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Status of Stocks & Multispecies Assessment Program

Alaska Marine Science Symposium Presentations

SSMA staff attended the Alaska Marine Science Symposium in Anchorage, Alaska during January. Abstracts from select presentations are provided with SSMA staff names presented in italics.

Processes Affecting the Productivity of Capelin and Pollock in the Gulf of Alaska

E. A. Logerwell, J. T. Duffy-Anderson, and M. T. Wilson
The Gulf of Alaska (GOA) is a highly productive ecosystem that is influenced by oceanographic forcing and climate variation. Studies that examine the distributional ecology of key fish species in the GOA relative to environmental variables provide an opportunity to better understand how external forcing factors influence and modify fish production. We present information on the effects of local hydrography on the distribution and feeding ecology of two key forage species in the GOA, juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus).

Multiple research cruises were undertaken in Barnabus Trough, located to the east of Kodiak Island. Biophysical sampling (temperature and salinity profiles, chlorophyll-a concentrations, acoustic backscattering) in these areas suggests that juvenile walleye pollock and capelin were spatially separated by a local hydrographic front in Barnabus Trough. These observed differences in habitat selection have implications for resource use and competition between pollock and capelin in that region. Our current research examines whether mesoscale hydrography is associated with differences in zooplankton composition, limits in geographic distribution, and fish feeding and diet. Results will provide a better understanding of the mechanisms by which climate variability influences fish populations in the Gulf of Alaska.

Reproductive Ecology of Atka Mackerel

Daniel Cooper and Susanne F. McDermott
A reproductive ecology study of Atka mackerel was funded by the North Pacific Research Board. Atka mackerel length and age at maturity were examined as part of the study. Ovaries were collected from International North Pacific Fisheries Commission (INPFC) areas 541 and 542 from 2002-04 during NMFS Atka mackerel tag recovery cruises. Maturity was determined by histology. Ages were assigned by NMFS AFSC age readers. A generalized linear model with binomial error distribution was used to test for maturity differences by area and over time. A logistic model was fit to the data using S-PLUS:

It was found that Atka mackerel female maturity is more dependent on age than length. The difference in length at 50% maturity between areas is likely due to different growth rates between areas.

A strong 1999 year class caused the maturity samples to include a high percentage of age-3 fish in 2002 and a low percentage in 2003. This is likely the cause of the fluctuations in length at maturity between years. Maturity at age was not significantly different between time periods at the alpha = 0.05 level (P=0.054). The 2003 estimate of 50% age at maturity differed the most from the one traditionally used in the stock assessment. Using the 2003 maturity estimate increases the 2003 spawning biomass by 14%. Variation in age and length at maturity can have an important influence on estimates of spawning biomass and therefore, stock assessment.

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