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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
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