Page 7 - Bearzi_2002

Corkeron et al. 1990, Waring et al. 1990, Mori- nylon, which makes them resistant to tear, either
zur et al. 1999, Goffman et al. 2001). While by yanking whole fish or by forcing an entry in
these nets may provide a concentrated food and/or out of the net. In the past, the dolphins
source that may be easy to exploit, cetaceans may used to badly damage the nets in order to reach
become entangled in operating nets and this op- the fish, to the point of being shot at by the fisher-
portunistic feeding behaviour is likely to be re- men. During the last few years, the dolphins have
learned (or forced to revert) to feed without dam-
sponsible for most cetacean captures in trawl nets aging the net, however, they apparently also ven-
(Overholtz and Waring 1991, Read 1996). How- ture into the net and incidental captures still occur
ever, there is little systematic knowledge of the (Kerem 2001). Some of the bycaught animals are
behavioural processes that cause cetaceans to be brought up inside the net and some (about 1/3) are
vulnerable to incidental takes in trawls (Fertl and found entangled in the free-floating lazy-line the
Leatherwood 1997). purpose of which is to secure the net in case the
Mid-water trawling seems to represent the main towing lines break.”
main threat, because it may target species that
represent typical components of cetacean diet. Apart from the remarkable incidence of by-
Moreover, these nets are usually dragged at rela- catch off the Israeli coast, and possibly in other
tively high speeds, with irregular and unpredict- Mediterranean areas for which data are lacking,
able changes of route that increase the chances of the main impact of trawl fisheries on Mediterra-
entanglement (Fertl and Leatherwood 1997). In nean cetaceans – particularly on coastal species
both European and U.S. waters the recent devel- feeding on demersal prey such as the common
opment of near-surface trawling (in particular bottlenose dolphin – may be due to direct or indi-
when nets are dragged by two fishing boats) has rect food-web interactions and habitat loss rather
further increased the risk of incidental captures of than bycatch (see in following pages, “Competi-
cetaceans (Crespo et al. 1995, Couperus 1997, tive interactions between cetaceans and fisher-
Morizur et al. 1999). ies”).
In the Mediterranean, interactions between
trawlers and several cetacean species reportedly Entrapment in purse seines. Purse seines are
occur, the main species involved being the com- widely used in the world's industrialised fisheries
mon bottlenose dolphin (Northridge 1984, Con- to capture a variety of pelagic species, from tuna
siglio et al. 1992, Silvani et al. 1992, Gannier to anchovies and sardines. The most dramatic
1995, Goffman et al. 1995, Marini et al. 1995, case of interaction between purse seines and ce-
Casale 1996, Mussi et al. 1998, Pace et al. 1998, taceans has occurred – and to some extent still
Bearzi et al. 1999, Mazzanti, In press). Based on occurs - in the eastern tropical Pacific, where
the available data, bycatch in trawling nets ap- strong affiliation between yellowfin tuna (Thun-
pears to be a relatively uncommon occurrence in nus albacares) and dolphins has led to extremely
most Mediterranean areas. However, high mortal- high mortality rates - with perhaps as many as
ity rates in bottom trawl nets have been reported seven millions dolphins killed since the late
from the Mediterranean coast of Israel. Of 67 1950s (Gosliner 1999). In this fishery, the asso-
common bottlenose dolphins found dead stranded ciation between tuna and dolphins is used to as-
or adrift, 26 (39%) were incidentally bycaught in sist in the location and capture of tuna schools.
trawl nets (Goffman et al. 2001). Contrary to As dolphins are more easily seen from vessels
what has been suggested from other areas (Fertl than tuna, fishermen search for schools of dol-
and Leatherwood 1997), bycatch off Israel affects phins and, after determining that they are associ-
animals regardless of gender and age classes ated with tuna, encircle the entire aggregation
(Goffman et al. 1995, 2001). with large purse seines. Dolphins may die if they
Goffman et al. (2001) make the following ob- become entangled or trapped in billows of the
servations for common bottlenose dolphins fol- net. Following regulations to prevent dolphin by-
lowing bottom trawlers off the Mediterranean catch, fishermen in the Pacific have been forced
coast of Israel: to release alive the dolphins that were encircled
by the net, but dolphin mortality could still occur
“Foraging is done by a unique method, a learned when efforts to release them failed, whether due
behaviour, of cutting out segments of fish that pro- to unpredictable dolphin behaviour, human error,
trude from the outer side of the net. The reason or unfavourable conditions of weather, current
may be the change of modern nets from cotton to speed, or lighting (Gosliner 1999, Reeves et al.,

Cetaceans of the Mediterranean and Black Seas – 9.7