Dolphins, Porpoises, and Whales - IUCN

Figure 3B
Bycatches in Fisheries
Although some incidental mortality of cetaceans has
occurred in fishing activities for a very long time, the significant
contributions of such mortality to the depletion of
cetacean populations have only been recognized during the
past 25 years. We are not aware of any instance before
the mid to late 1960s in which the magnitude of bycatches
was considered great enough to threaten populations.
Alarm over the killing of dolphins in the eastern tropical
Pacific tuna fishery (perhaps as many as seven million
since the late 1950s; IWC 1992a) stirred interest in other
forms of "incidental" mortality. The tuna-dolphin problem
is in fact best viewed as a case of deliberate utilization,
since the dolphin herds are chased and captured in the
purse seines in order to capture the schools of tuna associated
with them (see National Research Council 1992).
The first large-scale bycatch to have become well
known to cetologists, other than the kill of oceanic dolphins
in the tuna fishery, was that of Dall's porpoises in the
Japanese drift gillnet fishery for salmon in the North
Pacific (Fig. 4). Since that discovery, which was actually
made in the 1960s by Japanese investigators (Mizue and
Yoshida 1965) but not widely publicized until the mid
1970s (Ohsumi 1975), many additional cases have been
identified, such as the Italian swordfish driftnet fishery in
the Mediterranean Sea (Notarbartolo di Sciara 1990) and
the French tuna driftnet fishery in the northeastern Atlantic
(Goujon et al. 1993). Also, during the 1980s the scope of
the North Pacific drift gillnet problem widened, with more
nations becoming involved, additional target species being
taken such as squid, billfish, and tuna, and bycatches coming
to include other cetacean species such as northern right
whale dolphins and Pacific white-sided dolphins (IWC
1992a). Trawlers working in the Bay of Biscay are suspected
of making large bycatches of small cetaceans (A.
Collet, pers. comm.). In most cases, the cetaceans that
die are regarded as nuisances by the fishermen. Time and
effort are required to extricate the cetacean carcasses, and
fishing gear and catches are sometimes damaged. Since
the carcasses of incidentally caught animals are usually
discarded at sea, they provide no economic return and are
essentially "wasted." In some areas such as Peru, Sri
Lanka, and the Philippines, where artisanal gillnet fisheries
have made large bycatches of dolphins, the salvage and
use of carcasses has led to directed fisheries for cetaceans.