Depth related trends in proximate composition of demersal fishes in the eastern North Pacific

The proximate chemistry of the white muscle and liver of 18 species of demersal fish from the eastern North Pacific was studied to determine trends with depth, locomotory mode and buoyancy mechanism, foraging strategy and to elucidate energetic strategies. Data for 24 species from shallow water were taken from the literature and included for analysis of muscle water content. Benthopelagic species, primarily gadiforms, have significantly larger lipid-rich livers than benthic species. The benthopelagic species may use this lipid to add buoyancy, but it is also used as energy storage. Buoyancy mechanism was directly related to proximate composition. Fishes using gasbladders had normal muscle composition. The two species of benthopelagic fishes without gasbladders have either very high muscle lipid content (Anoplopoma fimbria) or gelatinous muscle (Alepocephalus tenobrosus) to aid in achieving neutral buoyancy. The macrourid, Albatrossia pectoralis, has a very small gasbladder and also has gelatinous muscle. Both of these benthopelagic fishes with gelatinous muscle feed on pelagic organisms. Gelatinous muscle was also found in two flatfishes that inhabit the oxygen minimum zone. For these fishes, high water content may serve to lower metabolic costs while maintaining large body size. Scavengers such as Coryphaenoides armatus and Coryphaenoides acrolepis have lipid rich livers and others such as A. fimbria and Pachycara sp. have high and variable muscle lipid content. Thus foraging mode also acts to influence proximate composition. Several depth-related trends in proximate composition were found. White muscle water content increased significantly with depth, and all four gelatinous species occurred at bathyal depths. This adds evidence in support of the hypothesis that decreasing light levels shorten reactive distances and relax the selective pressure for high locomotory capacity. In addition significant declines in liver protein content were observed, suggesting that the rates of metabolism in this organ also decline with depth. There was little evidence for food availability affecting proximate composition. There were no significant changes in either muscle or liver lipid or caloric density with depth. Total lipid stores actually increased significantly, but they were driven primarily by the abyssal scavenger C. armatus suggesting that foraging strategy rather than depth may be the most important factor determining total lipid stores.