Chemical composition of midwater fishes as a function of depth of occurrence off the Hawaiian Islands: Food availability as a selective factor?

The variation with depth in water, lipid, protein, carbon and nitrogen contents (% wet weight) of 42 species of midwater fishes, collected in November 1976 off the west coast of Oahu in the Hawaiian Archipelago, was measured. The Hawaiian fishes show significant relationships between these components and depth of occurrence. The slopes of these relationships are not significantly different from those reported for midwater fishes from off California, USA. However, the fishes from Hawaii have significantly lower lipid levels and higher protein levels than do the species from off California. The deep-living Hawaiian species (500 m and deeper) have significantly lower lipid (% wet weight), but there is no significant difference in protein (% wet weight). The difference in lipid contents at all depths appears to be an evolved characteristic, with the greater lipid levels off California being selected for by greater spatial and temporal variation in the food supply for these fishes off the California coast than off Hawaii. The higher protein contents in the shallow-living Hawaiian fishes appear to reflect greater muscle power selected for in these fishes by the greater water clarity, and therefore greater reactive distances, in the surface layers off Hawaii. These conclusions support the general hypothesis that the lower protein contents of bathypelagic fishes are not directly selected by food limitation at depth, but rather result from the relaxation of selection for rapid-swimming abilities at greater depths due to the great reduction at greater depths in the distance over which visual predator-prey interactions can take place. The lower lipid levels in the deeper-living species are apparently made possible by the reduced metabolic rates of these species which reduces their need for energy stores.