Shark Depredation and Unwanted Bycatch in Pelagic Longline

Chapter 1
Introduction and Methods
1.1. Bycatch and Depredation, Sharks and Pelagic
Longline Fisheries
Bycatch 1 in marine fisheries is an increasingly prominent
international ecological, social, and economic issue (Alverson et al.,
1994; IUCN, 1996a,b, 2000a,b; Hall et al., 2000; Cook, 2001; Gilman,
2001; Dobrzynski et al., 2002; FAO, 1999a,b,c, 2004a,b; Gilman
et al., 2005, 2006a,b,c,d, 2007). The issue has been considered
in a growing number of international conventions, treaties, and
resolutions, including Agenda 21 (1992); the Cancun Declaration
(1992); UN General Assembly Resolutions 49/118 (1994) and 50/25
(1995); the Rome Consensus on World Fisheries (1995); the Kyoto
Declaration and Plan of Action (1995), the United Nations Food
and Agriculture Organization International Code of Conduct for
Responsible Fisheries (1995) and International Plan of Action for
Reducing Incidental Catch of Seabirds in Longline Fisheries (1999),
and Agreement on the Conservation of Small Cetaceans in the Baltic
and North Seas Resolution on Incidental Take of Small Cetaceans
(2003) (Haward et al., 1998; Hall et al., 2000; Gilman, 2001).
Economic effects on fisheries from bycatch include the imposition of
a range of restrictions, closed areas, embargos, and possible closures;
fishery interactions, where bycatch in one fishery reduces target
catch in another, and bycatch of juvenile and undersized individuals
of a commercial species can adversely affect future catch levels
(Hall et al., 2000).
Bycatch raises ecological concerns, as some bycatch species of
marine mammals, seabirds, sea turtles, sharks 2 , and other fish species
are particularly sensitive to increased mortality above natural levels
because of their life history traits, including being long-lived, having
delayed maturity, and having low reproductive rates (Musick et al.,
2000; Bonfil, 2002; Gilman et al., 2005, 2006b). Bycatch can alter biodiversity
by removing top predators and prey species at unsustainable
levels. It also alters foraging habits of species that learn to take
advantage of discards (Hall et al., 2000).
Discarded bycatch is a social issue over waste. Alverson et al. (1994)
estimated that in 1994 about 27 million metric tons (27% of the
world catch), ranging between 17.9 and 39.5 million tons, of fish per
year were discarded at sea. FAO (1999c) estimated that 1998 global
marine fisheries fish discards totaled 20 million metric tons.
Depredation, the partial or complete removal of hooked fish and bait
from fishing gear, is conducted primarily by cetaceans and sharks
in pelagic longline fisheries. Economic losses from depredation can
be substantial (Lawson, 2001; Nishida and Shiba, 2002). Depredation
also raises ecological concerns as these interactions may change
cetacean and shark foraging behavior and distribution, increase
fishing effort, and confound fish stock assessments, as well as result
in deliberate injury and mortality of cetaceans and sharks by fishers
to discourage depredation and avoid future interactions (Gilman et
al., 2006d).
1
‘Bycatch’ is used in this report to refer to the retained catch of non-targeted species or ‘incidental catch’, plus all discards (McCaughran, 1992; Alverson et al., 1994). ‘Target’
catch is the catch of a species or species assemblage primarily sought in a fishery, while ‘non-target’ catch is the catch of a species or species assemblage not primarily sought.
‘Incidental’ catch is the portion of non-target catch that is retained, while ‘discards’ is the portion of non-target catch that is not retained (McCaughran, 1992; Alverson et
al., 1994).
2
The term ‘sharks’ is generally used in this report to refer to the Chondrichthyan fishes, which comprise elasmobranchs (sharks, skates and rays) and holocephalans
(chimaeroids).