Gulf Apex Predator-Prey Project
Kodiak Seafood and Marine Science Center
University of Alaska Fairbanks • School of Fisheries and Ocean Sciences • Kodiak, Alaska

GAP 2007

Seasonal prey use and partitioning by sympatric marine mammals in a Kodiak embayment

Project Summary

Sustainability of coastal marine resources is critical to the welfare of a variety of human and nonhuman consumers. Among these are a diverse range of marine mammal species that depend on accessing large volumes of prey to meet high energetic demands. Some of these prey species are of direct value to human consumers who harvest them for commercial, recreational, and subsistence purposes. Other marine mammal prey items are non-harvested forage species (including zooplankton) but key components of the food web that support managed and harvested fish stocks. Therefore removals of prey from coastal ecosystems, whether by humans or marine mammals, can directly or indirectly affect availability of prey resources to other consumers.

Adequately protecting, restoring, and managing these coastal marine resources requires an ecosystem-based understanding of spatial and temporal relationships among marine mammals, their prey, and habitat. Although many species of marine mammals co-occur and occupy apparently similar roles as apex marine predators, little is known about resource partitioning and 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 this study seeks to address predator-prey interactions and supporting ecological processes that occur on a geographically and seasonally restricted scale.

This is a multidisciplinary, multispecies study of the interactions between a suite of apex predators (fin whales, humpback whales, harbor seals, and Steller sea lions), their habitat, and seasonally abundant prey aggregations within Uganik Bay on the west side of Kodiak Island, Alaska. The study focuses on winter, an ecologically critical time for marine mammals and their prey, and for human consumers of these resources. A combination of techniques has been employed to concurrently assess prey availability within the bay and monitor the distribution and foraging behavior of resident and transitory pinnipeds and cetaceans sharing prey resources.

In fall 2007, personnel from the University of Alaska Fairbanks (UAF), Alaska Department of Fish and Game (ADFG), and the National Marine Fisheries Service (NMFS) began a cooperative effort to assess winter habitat and prey use by a suite of sympatric apex predators in Uganik Bay. The project’s main objective is to document the degree of dietary and foraging overlap among four sympatric marine mammal species (humpback whale, fin whale, Steller sea lion, and harbor seal) and to explore their use and potential partitioning of herring and other seasonally and spatially concentrated prey resources.

Our approach involves

  1. Coordination with ADFG seal researchers to monitor the movements and foraging behavior of harbor seals utilizing Uganik Bay during synchronous prey and predator surveys
  2. Exploration of diving habits of fin and humpback whales through real-time acoustic tag attachments
  3. Systematic vessel surveys of Uganik Bay utilizing hydro-acoustics and mid-water trawl methodologies to document distribution and abundance of prey resources
  4. Documenting relative abundance and distribution of sympatric apex predators in Uganik Bay in winter