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Chapter One: General Introduction
Many of the observed patterns are likely due to a complex interaction between
historical changes in marine environments (e.g., the impact of ice ages), resource
requirements and specialisations, and aspects of life history and demographics
(Hoelzel et al. 2002a). In this context, the use of molecular markers has played an
increasingly important role in the study of cetacean population structure and thus, in
the management and conservation of these species (e.g., Rosel et al. 1994, Baker &
Palumbi 1995, García-Martínez et al. 1999). Genetic data contain information on the
present and past structure of the populations that can not be obtained by any other
method (Avise 2004). As such, most examples of dolphin population structure
mentioned in this thesis have been revealed by molecular studies. Note, however,
that it is not the only tool, as photo-identification or telemetry data can provide
valuable and complementary information on the patterns of individuals’ movements
(Whitehead 2001).
1.3.2. Factors driving dolphin population structure
Different factors such as sea-surface temperature, behavioural specialisation,
isolation-by-distance, social system and historical processes are thought to shape the
structure of dolphin populations (Hoelzel et al. 2002a). This section presents a few
examples which illustrate these factors.
In the North Atlantic, the patterns of genetic differentiation in long-finned pilot whales
(Globicephala melas) suggest that population isolation occurs between areas that
differ in sea-surface temperature (Fullard et al. 2000). On the other hand, population
differentiation shows no correlation with geographical distance (Fullard et al. 2000).
Temperature also seems to be the primary factor determining the relative distribution
of two populations of short-finned pilot whales off the coast of Japan (Kasuya et al.
1988). However, these correlations might not illustrate a particular sensitivity of these
two species for sea-surface temperature, but instead may be the result of other
ecological factors such as prey behaviour (Sergeant 1962). Such a pattern has been
suggested for several other species of dolphins such as the dusky dolphin
(Lagenorhynchus obscurus) in Argentina (Würsig & Würsig 1980). In temperate
waters, water temperature could affect prey distribution and in turn affect the
distribution of dolphin species (Norris 1967, Kasuya et al. 1988). For pilot whales, it is
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