Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Economic, Practical, Ecological <strong>and</strong> Social Problems from <strong>Shark</strong> - Longl<strong>in</strong>e Interactions<br />
Results from this study reveal that there has been a large <strong>in</strong>crease<br />
<strong>in</strong> the dem<strong>and</strong> for shark f<strong>in</strong>s <strong>and</strong> meat <strong>and</strong> catch of sharks over the<br />
past several decades, <strong>and</strong> dem<strong>and</strong> for shark meat may cont<strong>in</strong>ue to<br />
<strong>in</strong>crease (Williams, 1997; McCoy <strong>and</strong> Ishihara, 1999; FAO, 2006;<br />
Alfaro-Shigueto et al., this volume; Clarke, this volume; Mangel <strong>and</strong><br />
Alfaro-Shigueto, this volume). For <strong>in</strong>stance, shark catch by weight<br />
<strong>in</strong> Chile fisheries has <strong>in</strong>creased an order of magnitude from about<br />
1000 tons <strong>in</strong> 1950 to over 10,000 tons <strong>in</strong> 2005 (FAO, 2006). Also,<br />
the shark catch <strong>in</strong> Peruvian fisheries <strong>and</strong> export market for frozen<br />
shark meat has grown, where the re-venue from shark meat exceeds<br />
revenue from f<strong>in</strong>s on a per-trip basis for a vessel <strong>in</strong> the Peru artisanal<br />
mahi mahi <strong>and</strong> shark longl<strong>in</strong>e fishery (Mangel <strong>and</strong> Alfaro-Shigueto,<br />
this volume). From 2000 to 2005 exports of shark meat from Peru<br />
tripled, with ma<strong>in</strong> export markets <strong>in</strong>clud<strong>in</strong>g Uruguay, Spa<strong>in</strong>, Brazil<br />
<strong>and</strong> Colombia (PROMPEX, 2006; Mangel <strong>and</strong> Alfaro Shigueto,<br />
this volume). Clarke (this volume) identifies a trend <strong>in</strong> exp<strong>and</strong><strong>in</strong>g<br />
dem<strong>and</strong> for shark meat <strong>in</strong> a few regions <strong>in</strong> Japan where offshore <strong>and</strong><br />
nearshore vessels l<strong>and</strong> their catch as well as at several foreign seaports<br />
where distant water longl<strong>in</strong>e vessels l<strong>and</strong> their catch, <strong>in</strong>clud<strong>in</strong>g Cape<br />
Town (South Africa), Callao (Peru), Las Palmas (Spa<strong>in</strong>), Balboa<br />
(Panama), Cartagena (Venezuela) <strong>and</strong> Port Louis (Mauritius), <strong>and</strong> a<br />
concomitant <strong>in</strong>crease <strong>in</strong> retention <strong>and</strong> l<strong>and</strong><strong>in</strong>g of shark carcasses by<br />
the Japan longl<strong>in</strong>e fisheries. The shark meat l<strong>and</strong>ed <strong>in</strong> Callao, Cape<br />
Town <strong>and</strong> Las Palmas may be exported to European markets <strong>in</strong> Italy<br />
<strong>and</strong> Spa<strong>in</strong> (Clarke, this volume).<br />
5.2. Ecological Concerns<br />
There is an ecological basis for concern over shark <strong>in</strong>teractions<br />
<strong>in</strong> pelagic longl<strong>in</strong>e fisheries. In the last decade, as elasmobranch<br />
catches have <strong>in</strong>creased <strong>in</strong> both directed <strong>and</strong> <strong>in</strong>cidental fisheries,<br />
there has been <strong>in</strong>creas<strong>in</strong>g concern about the status of some<br />
shark stocks, the susta<strong>in</strong>ability of their exploitation <strong>in</strong> world<br />
fisheries, <strong>and</strong> ecosystem-level effects from shark population<br />
decl<strong>in</strong>es (e.g., FAO, 1999b; Bonfil, 2002; Baum et al., 2003; Baum<br />
<strong>and</strong> Meyers, 2004; Ward <strong>and</strong> Meyers, 2005). 4 Most shark species<br />
are predators at the top of the food cha<strong>in</strong> <strong>and</strong> characterized by<br />
relatively late maturity, long life, slow growth, low fecundity <strong>and</strong><br />
productivity (small <strong>and</strong> <strong>in</strong>frequent litters), long gestation periods,<br />
high natural survivorship for all age classes, <strong>and</strong> low abundance<br />
(K-selected life history strategies) relative to bony fish such as tunas<br />
<strong>and</strong> billfishes <strong>and</strong> to organisms at lower trophic levels (Fowler et<br />
al., 2005). Some shark species may also aggregate by sex, age <strong>and</strong><br />
reproductive stage (Heberer <strong>and</strong> McCoy, 1997; Cailliet et al., 2005).<br />
These life history characteristics make sharks particularly vulnerable<br />
to overexploitation <strong>and</strong> slow to recover from large population<br />
decl<strong>in</strong>es (Musick et al., 2000; Cailliet et al., 2005). Directed shark<br />
fisheries <strong>in</strong> North America provide examples of overfish<strong>in</strong>g<br />
<strong>and</strong> population decl<strong>in</strong>es, such as occurred <strong>in</strong> directed fisheries<br />
for the porbeagle (Lamna nasus) (Casey et al., 1978), soupf<strong>in</strong><br />
shark (Galeorh<strong>in</strong>us zyopterus) (Ripley, 1946), <strong>and</strong> sp<strong>in</strong>y dogfish<br />
(Squalus acanthias) (Rago et al., 1998). Also, for example, the lack<br />
of monitor<strong>in</strong>g of primarily discarded bycatch of the barndoor<br />
skate (Dipturus laevis) <strong>in</strong> the western North Atlantic bottom trawl<br />
fisheries resulted <strong>in</strong> a large population decl<strong>in</strong>e (Musick, 2005)<br />
The ma<strong>in</strong> threats faced by chondrichthyans are various fish<strong>in</strong>g<br />
activities <strong>and</strong> habitat degradation <strong>and</strong> loss (Stevens et al., 2005).<br />
Reviews of assessments of the threatened status of sharks <strong>and</strong> related<br />
taxa undertaken to date <strong>in</strong>dicate that the taxa at highest risk <strong>in</strong>clude<br />
commercially exploited species of deepwater sharks, species restricted<br />
to freshwater <strong>and</strong> brackish water habitats <strong>and</strong> coastal endemics whose<br />
entire range overlaps with fish<strong>in</strong>g effort (CITES, 2003). However, a lack<br />
of both fundamental biological <strong>in</strong>formation <strong>and</strong> fishery-dependent<br />
data for most shark species (Cailliet et al., 2005; Musick, 2005) means<br />
that there is a high degree of uncerta<strong>in</strong>ty <strong>in</strong> the status of these species.<br />
The biology of the chondrichthyan fishes is the least understood of all<br />
the major mar<strong>in</strong>e vertebrate groups, where detailed <strong>in</strong>formation on<br />
life history <strong>and</strong> reproductive dynamics is not available for all but a few<br />
of species important for directed fisheries (Cailliet et al., 2005). There<br />
is a general lack of reliable <strong>and</strong> sufficiently detailed fishery-dependent<br />
data on shark species to enable susta<strong>in</strong>able management (Shotton,<br />
1999; Musick, 2005). <strong>Pelagic</strong> longl<strong>in</strong>e fisheries operat<strong>in</strong>g on the high<br />
seas are not likely <strong>in</strong>teract<strong>in</strong>g with these shark species identified as<br />
highest-risk, while some coastal pelagic longl<strong>in</strong>e fleets could be<br />
catch<strong>in</strong>g at-risk coastal endemics. In particular, blue sharks (Prionace<br />
glauca), the dom<strong>in</strong>ant species of shark caught <strong>in</strong> most pelagic longl<strong>in</strong>e<br />
fisheries operat<strong>in</strong>g on the high seas (e.g., Williams, 1997; Francis et al.,<br />
2001), are less vulnerable to overfish<strong>in</strong>g relative to other shark species<br />
due to their be<strong>in</strong>g relatively prolific <strong>and</strong> resilient (Smith et al., 1998;<br />
Cortes, 2002). Blue sharks comprise the largest proportion of shark<br />
species caught <strong>in</strong> all 12 of the fisheries <strong>in</strong>cluded <strong>in</strong> this study, rang<strong>in</strong>g<br />
from 47% - 92% of shark catch <strong>in</strong> fisheries where this <strong>in</strong>formation is<br />
available. Kleiber et al. (2001) conducted a stock assessment of blue<br />
sharks <strong>in</strong> the North Pacific <strong>and</strong> concluded that blue sharks are not<br />
be<strong>in</strong>g overfished <strong>in</strong> the North Pacific. However, more recent research<br />
by Clarke et al. (2006) suggests that blue sharks globally are be<strong>in</strong>g<br />
captured at levels close to or possibly exceed<strong>in</strong>g maximum susta<strong>in</strong>able<br />
yield. Clarke et al. (2006) estimated global shark catches us<strong>in</strong>g shark<br />
f<strong>in</strong> trade records, <strong>and</strong> found that shark biomass <strong>in</strong> the f<strong>in</strong> trade is three<br />
to four times higher than shark catch figures reported by the Food<br />
<strong>and</strong> Agriculture Organization of the United Nations, which is the sole<br />
exist<strong>in</strong>g global database. Additional stock assessments for other pelagic<br />
sharks have been conducted only by the International Commission for<br />
the Conservation of Atlantic Tunas for blue <strong>and</strong> shortf<strong>in</strong> mako sharks<br />
<strong>in</strong> the North <strong>and</strong> South Atlantic (Anonymous, 2005).<br />
4<br />
Atlantic blue sharks are among those species reported to have undergone considerable population decl<strong>in</strong>es (Baum et al., 2003). Consistent with previous arguments<br />
aga<strong>in</strong>st treat<strong>in</strong>g CPUE as an <strong>in</strong>dex of abundance (Cooke <strong>and</strong> Bedd<strong>in</strong>gton, 1984), the reported blue shark decl<strong>in</strong>e (~60% s<strong>in</strong>ce 1986) postulated by Baum et al. (2003) has<br />
been questioned by several authors (e.g. Burgess et al., 2005a,b; Campana et al., 2005) on the basis that potential reasons for drops <strong>in</strong> CPUE aside from abundance<br />
decl<strong>in</strong>es were not accounted for, such as underreport<strong>in</strong>g, changes <strong>in</strong> fish<strong>in</strong>g grounds <strong>and</strong> changes <strong>in</strong> fish<strong>in</strong>g gear such as not us<strong>in</strong>g wire leaders (Brooks et al., 2005). It is<br />
acknowledged, however, that the species has likely endured some level of decl<strong>in</strong>e <strong>in</strong> recent years (Brooks et al., 2005; Campana et al., 2006).<br />
21