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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.

Cetaceans of the Mediterranean and Black Seas – 9.13