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Marine Ecology and Stock Assessment Program

Energy Density and Length of Juvenile Pink Salmon in the Eastern Bering Sea from 2004 to 2007

Juvenile pink salmon were examined in the eastern Bering Sea from 2004 to 2007 to assess the influence of ocean temperature on whole body energy content (WBEC), length, and diet. Fish were collected during the U.S. BASIS surveys in the eastern Bering Sea. Warmer spring and summer sea surface temperatures prevailed from 2004 to 2005 on the eastern Bering Sea shelf, whereas cooler spring and summer sea surface temperatures occurred from 2006 to 2007.

Juvenile pink salmon changed diet between the warm and cool years. Walleye pollock (Theragra chalcogramma) dominated the diet (> 50% wet mass) in warm years, while walleye pollock were nearly absent from the diet in cool years. Juvenile pink salmon lengths were significantly longer in warm years but WBEC was significantly lower. We interpret our results to indicate that length is not always a reliable measure of energy status.

By Alex Andrews

Geographical Variation in Carbon and Nitrogen Stable Isotope Ratios in Spiny Dogfish in the Northeastern Pacific Ocean

Spiny dogfish (Squalus acanthias) biomass has increased in the Gulf of Alaska, yet little is known about the ecological niche that dogfish fill in this ecosystem. Trophic position is an important indicator of the ecological role of an organism. To explore the trophic position of dogfish we analyzed the nitrogen and carbon stable isotope ratios of 60 dogfish from five locations between Washington State and the Gulf of Alaska.

The mean δ15N values for dogfish ranged from 12.0 (central Gulf of Alaska) to 13.4 (Howe Sound, British Columbia) and the mean δ13C values ranged from 21.3 (Yakutat Bay, Alaska) to 17.9 (Puget Sound, Washington). Sites to the north tended to be significantly depleted in the heavy isotopes of both nitrogen and carbon. The differences in nitrogen isotope ratios among sites were attributed to potential changes in dogfish feeding behavior and trophic position.

Differences in carbon isotope ratios suggested that dogfish utilize different food webs along the northeastern Pacific Ocean shelf. Additionally it was hypothesized that feeding differentially in offshore versus inshore food webs or targeting pelagic versus benthic prey species may explain the isotopic variability. These results are preliminary and require additional tests before conclusions can be made about the trophic position of dogfish in this region. Future work will explore stable isotope variability at lower trophic levels to test the hypothesis that entire food webs are isotopically shifted owing to differences in isotopic fractionation at the base of the food web. Also, trophic level differences among dogfish size classes and between sexes will be explored among a greater diversity of locations to better describe the ecological consequences of increased biomass of dogfish in the northeastern Pacific Ocean.

By Alex Andrews

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