Activities and Conditions Which
Indirectly Affect Marine Mammals
Globally,
loss of habitats and habitat destruction are perhaps the most serious
threat confronting all species of wildlife today. Many of the activities
and conditions which indirectly effect marine mammals are first experienced
as an alteration or disturbance to their habitat. Changes in prey species
abundance and water quality are examples of conditions which may indirectly
effect marine mammals. Since changes in environmental conditions may
be caused by the cumulative effects from many different sources of perturbation,
it is important to identify and protect the basic ecological processes
which constitute marine mammal habitat.
Research
to investigate the fundamental relationships and conditions that may
be used for defining preferred or "suitable" habitats for
marine mammals is essential to their future well-being (Mate, 1993).
This simple proclamation underscores the importance of habitat studies
for marine mammal conservation. The information on the habitat ecology
of marine mammals is limited, but some understanding of important ecological
relationships has been gained from long-term studies of cetacean occurrence
and distribution, feeding behavior, prey distribution and abundance,
and analysis of physical oceanographic conditions associated with habitat
use including water depth and temperature, salinity, nutrient gradients,
and bottom types (Watts, 1985, Kenney, et al. 1993, Waring, 1993,).
Cetacean distribution and abundance has been attributed primarily to
the abundance and availability of preferred prey species (Kenney, et
al 1991, Mayo, et al. 1988a, Overholtz and Nicholas, 1979), although
studies of humpback distributions on Georges Bank concluded that factors
other than simple prey distribution, including bottom topography and
foraging behavior may also influence distribution (Payne, et al 1986).
The precise factors that make the Stellwagen Bank area attractive habitat
for cetaceans have not been documented, but the area's extraordinary
productivity which supports an abundance of preferred prey species,
e.g., sand lance (Ammodytes americanus), as well as the bottom topography
and benthic environments of the Bank are undoubtedly important elements
in defining the cetacean habitat requirements in the Sanctuary.
Recently,
geographic information systems have been used to provide computer analysis
and mapping of cetacean habitat use in the GOM (Northridge, 1993). This
approach may be helpful for describing overall habitat characteristics
by comparing observational data for cetaceans with routine fish surveys,
commercial fishing catch data, and oceanographic conditions.
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