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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
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