Graduate Thesis Or Dissertation

 

Variation in the energy density of forage fishes and invertebrates from the southeastern Bering Sea Public Deposited

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  • The quality and availability of forage fishes and invertebrates can affect the behavior and productivity of predators that rely on these resources. This study measures the proximate composition of forage fishes and invertebrates from the southeastern Bering Sea to estimate prey energy density (quality) using a method that is ecologically relevant. During the summers of 2008 and 2009, transect surveys were conducted near the Pribilof Islands in the southeastern Bering Sea to collect forage fishes and environmental data during the time when seabirds and fur seals were rearing their young on those islands. All forage species caught in sufficient numbers by trawling were analyzed for proximate composition to estimate prey quality, in terms of total energy density. Proximate composition of forage species from this area varied widely between years and among sampling locations. Much of the observed variation in prey energy density was explained by lipid content, which was inversely related to water content. Invertebrates exhibited greater variation in lipid content and energy density than did forage fishes, although their overall prey quality was similar to that of many forage fishes. Among forage fishes, northern lampfish (Stenobrachius leucopsarus) and northern smoothtongue (Leuroglossus schmidti) were found to have especially high lipid content and energy density. Estimates of total energy per individual for all prey types provide an alternate ranking of prey quality when compared to energy density, which may affect predators with certain foraging strategies. Juvenile walleye pollock (Theragra chalcogramma), of both age 0+ and age 1+, were the most numerically abundant forage fish species caught and are known to be an important prey type in this area. Age 0+ pollock caught in 2009 were larger and had higher energy density than those caught in 2008, however the relationship between fish size and energy density was the same during the two years. The energy density of individual pollock from both years varied among sampling locations, suggesting that the quality of individual prey items is affected by variation in the environment. However, point-specific measurements of ocean habitat did not explain the observed variation in energy density of juvenile pollock. Model selection indicated that size was by far the most important determinant of total energy density in 0+ pollock. This study observed a wide range in energy density and variation in the quality of these forage fishes and invertebrates will likely affect habitat use by predators seeking to optimize their use of this important prey resource.
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