Food availability as a selective factor on the chemical compositions of midwater fishes in the eastern North Pacific

In September and October 1980 we examined the relationships between food availability, depth, and chemical composition among 12 midwater fish species, from three adjacent areas of the eastern North Pacific: the eastern gyre, the California Current, and the transitional region between them. By comparing trends in chemical composition across a geographical productivity gradient, the influence of food availability could be examined both with and without depth as a dependent variable. In general, caloric density, lipids, and water content showed consistent trends along both vertical and horizontal gradients of food availability. Lipids and caloric contents were lowest among bathypelagic species and among fishes from the gyre. Water content was highest in the gyre and among deeper-living species. While protein content as a percentage of the total wet weight declined with depth, there was no clear trend among mesopelagic species between stations. Protein as percent of the total organic content showed no decrease with depth, but was significantly higher in the gyre among mesopelagic species. These findings suggest that compositional trends in water, lipids, and caloric content are correlated with food availability, regardless of any depth-related factors. In contrast, protein did not vary consistently with food availability and thus may be linked to other factors.