Food limitation will be considered by examining population densities and size class structures of dominant prey species. Considerable dietary overlap and potential competition for food exists among the apex predators of the nearshore system. There is also strong evidence to suggest that population densities of many nearshore vertebrate predators are limited by food. For example, after initial immigration of sea otters into eastern PWS in 1979, the population increased rapidly over the next several years. Concurrent with this increase in otters was a rapid decline in many preferred prey items, including Dungeness crabs. Following prey reduction, sea otter populations declined and became relatively stable. Similar patterns have been observed at Amchitka and the Commander Islands. Evaluation of abundance and size distribution data for prey items may also be useful for providing additional indirect evidence for estimating recovery of some predator species. For example, it is well documented that sea otters prefer sea urchins as prey and that in the presence of strong predation by sea otters, both the abundance and average size of sea urchins is reduced. Other suggestions of food limitation exist for sea ducks.
There is circumstantial evidence that pigeon guillemots nesting at Naked Island in central PWS are food-limited. Historically, Naked Island has been an important breeding area for pigeon guillemots, supporting about 20% of the sound-wide breeding population. Concurrent with a 50% decline in the numbers of breeding guillemots at Naked Island, there has been a decline in growth rates of nestlings. These declines have been associated with a major diet shift for adult guillemots feeding nestlings. Before the EVOS, over one third of the prey fed to guillemot nestlings was sand lance. After the EVOS, nestling diets on Naked Island were dominated by juvenile gadid, a comparatively low-quality prey. The apparent decline in the availability of a high-quality prey type (sand lance), and its replacement in the diet by prey types with half the energy density (cod, pollock), suggest that pigeon guillemot productivity on Naked Island is constrained at least in part by food availability.
There is evidence that population densities of at least some important vertebrate prey species declined as a result of the EVOS. For example, mussels, an important component of the diets of sea otters and sea ducks, were less abundant at oiled sites relative to unoiled sites following the EVOS. Many of the prey species of the nearshore vertebrate predators, including crabs, limpets, chitons, and mussels have failed to recover fully in some habitats.
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Demography
Continued Hydrocarbon Exposure
NVP
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Last Reviewed: March 26, 2003