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Alaska Fisheries Interaction Team Atka Mackerel Food Habits

The Status of Stocks and Multispecies Assessment Program, Fishery Interaction Team, was formed to investigate the effects of commercial fishing on top trophic level consumers.

Left: fish head. Right: Two scientists looking into a blue bowl
Atka macherel research study area.

        Atka mackerel research study area.

During the Atka mackerel mark-recapture research charters from 2002 to 2004, approximately 2,700 stomach samples were collected. These samples were analyzed in the Resource and Ecology and Ecosystem Modeling laboratory and  Atka mackerel food habits were extensively examined (in fulfillment of a Master’s thesis). The primary focus of this research is on the feeding ecology of Atka mackerel in two Aleutian Island passes. Previous research has shown disparate Atka mackerel growth along a longitudinal gradient with size-at-age decreasing from east to west across the Aleutian chain.  In general, latitudinal growth differences in fish have been well documented, commonly due to genetic and temperature clines. Longitudinal growth differences are less common and in the case of Atka mackerel, genetic variation between our study areas was excluded as a factor. The hypotheses are based on current evidence that the observed longitudinal growth difference is a combination of up to three factors, prey quantity, prey quality and temperature.   

The Wisconsin 3.0 bioenergetics model parameterized for an adult walleye pollock, Theragra chalcogramma was used to examine the effects of diet composition, consumption, and temperature on Atka mackerel growth at two study sites: Seguam Pass in the east and Amchitka Island in the west. Either growth or consumption is estimated in bioenergetics modeling. This study used observed Atka mackerel growth to estimate consumption rates. Estimated consumption rates were corroborated with observed consumption estimates to determine the validity of using a bioenergetics model parameterized for walleye pollock.  The model was then used to simulate growth under various scenarios to examine the effects of temperature and diet on Atka mackerel growth. The goal of this study is not to accurately model Atka mackerel consumption as a population and the impact on the prey field, but rather form the basis to make a comparison between each study area.

The secondary focus of this research was to examine the spatial and temporal variation in Atka mackerel food habits. In these 2 study areas The Magnuson-Stevens Act mandates that all Fishery Management Plans identify those habitats, including water column and substrates necessary to fish for spawning, feeding or growth to maturity. Average stomach fullness was used as a proxy for feeding intensity.

Patterns in feeding intensity were observed in relation to trawl exclusion zones that were established a protection measure for the Steller sea lion. The boundaries for trawl exclusion zones were established as 10 nm or 20 nm radii around Steller sea lion rookeries and haulouts and are not based on fish habitat. However, they can serve as zones representing disturbed and undisturbed habitat as these zones exclude all commercial bottom and pelagic trawling. By theory the inside of the trawl exclusion zone is relatively undisturbed in comparison to the outside of the trawl zone. The inside of the trawl zone may be an important factor in both feeding and spawning because Atka mackerel are considered a demersal species.

In both study areas there appeared to be distinct feedings grounds. Additionally, one of the areas showed significantly higher average stomach fullness inside the trawl exclusion zones. Essential fish habitat includes both the water column as well as bottom habitat. To understand the mechanisms that may be contributing to the observed spatial patterns in feeding intensity, several variables were examined: bottom depth, average fish length per observation, time of day and temperature. Using both surface and bottom temperature, areas were identified where the water column was well mixed or stratified. The only significant factor explaining observed feeding patterns was water column structure (mixed vs. stratified). It appeared that Atka mackerel prefer areas of strong mixing for feeding. However, in the fall months feeding intensity declines sharply. During this time, egg cannibalism contributes significantly to the Atka mackerel diet. The spatial pattern observed at the Amchitka study site showed the only feeding taking place occurred inside the trawl exclusion zone where egg cannibalism was prevalent. This may be due to the fact that Atka mackerel nests were located inside the trawl exclusion zone.