Prey use and foraging patterns of Kodiak’s sympatric marine mammals
Ecosystem-based management of coastal marine resources requires an understanding of spatial and temporal relationships among apex predators (including marine mammals), their prey, and habitat. Although many species of marine mammals co-occur and may occupy similar roles as apex predators, little is known about their partitioning of prey resources or the degree of competitive interactions among sympatric species. Because ecosystems are complex, it is difficult to study these relationships over large geographic areas and long periods of time. Therefore we undertook a three-part study to address predator-prey interactions and foraging overlap on a geographically restricted scale in Kodiak Island’s near-coastal waters. In this study we examined prey use and foraging patterns of Kodiak’s gray, fin, and humpback whales, harbor seals, and Steller sea lions. We employed a combination of techniques to concurrently assess prey availability and monitor the distribution and foraging behavior of resident and transitory pinnipeds and cetaceans sharing common prey resources.
The highly productive nearshore waters of the Kodiak archipelago support significant seasonal aggregations of zooplankton (euphausiids, copepods) and shrimp which in turn support spawning and nursery aggregations of salmon, herring, pollock, and capelin. These resources also support a highly mobile, opportunistic, and ecologically diverse group of marine mammals. Among Kodiak’s baleen whales, gray (Eschrichtius robustus), humpback (Megaptera novaeangliae), and (assumedly) fin whales (Balaenoptera physalus) are seasonal migrants that must store enough energy while foraging in Kodiak’s productive waters to sustain them during the energetically demanding months during which they migrate, fast, and mate in warmer and less productive low latitude waters. Kodiak’s pinnipeds (harbor seals [Phoca vitulina] and Steller sea lions [Eumetopias jubatus]) are mobile but nonmigratory piscivores whose diets and distribution patterns reflect their opportunistic exploitation of seasonally and regionally abundant prey sources. This diversity of sympatric marine mammals in Kodiak waters allows assessment of potential dietary overlap for subsequent exploration of niche separation and prey partitioning in a region of high prey abundance. Long-term monitoring of marine mammal diets, distribution, and foraging patterns in this area allows examination of trophodynamic interactions and species-specific effects of changes in those patterns.
In this project we continued GAP’s multiyear examination of the distribution, diet, and foraging patterns of marine mammals in the Kodiak area as a means of exploring species-specific responses to changes in prey availability. Specifically, our objectives were to investigate
- Fin and humpback whale foraging overlap in Marmot Bay
- Gray whale presence and feeding in Ugak Bay
- Dietary overlap of sympatric harbor seals and Steller sea lions