A synthesis of findings
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 (GOA). Productivity here supports a huge biomass of mid-trophic level species (MTL) that, in turn support 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 (GAP) study 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 multi-year snapshots and long time-series 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 1999–2005. 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 in some years but may, in fact, respond differently to relatively abundances of fish versus zooplankton resources.
Such multi-year studies are key to monitoring temporal variability and assessing the direction, magnitude, and consequences of long-term trends. GAP2010 was proposed and funded as a means of compiling this decade-worth of GAP data, exploring complementary datasets, and incorporating them all into a model of trophic interactions in the Kodiak area. Specifically, GAP2010 takes a top-down approach to explore the impact of balaenopterids and other consumers on the nearshore marine ecosystem that includes waters of Kodiak and the western GOA. Using empirical data from GAP and other studies as input in consumption models, GAP2010 explores potential trophic-level responses to changes in abundance, diet, and/or consumption rate of major consumers such as balaenopterids. With this, the secondary and tertiary effects of those changes on prey populations and subsequent changes in carrying capacity (K) for Steller sea lions and other UTL consumers can be explored.
GAP2011 supported a continuation and enhancement of GAP2010’s efforts to monitor and model the interrelationships among apex predators and their use of Kodiak’s coastal environment. While continuing to add data to the long time-series on marine mammal diets and foraging behavior GAP2011 developed “Variability Index Sites” as a means of monitoring nearshore oceanographic variability in foraging “hotspots.” In addition, we renewed collection of Steller sea lion diet samples needed to examine current prey use and to provide a comparison of prey use from 1999 to 2005, prior to a recent period of oceanographic cooling.
Although we will maintain minimal sampling needed for long-term monitoring of whale distribution and sea lion diets, GAP2012 will focus on the synthesis, modeling, and publication of results gathered in these and the previous 10 years’ studies of the seasonal distribution, residency, and foraging patterns of marine mammals and their prey in waters of the Kodiak Archipelago.
Specific objectives in GAP2012 include
- Synthesize isotope data to
- develop a Gulf of Alaska isotope clearinghouse and “isoscape”
- determine the degree of fine-scale temporal and spatial overlap between/among fin and humpback whales from Kodiak and the Shumagin Islands
- Synthesize humpback whale sighting histories and association data to
- examine associations and social structure of humpback whales within the Kodiak and Shumagin Island feeding aggregations
- Synthesize dive data from foraging fin and humpback whales to
- describe foraging behaviors and prey preferences of fin and humpback whales
- Synthesize pinniped diet data to
- document patterns of prey use and overlap between Steller sea lions and harbor seals
- compare prey distribution patterns to the diet of Kodiak’s Steller sea lions
- Synthesize prey quality data
- use fatty acid composition to describe and compare quality of primary prey species