Shifting gears: enzymatic evidence for the energetic advantage of switching diet in wild-living fish

Large variations in the activity and scaling patterns of enzymes involved in anaerobic metabolism exist and appear to be related to species differences in the locomotory habits of fish. Here, we show how the scaling of muscle lactate dehydrogenase (LDH) activity is highly variable in fish, not only among species, but also among populations of yellow perch (Perca flavescens) and lake trout (Salvelinus namaycush) exhibiting large differences in the scaling of fish activity costs. These differences in LDH scaling properties were significantly related to differences in diet ontogeny. Scaling coefficients and adjusted R-2 values of LDH versus body size relationships were both threefold higher in fish that do not make important diet shifts among planktivory, benthivory, and piscivory than in those that do. We argue that fish activity and related glycolytic potential are reset to lower values whenever fish are able to switch diet to larger prey while growing; we implicate the burst component of foraging (mostly attacks) as being responsible for changes in activity costs. Our results suggest that anaerobic power requirements in fish are highly plastic and adapted to local and recent food web conditions. We discuss these findings in relation to optimal foraging theory and the energetic basis of prey-size selection.