Changes in Prey Composition, Distribution, and Abundance
It has
been suggested that the abundance and distribution of preferred prey
species has played a significant, perhaps critical, role in structuring
baleen whale populations in the Gulf of Maine (Payne, et al. 1990).
The distribution of humpback whales has been shown to be significantly
correlated with the number of sand lance obtained from standardized
trawl tows (Payne, et. al., 1986). Humpback sightings from 1978 - 1986
showed a shift in distribution from the upper Gulf of Maine-lower Bay
of Fundy region to the southwestern Gulf concurrently with an increase
in sand lance in this area during the same period. This shift in distribution
coincided with a dramatic increase in the concentrations of sand eels
throughout the shelf waters of the eastern United States. The sand eel
populations apparently expanded in response to the collapse of the Atlantic
herring stocks in the mid-1970's due to over-fishing from foreign, distant
water factory fleets (Meyer, et. al., 1979, Sherman, et. al., 1981).
Further
study demonstrated that significant changes in the biomass of sand lance
and the abundance of copepods co-occurred with a shift in the occurrence
and abundance of four species of baleen whales (northern right, humpback,
sei, and fin) in the southern Gulf of Maine (Payne, et. al., 1990).
Peak years in the abundance of C finmarchicus were the lowest years
in abundance for sand lance. Right whales and sei whales were common
in the region only during 1986, when C. finmarchicus reached a regional
maximum and sand eels were at a regional minimum. These distributional
shifts in cetaceans have been characterized as an ecological response
to human-induced changes in the abundance of planktivorous finfish caused,
in part, by over harvesting (ibid.).
Interestingly
since the drastic decline in foreign fisheries on Georges Bank in the
late 1970's, Atlantic herring populations have been able to recolonize
much of the area's spawning territory during the period from 1988-1993
(NMFS, 1993c). During the final two years (1992-1993), the abundance
of sand lance (a member of the pelagic, small-fish community on Georges
Bank) has been well below the average for previous years. This change
in the abundance of species which feed at the same trophic level is
referred to as a "biomass flip". Whether or not this shift
in the abundance and distribution of cetacean prey will trigger a similar
shift in the distribution of humpbacks and other cetaceans that feed
on these small pelagic species remains to be seen, but preliminary analysis
of 1994 sighting data suggests that the distribution of humpbacks is
moving north.
Many
species of marine mammals appear to follow the movements of their prey,
which in turn may be linked to physical oceanographic conditions including
circulation patterns, water temperature, and salinity. For example,
northern right whale distributions and feeding patterns have been associated
with relatively discrete, very high density patches of zooplankton in
Cape Cod Bay (Mayo and Marx, 1990). Although the factors responsible
for the development of the high density patches in Cape Cod Bay have
not been described, the formation of similar prey aggregations in the
Great South Channel east of Cape Cod has been attributed to upstream
advection from the Gulf of Maine and concentration within a large-scale,
near-surface area of convergence (Kenney, et. al., 1993, Wishner, et.
al., 1988).
The U.S.
EPA (1993) conducted an assessment of the potential for adverse effects
on endangered cetaceans and their prey due to the discharge of nutrients
and toxic chemicals associated with the planned construction of the
Massachusetts Water Resources Authority's sewage outfall in Massachusetts
Bay. Nutrient aspects of the investigation focused on whether loadings
from the new outfall would lead to excessive phytoplankton growth or
other resultant environmental changes such as shifts in the abundance
and distribution of preferred prey, stimulation of toxic phytoplankton
blooms, or depletion of dissolved oxygen, any of which could ultimately
lead to adverse effects on endangered species of whales.
With
respect to the question of nutrient enrichment, the assessment concluded
that due to enhanced treatment and greater dilution, the concentrations
of nutrients at the new outfall were expected to be lower than those
at the existing outfalls in Boston Harbor, although detectable increases
in nutrients are expected to occur in the western portions of Cape Cod
Bay, south of the new outfall. Because the effluent-derived nutrient
concentrations (in the Bays) were predicted not to change appreciably
over existing conditions, phytoplankton production and community structure
should not change in areas (outside of the immediate zone of dilution)
which are important to endangered species (NMFS, 1993, Anderson, 1993).
This conclusion is not universally accepted by the scientific community.
For example, one reviewer responded that although "the Assessment
concludes that baywide changes in primary productivity and phytoplankton
biomass are probably unlikely. . . it is generally recognized that environmental
and biotic patchiness are major determinants of food web dynamics and
regulation" (Smayda, unpub., 1993)
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