Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
3.1.1.2 Marine Mammal Distribution—Habitat and Environmental Associations
Marine mammals inhabit most marine environments, from deep ocean canyons to shallow estuarine
waters. They are not randomly distributed. Marine mammal distribution is affected by demographic,
evolutionary, ecological, habitat-related, and anthropogenic factors (Bowen et al. 2002; Bjørge 2002;
Forcada 2002; Stevick et al. 2002).
Movements are often related to feeding or breeding activity (Stevick et al. 2002). A migration is the
periodic movement of all, or significant components of an animal population from one habitat to one or
more other habitats and back again. Migration is an adaptation that allows an animal to monopolize areas
where favorable environmental conditions exist for feeding, breeding, and/or other phases of the animal’s
life history. Some baleen whale species, such as humpback whales, make extensive annual migrations to
low-latitude mating and calving grounds in the winter and to high-latitude feeding grounds in the summer
(Corkeron and Connor 1999). These migrations undoubtedly occur during these seasons due to the
presence of highly productive waters and associated cetacean prey species at high latitudes and of warm
water temperatures at low latitudes (Corkeron and Connor 1999; Stern 2002). The timing of migration is
often a function of age, sex, and reproductive class. Females tend to migrate earlier than males and
adults earlier than immature animals (Stevick et al. 2002; Craig et al. 2003). Not all baleen whales,
however, migrate. Some individual fin, Bryde’s, minke, and blue whales may stay year-round in a specific
area.
Cetacean movements can also reflect the distribution and abundance of prey (Gaskin 1982; Payne et al.
1986; Kenney et al. 1996). Cetacean movements have also been linked to indirect indicators of prey,
such as temperature variations, sea-surface chlorophyll a concentrations, and features such as bottom
depth (Fiedler 2002). Oceanographic conditions such as upwelling zones, eddies, and turbulent mixing
can create regionalized zones of enhanced productivity that are translated into zooplankton
concentrations, and/or entrain prey.
As noted by MacLeod and Zuur (2005), however, even in the best studied marine mammal species,
determining the fundamental reasons behind the linkage between habitat variables and distribution can
be problematic, and often requires extensive datasets. For example, though topography might increase
primary productivity, and as a result, provide a local increased availability of prey, not every marine
mammal species is necessarily concentrated in that area. Additional factors may be involved, such as
habitat segregation between other species that share the same ecological niche (MacLeod and Zuur
2005). The degree of similarity in diet between two or more predators that occur in the same habitat will
affect the level of competition between these predators. Competition between predators can result in the
exclusion of one or more of them from a specific habitat. For example, MacLeod et al. (2003) suggested
that an example of niche segregation might be that Mesoplodon occupy a separate dietary niche from
bottlenose whales (Hyperoodon) and Cuvier’s beaked whales (Ziphius), though they share the same
distribution. In contrast, Hyperoodon and Ziphius appear to occupy very similar dietary niches but have
geographically segregated distributions, with Hyperoodon occupying cold-temperate to polar waters and
Ziphius occupying warm-temperate to tropical waters.
Since most toothed whales do not have the fasting capabilities of the baleen whales, toothed whales
probably follow seasonal shifts in preferred prey or are opportunistic feeders, taking advantage of
whatever prey happens to be in the area. Small-scale hydrographic fronts may act as convergence
zones. Bottlenose dolphins have demonstrated a spatial association with the area near the surface
features of tidal intrusion fronts, which could be related to increased foraging efficiency resulting from the
accumulation of prey in the frontal region (Mendes et al. 2002).
Occurrence of cetaceans outside the area with which they are usually associated may reflect fluctuations
in food availability. Some studies have correlated shifts in the distribution of some baleen whale and
toothed whale populations with ecological shifts in prey patterns after intense fishing efforts by
commercial fisheries in the western North Atlantic (Payne et al. 1986, 1990; Kenney et al. 1996).
DeMaster et al. (2001) predicted, based upon current data on human population growth and marine
mammal fisheries interactions, that in the future, the most common type of competitive interaction would
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