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1997): 1) presence in the nets or in their prox- ranean areas, and it is very likely that the existing
imity of organisms representing potential ceta- estimates are lower than the actual toll (Silvani et
cean prey; 2) water turbidity making the fishing al. 1992, UNEP/IUCN 1994).
gear less visible; 3) ambient noise in the marine
environment that may mask or confuse the ech- Entrapment in trawl nets. Trawl nets are towed
oes produced by fishing gear, thus reducing their horizontally or obliquely, and consist of a cone-
detectability for echolocating cetaceans; 4) loca- shaped net with a cod-end or bag for collecting
tion and three-dimensional position of fishing fish or other target species. Trawling nets target
gear; and 5) cetacean capability to detect the net demersal and bentho-pelagic stocks, as well as
filaments by means of echolocation. Moreover, mid-water species. Typical target species may
lack of experience by juvenile or immature indi- include species such as hake, pollock and other
viduals, together with their bent for playful groundfish, shrimp, prawn, and a variety of squid
and/or scouting behaviour, may make them more (Read 1996).
vulnerable to entrapment in gillnets (Mann et al. The significance of cetacean mortality in trawl
1995, da Silva 1996, Fertl and Leatherwood nets has only recently begun to be recognised
1997). (e.g., Jefferson et al. 1992, Crespo et al. 1994,
Bottom gillnet fisheries are very common Couperus 1997, Crespo et al. 1997, Dans et al.
throughout the Mediterranean basin, with around 1997, Fertl and Leatherwood 1997, Crespo et al.
50,000-100,000 boats reportedly involved (IWC 2000). Incidental takes of cetaceans exist in most
1994). Target species are largely represented by areas where trawling occurs (Fertl and Leather-
demersal and bentho-pelagic fish and crusta- wood 1997), and several cetacean species are
ceans. Although few entrapments in bottom gill- known to become incidentally caught in the nets.
nets have been documented in the Mediterranean, A preliminary review of global data indicates that
this may be in part due to under-reporting (Di 25 cetacean species (two mysticetes, 23 odonto-
Natale and Notarbartolo di Sciara 1994). Being cetes) have died in working trawls or discarded
so widespread throughout the Mediterranean trawling gear (Fertl and Leatherwood 1997). In
coastline, this fishery may actually result in occa- extra-Mediterranean areas, bycatch in trawl nets
sional mortality of coastal species. Incidental may affect species including Tursiops, Del-
catches of short-beaked common dolphins and phinus, Stenella, Lagenorhynchus, and Globi-
common bottlenose dolphins in gillnets report- cephala (Jefferson et al. 1992, Waring et al.
edly occurred in Italy and Turkey, and are sus- 1990, Kuiken et al. 1994, Read 1996, Tregenza
pected to occur in several other Mediterranean and Collet 1998, Morizur et al. 1999). Recent
countries (Di Natale and Notarbartolo di Sciara mass strandings of small odontocetes - particu-
1994, UNEP 1998a). larly short-beaked common dolphins and Atlantic
Bycaught cetaceans are usually removed from white-sided dolphins - on the western and north-
the nets dead or alive - either by disentan- ern coasts of Europe have been related to pelagic
gling/cutting the net or by amputation of cetacean trawl fishing, and the potential of these mortality
fins or flukes. Occasionally, small cetaceans may events at the population level has been probably
be brought to the port for human consumption. underestimated (Kuiken et al. 1994, Berrow and
The proportion of live/dead bycatch is unknown, Rogan 1997, Couperus 1997, Tregenza and Col-
and remarkably few studies have been conducted let 1998, Morizur et al. 1999). In the U.S. waters
to evaluate mortality trends in bottom gillnet of the Mid-Atlantic Bight, Globicephala and
fisheries. Scientific data are scarce and for most Delphinus have been heavily bycaught by mid-
Mediterranean countries only anecdotal reports water trawl fisheries for mackerel (Scomber
exist, making it difficult to assess the current im- scombrus) in the 1980s and early 1990s (Waring
pact of this threat to coastal cetaceans. The 1994 et al. 1990).
IWC report estimated “likely annual ranges of It has been suggested that cetaceans bycaught
marine mammal mortality” of 1-10 Risso’s dol- in trawl nets are probably aware of the net and
phins, 0-5 short-beaked common dolphins, 50- the boat’s activity (Fertl and Leatherwood 1997).
200 common bottlenose dolphins, 1-20 striped In many areas around the world, cetaceans have
dolphins and low numbers of other cetacean spe- learned to follow bottom trawlers to take advan-
cies in coastal set gillnet fisheries (IWC 1994). tage of fish caught by the net, stirred up by the
However, the incidence of accidental captures in net, attracted by the net, or discarded from the
gillnets is reportedly significant in some Mediter- nets after trawling (e.g., Leatherwood 1975,

Cetaceans of the Mediterranean and Black Seas – 9.6