Dolphins, Porpoises, and Whales - IUCN

density in brackish, mangrove-lined estuaries and apparently
limited movement close along exposed coastlines (cf.
Spaans 1990). The total population size has not t)een estimated
but is certainly low compared with those of the more
widely distributed and gregarious delphinids. Maigret
(1980) estimated that no more than 100 hump-backed dolphins
were present in Banc d'Arguin, Mauritania, or in the
Saloum Delta, Senegal (also see Beaubrun 1990).
Klinowska (1991) summarized the potential threats to
the species, including accidental capture in fishing gear and
reduction of prey populations by overfishing or habitat
destruction. Directed hunting for these dolphins is not
known to occur systematically in any area but may occur
in at least Senegal (T.A. Jefferson, pers. comm.). Nevertheless,
study of Atlantic hump-backed dolphins is accorded
a high priority because of their restricted range, narrow ecological
niche, and presumed low population size.
Pantropical Spotted Dolphin
{Stenella attenuata)
This widespread, abundant dolphin was only recently distinguished
from Stenella frontalis, with which it is
broadly sympatric in much of the Atlantic part of its range
(Perrin et al. 1987, Perrin and Hohn 1994). The pantropical
spotted dolphin occurs in groups of a few individuals
to several thousand animals, and it inhabits both nearshore
and oceanic habitats in tropical and warm-temperate
areas. Along with Stenella longirostris this species has
borne the brunt of the massive killing by tuna purse seiners
in the eastern tropical Pacific (ETP). More than 60%
of the total dolphin kill in this fishery has involved animals
of the northern offshore stock oi Stenella attenuata
(IWC 1992a). The current northeastern stock of approximately
730,000 is estimated to be less than a quarter of
its size in 1959 (Wade 1993a). Declines in the western/
southern offshore and coastal ETP stocks are less well
documented. It is important to recognize that the impact
of purse seining on dolphins extends beyond direct mortality
and injury to include disruption of social relationships
and behavior, increased vulnerability to predation,
and "stress" effects (e.g. My rick 1988). (These concerns
are equally relevant to other species involved in the purseseine
fishery.) According to W.F. Perrin (pers. comm.)
the conservation problem caused by mortality of spotted
dolphins in the ETP tuna fishery has been largely solved,
with the estimated level of mortality less than 1% of estimated
current population size.
In addition to the serious conservation problems caused
by tuna purse seining, pantropical spotted dolphins have
been heavily exploited in the Japanese drive and cull fisheries
(Kasuya 1985a, 1985b, Kishiro and Kasuya 1993)
and in a variety of other local or regional fisheries.
30
Clymene Dolphin
(Stenella clymene)
The identity of this dolphin as a species separate from
Stenella longirostris was established by Perrin et al.
(1981). Its known range is in the tropical and subtropical
Atlantic Ocean, including the Gulf of Mexico and
Caribbean Sea (Perrin and Mead 1994). There is no good
information on its overall abundance. However, the average
herd size in the Gulf of Mexico is only about 30, and
the largest herd known had about 50 animals (T.A.
Jefferson, pers. comm.). Clymene dolphins may be killed
in tuna purse seines in the eastern tropical Atlantic. Rough
estimates of the total dolphin kill in this fishery have
ranged from 3,300 to 5,000 per year, but there has been
very little observer coverage. The actual kill may be much
higher, and if it is, this mortality could represent a significant
conservation problem. These dolphins are also harfXDoned
in small numbers by artisanal dolphin hunters in
the Lesser Antilles and are taken incidentally in gillnets off
Venezuela (Perrin and Mead 1994).
Striped Dolphin
(Stenella coeruleoalba)
Striped dolphins have a cosmopolitan distribution in tropical
and temperate waters (Perrin et al. 1994b). Although
they remain abundant overall, several populations have
been heavily exploited and show signs of serious decline.
The most critical situations are in the western North
Pacific and the Mediterranean Sea. Catches of striped dolphins
in the Japanese drive fishery have declined dramatically
since the 1950s, and Japanese scientists estimated
that the population had been reduced by 50% or more by
the late 1970s (Kasuya and Miyazaki 1982, Kasuya
1985a). During the 1980s the fishermen consistently fell
far short of reaching their quota of 5,000 striped dolphins
per year (IWC 1991). In 1992 the IWC's Subcommittee on
Small Cetaceans called for a detailed assessment of the
status of striped dolphins off Japan and for an "interim halt"
to their direct exploitation in the drive fishery (IWC 1993).
The annual catch off the Pacific coast of Japan at Taiji has
declined to only about 1,000 dolphins in recent years,
which is still below the quota (Kishiro and Kasuya 1993).
This development has occurred in spite of the continuing
strong demand for dolphin meat (T. Kasuya, pers. comm.).
The principal threats facing striped dolphins in the
Mediterranean are pollution and incidental capture in purse
seines and pelagic drift gillnets (IWC 1992a). A major
die-off of striped dolphins occurred in the western
Mediterranean during 1990-1992. Although the proximate
cause was a disease syndrome linked to a morbillivirus
infection (Van Bressem et al. 1991), high body burdens