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these and other reasons, most cetacean scientists  fitness by reducing the range of behavioural
               have been focusing their attention on threats that   flexibility that is necessary to react with appro-
               are less complex and relatively easier to docu-  priate strategies to other environmental fluctua-
               ment.                                          tions, or to a further worsening of conditions
                  Although complex food-web dynamics are  (e.g., further prey reduction, increased human
               difficult to study, it is clear that reduced prey  disturbance, etc.).
               availability caused by overfishing of Mediterra-  As stressed by Chapman and Reiss (1999)
               nean fish stocks and other causes, may impact  “the lack of sufficient food to maximise repro-
               cetaceans in a number of ways.  Several Mediter-  ductive potential may  be the most important
               ranean cetaceans -  particularly coastal species  regulator of population size in animals”.  As a
               such as short-beaked common dolphins and  general rule, increased time spent searching for
               common bottlenose dolphins -  compete for prey  food and feeding reduces the time that can be de-
               species of commercial interest that have been  voted to social and reproductive activities,
               heavily exploited by human fisheries during the  including mating, weaning, and caring for the
               last decades.  Dolphins, as top predators, can be   offspring,  with  negative  repercussions  on
               affected due to a decreased prey biomass or to a   reproductive success (Wilson  1979, Valiela
               reduced mean size or nutritional value of individ-  1995).  More dramatic effects may be recorded in the
               ual prey items.                                long-term, if access to prey resources is consis-
                  Moreover, fish distribution may become more  tently impaired by human competition, habitat
               scattered, and seasonal and yearly trends of  degradation, or both.  This may ultimately result
               abundance may show wider fluctuations due to   in: 1) increased levels of stress, 2) loss of weight
               the combined effects of overfishing, pollution  and physical strength accounting for emaciation
               and environmental variables (FAO  1997b, Bom-  (e.g., in common bottlenose dolphins: Politi et al.
               bace  1990, Stergiou et al.  1997, Degobbis et al.  2000) or starvation, 3) reduced reproductive
               2000).  Marine mammals with widespread distri-  rates, due to behavioural modifications and nega-
               butions may react to worsening habitat conditions  tive feedback mechanisms, 4) behavioural re-
               by leaving their core areas either permanently or  sponses leading to dispersion or emigration to-
               temporarily, as changes in the distribution of key  wards areas with higher food availability, 5) in-
               prey represent primary factors determining dol-  creased inter-  and intra-specific competition and
               phin movements and habitat preferences (Evans  aggressive behaviour (e.g., in common bottlenose
               1971, Wells  et al.  1990, Hanson and Defran  dolphins: Ross and Wilson  1996, Patterson et al.
               1993, Maze and Würsig    1999).  As cetacean  1998), 6) increased susceptibility to disease due
               feeding preferences are related to prey ecology  to reduced immune responses (e.g., in striped
               and availability in their own habitat, diet modifi-  dolphins: Aguilar and Raga  1993), and 7) higher
               cations may occur as a response to fishery exploi-  mortality rates (Baker  1978, Sinclair  1983,
               tation (Northridge  1984, Estes et al.  1998).  The  Swingland  1983, Fowler  1987, Apanius  1998,
               long-term, population-level, impact of changes in  Hofer and East 1998, von Holst 1998).
               distribution and feeding habits due to reduced    In addition, reduced food prey availability
               prey availability is largely unknown, and de-  may increase or exasperate the extent of interac-
               serves further investigation.                  tions between cetaceans and fishermen, and ex-
                  Behaviourally flexible cetacean populations  pose the former to higher risks of intentional
               affected by a temporarily lower prey abundance,  takes  and  harassment  (Northridge  1984,
               or by shifts in food prey availability, may react in  UNEP/IUCN  1994, Fertl and Leatherwood
               part by devoting more time to foraging or by dis-  1997).  Unfortunately, no clear evidence is cur-
               playing a wider range of feeding strategies (e.g.,  rently available to address this issue.  It has been
               Shane  1990, Bräger  1993).  The capability of  noted (Reeves et al.  2001) that conflict occurs in
               some cetacean species to adapt to fluctuations in  certain areas where target fish stocks are rela-
               the abundance of some prey by feeding on other  tively abundant (e.g., in the Asinara Island, Italy)
               prey is clearly an important requisite to withstand  whilst in some other areas where target fish
               seasonal and yearly variations in food supply  stocks are depleted there is little or no conflict
               (Northridge  1984).  A consistently lower prey  between dolphins and fisheries (e.g., in the
               availability, however, implies higher energetic  Kvarneric, Croatia).  The complexity of ecosys-
               costs for the dolphins to secure their daily food  tem dynamics may be responsible for the lack of
               intake.  This has the potential to affect population  simple cause-effect evidence.

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