Dolphins, Whales and Porpoises: 2002-2010 Conservation - IUCN

Figure 4. Risso’s dolphin is one of many species of cetaceans taken in
Sri Lankan waters, where a directed fishery for dolphins and whales
emerged as markets developed for cetacean meat obtained as fishery
bycatch, 1985. Photo: Steve Leatherwood.
(making it difficult to quantify removals), and
fishermen remain incredulous of the idea that
bycatch is a serious problem.
in space and time with pelagic trawl fishing. It is clear that
mortality of small delphinids in pelagic trawl fisheries has not
been sufficiently recognized or studied in European waters,
even though it could be having population-level effects
(Tregenza and Collet 1998).
In most cases, fishermen regard the cetaceans that die
incidentally in fishing gear as nuisances. Time and effort are
required to extricate the carcasses, and the gear and catch are
sometimes damaged. Since 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 gillnetting has
caused the deaths of large numbers of small cetaceans,
markets have emerged for cetacean meat, leading to directed
hunts (Figure 4).
Incidental mortality of cetaceans also results from entanglement
in derelict fishing gear (“ghost nets”) and ingestion
of plastic bags (Cagnolaro and Notarbartolo di
Sciara 1992). Marine debris pollution is a global problem,
and its impact on marine animal populations is extremely
difficult to evaluate (Laist et al. 1999).
There is a clear and longstanding need for fishery
agencies and managers at all levels to incorporate bycatch
monitoring and bycatch reduction measures into management
regimes. It is a major challenge for fishery managers to
convince fishermen that bycatch is a problem. This may
pertain especially to cetacean bycatch where the cetacean
population has already been reduced to low densities and
therefore a bycatch is a rare event (e.g., harbor porpoises in
the Baltic Sea). Very low bycatch rates are difficult and
costly to measure, and it is similarly difficult and costly to
obtain precise abundance estimates in areas where cetaceans
occur in low densities. Therefore, without bycatch mitigation,
cetaceans remain scarce (making it difficult to obtain
good abundance estimates), the bycatch remains small
Indirect effects of industrial
fisheries
Large-scale industrial fisheries may have
serious long-term consequences for cetacean
populations quite apart from the deaths caused
by entanglement in fishing gear. Unfortunately,
the indirect effects are extremely hard to document
and have rarely been evaluated. Of greatest
concern are high-seas fisheries that extract
vast amounts of fish and squid biomass from
the world’s oceans, and transform biological
communities in the process (e.g., Jakobsson
1985). Fleets of large bottom and mid-water
trawlers and jigging vessels, especially those
with factories on board, possess fishing capacities
that allow them to exploit biological systems
at unprecedented levels and rates. Trawlers target
particular species but are indiscriminate in what they take.
Large bycatches of non-target species are always associated
with trawl fisheries. Squid-jigging vessels are highly
selective and have little or no bycatch, but they can account
for large biomass extraction. In some instances, small-scale
coastal and freshwater fisheries have been shown to have
similarly devastating system-level effects (e.g., Alcala and
Vusse 1993). In the Mediterranean Sea, the combination of
some 50,000–100,000 small gillnet fishing boats, plus large
bottom trawlers, has depleted numerous fish, crustacean,
and mollusk populations, and much the same can be said of
the North Sea.
Market policies and foreign investment in most Latin
American and Caribbean countries have created incentives
for fisheries to expand into little-exploited or nearly pristine
areas. These regions presently provide more than 20% of
total world fishery landings. From the late 1980s to late
1990s, the fleet of large trawlers targeting common hake
(Merluccius hubbsi) and shrimp in the south-western
Atlantic Ocean grew to about 200 vessels, and biomass
extraction increased from about 0.3–1.2 million tons per
year (Crespo, unpublished data). During the mid-1990s,
some seven tons of bycatch were discarded (dumped back
into the sea) per day per vessel, with each vessel fishing for
an average of 300 days per year. The hake fishery involves
the capture of more than 40 non-target species in coastal
waters and at least 20 in offshore shelf waters. Therefore,
even if the hake and shrimp stocks targeted by trawlers were
themselves unimportant as prey for cetaceans (in fact they
are important, Koen Alonso et al. 1998, 2000), some of the
by-caught species certainly would be. This situation is only
one example of what is undoubtedly a more widespread
phenomenon.
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