Marine mammals foraging in a dynamic environment
Kodiak archipelago waters are highly productive due to a combination of local bathymetry, winds, and water transport (currents, tides, upwelling, and eddies) in the western Gulf of Alaska. Productivity here supports a huge biomass of mid-trophic level species that in turn supports an abundance and variety of upper trophic level (UTL) species. As a result, a diverse and abundant suite of apex predators—including seabirds, fish, cetaceans, pinnipeds, and humans—forage seasonally or year-round in Kodiak waters. Management of these species and conservation concerns regarding the environment that supports them have driven decades of biotic surveys and oceanographic studies in the Kodiak region.
Since 1999, the Gulf Apex Predator-Prey Project (GAP) has investigated interactions between Kodiak’s apex predators, their prey, and environment. GAP’s interrelated studies have focused on Steller sea lion concerns and also broadly assessed the degree of temporal variability and dietary overlap among Kodiak’s sympatric apex predators. The distinct but related research projects have sought to explore the processes that drive populations of their prey within a dynamic marine environment. Through 2006, GAP studies involved a multi-taxa approach, collecting synchronous data on fish, seabird, and marine mammal diets and populations in nearshore Kodiak waters. Since 2007, GAP studies have focused on documenting the distribution, identity, foraging patterns, and prey use by Kodiak’s marine mammals.
Combined, GAP studies have provided both multiyear snapshots and long time-series of data demonstrating that Kodiak’s apex predators are responding to highly variable marine prey resources. Spatial and temporal variability in zooplankton and fish prey fields were documented on several scales in nearshore waters. The diet of tufted puffins and the productivity of black-legged kittiwakes were both found to be significantly affected by annual variability in nearshore forage fish availability in 2003–2005. Dominant items in the diet of arrowtooth flounder near Kodiak were found to vary both within and between years as prey availability changed. The relative importance of prey items in Kodiak area Steller sea lion diets was found to vary regionally, seasonally, and annually between 1999 and 2005. Aerial surveys conducted between 1999 and 2010 documented changes in the distribution of three whale species that feed in different trophic levels or habitats—as highly mobile foragers, their distribution likely reflects changes in prey fields or switching to alternative prey. A recent tagging study suggests fin and humpback whales seen in close proximity may target the same dense prey fields.
GAP 2010 was proposed and funded as a means of compiling this decade-worth of GAP data, exploring complementary data sets, and incorporating them all into a model of trophic interactions in the Kodiak area. Specifically, GAP 2010 takes a top-down approach to exploring the impact of balaenopterids on the nearshore marine ecosystem that includes waters of Kodiak and the western Gulf of Alaska. When available, empirical data from GAP and other studies will be used as input to model potential trophic-level responses to changes in balaenopterid abundance, diet, and/or consumption rate. With this, the secondary and tertiary effects of those changes on potential prey populations and subsequent changes in carrying capacity (K) for Steller sea lions and other UTL consumers will be explored. In GAP 2011 we propose a continuation and enhancement of GAP 2010’s efforts to monitor and model the interrelationships among apex predators and their use of Kodiak’s coastal environment.
In GAP 2011 we propose efforts that support the overall goal of identifying and quantifying (where possible) the ecosystem processes, drivers, sensitivities, and variability that affect marine mammals and their prey. This will include continued trophic-level monitoring of fin, humpback, and gray whale foraging, providing empirical data to strengthen any GAP models. In addition, we propose collecting Steller sea lion diet samples needed to examine current prey use and to provide a comparison to prey use from 1999 to 2005, prior to a recent period of oceanographic cooling.
Specific objectives in GAP 2011 include (1) continued monitoring of marine mammal diets and foraging behavior, (2) initial monitoring of oceanographic variability in nearshore “Variability Index Sites,” and (3) enhancement of GAP trophodynamic modeling efforts.