kerstetter and Graves,
2006B). Although hooking has been found to be deeper (e.g. esophageal
or stomach) in J-style than in circle hooks independent of bait type
and offset (in circle hooks), catch mortalities of blue sharks have not
differed accordingly (Yokota et al., 2006; Kerstetter and Graves, 2006B;
Kerstetter et al., 2006). The lack of difference is likely ascribable to
the species already displaying high hooking survivorship, whereby
discerning a difference due to hook type is difficult. Anecdotally, the
conventional “Japanese (tuna) hooks”, which are currently prohibited
from use, may actually reduce shark catch rates more than the circle
hooks (John Hoey, personal communication).
Interestingly, it has been found that the use of mackerel baits reduced
the blue shark catch rates associated with both circle (31%) and J-style
(40%) hooks (Watson et al., 2005). Thus, a switch in bait regimes may
hold greater influence on mitigating shark bycatch than hook type.
In the best interest of management, conservation, and industry,
investigating the effects of both hook and bait types in additional
species of sharks caught by PLL is essential. The management regime
shift to circle hooks in the U.S. PLL fishery in 2004 has likely resulted
in a still-unquantified change in shark catch rates. Future analyses
of shark CPUE changes via POP/logbook data will require careful
comparison to account for the change in catchability between the
two hook types. Anecdotally, there are on-going studies assessing
the catch rate and hooking injury effects of circle hooks on a wide
array of shark species caught in PLL operations.
A9.3.4. Ongoing work on hook repellants
Historically, there have been numerous measures designed, tested
and implemented to repel sharks (see Sisneros and Nelson, 2001).
The primary impetus behind the majority of these has been to deter
shark attacks on humans. In response to the sometimes negative
implications of incidental shark takes in the PLL fishery however,
attention related to technologies with potential hook-repelling
implications has heightened in recent years.
Focusing prominently on semiochemical repellants, SharkDefense,
LLC (Oak Ridge, NJ) is actively engaged in field-testing products
with applications for the PLL fishery. According to their website1
(http://www.sharkdefense.com/Fisheries/fisheries.html), the company
is presently augmenting previously successful demonstrations
of chemical aversions in a mix of several (small and large coastal)
tropical reef-oriented species by field-testing semiochemicals and
other chemical synthetics repellants in pelagic species such as blue
sharks. Thus far, results have been promising. Presumably, this can
be ascribed to an apparent aversion in sharks to certain chemicals,
including ammonium acetate (a major component in decaying shark
flesh) and other semiochemicals emitted from predators (Sisneros
and Nelson, 2001). However, the methods of application and the
deployments of such technologies in real-time remain uncertain.
Field investigation into the efficacy of these practices has also proven
challenging in that it is supposedly difficult to administer and
monitor the repellants in a controlled fashion.
Fig. A9.7. Illustration of a magnet
prototype that received first prize
in the 2006 World Wildlife Fund
Smartgear Competition
(http://www.worldwildlife.org/
oceans/projects/smartgear.cfm).
Most recently, Michael Hermann, a partner of SharkDefense, was
awarded the World Wildlife Fund’s (WWF) 2006 International
‘Smart Gear” Competition grand prize for a technology intended
to deter sharks from hooks. The invention, a highly powerful but
small magnet deployed above the shaft of a hook (Fig. A9.5) is
meant to repel sharks through an overstimulation of their acutely
sensitive electromagnetic receptors. At present, the magnets have
successfully deterred Negaprion brevirostris (the lemon shark) and
Ginglymostoma cirratum (nurse shark) with little interference on
targeted species. Again however, the efficacy of this deterrent under
actual PLL conditions, and on pelagic species which certainly employ
very different foraging strategies, has yet to be documented.
A9.3.5. Next steps
To better evaluate the extent of shark bycatch in the Atlantic and
to derive means to reduce incidental interactions with sharks in the
PLL fishery, a more thorough analysis of the available data sources
(POP and logbook) must be conducted. Of utmost importance is the
treatment of seasonality, seawater temperature profiles, depth-offishing,
and geographic area as primary variables possibly impacting
shark catches (John Hoey, personal communication). The better
conditions leading to increased takes of specific shark species are
understood, the more feasible it will be to establish mitigation tools
and management strategies.
The reconciliation of POP and logbook data also represents a major
step to better understanding the extent of shark takes and would
expound upon advisable future management strategies in relation
to sharks. Although there has been some concordance in these data
(e.g. Brooks et al., 2005), the fishing industry has anecdotally alluded
to an often wide discrepancy between perceptions by fishermen
and observers. For example, one anonymous fisherman vicariously
described a true case of divergence over how many blue sharks
had been caught across several sets on the Grand Banks: “On what
I think was the first trip he took an observer on back in the early
‘80s, the observer asked how many blue [sharks] they caught. [The]
“unnamed fisherman” guessed about 250. The observer said, 80. The
numbers might not be exactly those, but the “unnamed fisherman”
was really taken aback that he’d misjudged the number so wildly.
I think it just emphasizes how perception seems to alter reality.”
This example illustrates that amidst the demand and commotion of
fishing operations, the under- and/or over-reporting of non-targeted
catch is plausible, in which cases the resulting data can grossly
misrepresent what is actually occurring.
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