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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<strong>Shark</strong> <strong>Depredation</strong> <strong>and</strong> <strong>Unwanted</strong> <strong>Bycatch</strong> <strong>in</strong> <strong>Pelagic</strong> Longl<strong>in</strong>e Fisheries<br />
Modified techniques that have been used to l<strong>and</strong> more sharks<br />
<strong>in</strong>clude fish<strong>in</strong>g <strong>in</strong> more shallow depths, <strong>in</strong>creas<strong>in</strong>g the numbers of<br />
hooks deployed between floats, <strong>and</strong> generally sett<strong>in</strong>g gear closer to<br />
the bottom, presumably to l<strong>and</strong> more coastal sharks dur<strong>in</strong>g opened<br />
seasons (Hoey <strong>and</strong> Moore, 1999). These strategies are likely to yield<br />
high shark catch rates under certa<strong>in</strong> conditions, at least <strong>in</strong> relation to<br />
large coastal species. For <strong>in</strong>stance, POP data have revealed that 67% of<br />
silky sharks taken <strong>in</strong> the GOM were caught <strong>in</strong> depths <strong>in</strong>ferior to 1000<br />
meters, while the majority of dusky sharks (SAB) were captured <strong>in</strong> less<br />
than 500 meters. As a basis of comparison, blue sharks <strong>and</strong> tuna were<br />
caught most frequently beyond the 1000 meter depth contour, while<br />
mako <strong>and</strong> common thresher sharks just around that threshold (Hoey<br />
<strong>and</strong> Moore, 1999). In the GOM, conditions for highest silky shark<br />
catches closely resembled those for swordfish, morn<strong>in</strong>g gear retrievals<br />
from less than 1000 meter depths. The avoidance by tuna-target<strong>in</strong>g<br />
vessels of areas where swordfish catches are high (shallow depths,<br />
< 500 meters, <strong>in</strong> the GOM, SAB, <strong>and</strong> FEC) should thus simultaneously<br />
reduce <strong>in</strong>cidental silky shark catches (Hoey <strong>and</strong> Moore, 1999).<br />
Additional studies have also documented the effects of gear shifts on<br />
CPUE. For example, the use of rope/steel (“Yankee”) gangions has<br />
yielded a lower CPEU of juvenile s<strong>and</strong>bar sharks than when us<strong>in</strong>g<br />
monofilament gangions (Branstetter <strong>and</strong> Musick, 1993). In another<br />
study, percent-capture of blue shark with the use of monofilament<br />
gangions (66%) exceeded that when employ<strong>in</strong>g multifilament<br />
gangions (34%) (Stone <strong>and</strong> Dixon, 2001). Shortf<strong>in</strong> mako shark catches<br />
adhered to the same pattern (60% <strong>and</strong> 40% for ‘mono’ <strong>and</strong> ‘multi’<br />
respectively). Stone <strong>and</strong> Dixon (2001) surmised that the relative<br />
aversion to the multifilament gangion could have been a function<br />
of strong visual acuity, a trait shared by pelagic predators that often<br />
hunt nocturnally. Investigat<strong>in</strong>g the effects of these alternate gears<br />
on additional elasmobranch species is certa<strong>in</strong>ly warranted for<br />
st<strong>and</strong>ardiz<strong>in</strong>g historical catch rates.<br />
A9.3.3. Effects of J-style versus circle hooks<br />
As said, shifts <strong>in</strong> gear regimes can <strong>in</strong>fluence the catchability of<br />
both targeted <strong>and</strong> non-targeted species. A plethora of studies have<br />
addressed the effects on catch rates, location of hook<strong>in</strong>g (e.g. <strong>in</strong>ternal<br />
versus external; gut versus mouth hook<strong>in</strong>g), <strong>and</strong> result<strong>in</strong>g <strong>in</strong>juries<br />
<strong>and</strong> mortalities <strong>in</strong>duced by differ<strong>in</strong>g hook types <strong>in</strong> teleosts (e.g.<br />
Bacheler <strong>and</strong> Buchel, 2004; Cooke et al., 2005), pelagic teleosts (e.g.<br />
Domeier et al., 2003; Kerstetter <strong>and</strong> Graves, 2006A; Kerstetter et al.,<br />
2006; Kerstetter <strong>and</strong> Graves, 2006B) <strong>and</strong> <strong>in</strong> sea turtles (see Garrison,<br />
2003; Watson et al., 2005). Although sharks have only been directly<br />
(Yokota et al., 2006) or secondarily (Watson et al., 2005; Kerstetter<br />
<strong>and</strong> Graves, 2006B) addressed <strong>in</strong> a few such studies, methodologies<br />
<strong>in</strong> which to do so <strong>in</strong> future studies are now well established.<br />
In general, these studies have found that the use of 18/0 (non-offset)<br />
circle hooks compared to 9/0 <strong>and</strong> 10/0 J-style hooks (<strong>in</strong> some cases<br />
<strong>in</strong> conjunction with mackerel bait) reduce the bycatch of sea turtles,<br />
<strong>in</strong>crease the catch of target species such as yellowf<strong>in</strong> <strong>and</strong> bigeye<br />
tuna, <strong>in</strong>crease the number of animals hooked <strong>in</strong> the mouth or jaw as<br />
opposed to the esophagus or gut, <strong>and</strong> mitigate the degree of hook<strong>in</strong>g<br />
mortality <strong>and</strong> (estimated) post-release mortality of bycatch species.<br />
In response to these results, NMFS imposed a policy <strong>in</strong> August 2004<br />
requir<strong>in</strong>g all PLL vessels, time <strong>and</strong> area-<strong>in</strong>dependent (exclud<strong>in</strong>g the<br />
NED), to use only 16/0 or larger non-offset circle hooks <strong>and</strong>/or 18/0<br />
or larger circle hooks with an offset not exceed<strong>in</strong>g 10 degrees (NMFS,<br />
2005; NMFS, 2006). Moreover, only whole f<strong>in</strong>fish <strong>and</strong> squid baits<br />
may be utilized <strong>in</strong> the majority of areas, while only whole mackerel<br />
<strong>and</strong> squid baits may be possessed <strong>and</strong>/or utilized <strong>in</strong> the NED (when<br />
opened) (NMFS, 2006).<br />
The potentially positive impact of circle-hook usage on hook<strong>in</strong>g<br />
mortality <strong>and</strong> ultimate discard survivability is not a trivial issue<br />
for sharks. Significant <strong>in</strong>terspecific differences <strong>in</strong> catch mortality<br />
rates have been found <strong>in</strong> sharks captured by PLL operations (Table<br />
A9.8) (Beerkircher et al., 2002). Even if a pelagic species is discarded<br />
without overt physical depredatory damage, it can still succumb<br />
due to physiological stress (Moyes et al., 2006), <strong>and</strong> cryptic <strong>in</strong>juries<br />
<strong>in</strong>curred while on or while be<strong>in</strong>g removed from the hook. Exclud<strong>in</strong>g<br />
tiger sharks <strong>and</strong> rays addressed <strong>in</strong> Beerkircher et al. (2002), most<br />
species demonstrated moderate to high rates of mortality upon gear<br />
retrieval. Even the resilient blue shark has exhibited modest (7-19% as<br />
cited earlier) degrees of hook<strong>in</strong>g mortality. Thus, although difficult to<br />
dist<strong>in</strong>guish by <strong>in</strong>vestigation, there is room for survival enhancement<br />
<strong>in</strong> even the most resilient species. The less <strong>in</strong>jurious potential of<br />
circle hooks certa<strong>in</strong>ly bodes well for the variety of by-caught sharks<br />
if adher<strong>in</strong>g to the same superficial hook<strong>in</strong>g trends as <strong>in</strong> other species.<br />
Among the studies consider<strong>in</strong>g shark catchability as a function of hook<br />
<strong>and</strong>/or bait type, the results are <strong>in</strong>conclusive regard<strong>in</strong>g catch rates <strong>and</strong><br />
the use of circle hooks. Watson et al. (2002) found circle hooks to yield<br />
higher catch rates of blue sharks than with J-style hooks, <strong>in</strong>dependent<br />
of bait type. This discrepancy, however, was acknowledged as a<br />
possible function of <strong>in</strong>creased bite-offs of the monofilament leaders<br />
(<strong>and</strong> thus high unaccounted blue shark catch) due to the swallow<strong>in</strong>g<br />
Table A9.8. Catch status of elasmobranchs observed <strong>in</strong> the pelagic longl<strong>in</strong>e<br />
fishery off the southeastern U.S., 1992–2000 (Beerkircher et al., 2002)<br />
Species Alive Dead Unknown Damaged<br />
%<br />
Dead<br />
Silky 487 949 0 10 66.3<br />
Dusky 348 325 0 6 48.7<br />
Night 110 451 0 11 80.8<br />
Blue 381 49 0 4 12.2<br />
Tiger 255 8 0 0 3.0<br />
Scalloped<br />
hammerhead<br />
Oceanic<br />
whitetip<br />
77 117 1 5 61.0<br />
95 36 0 0 27.5<br />
Rays 113 0 0 0 0.0<br />
S<strong>and</strong>bar 82 29 0 1 26.8<br />
Bigeye<br />
thresher<br />
Shortf<strong>in</strong><br />
mako<br />
38 43 0 1 53.7<br />
52 28 0 0 35.0<br />
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