Optimal diets in complex environments: feeding strategies of two herbivorous fishes from a temperate rocky intertidal zone.

The seasonal diets of Cebidichthys violaceus and Xiphister mucosus, two herbivorous fishes from the complex, multifood environment of the central California rocky intertidal zone, were partially consistent with three predictions of optimal diet models based on energy maximization: 1) The prediction that at high food densities a forager should concentrate solely on the energetically most valuable items was incompletely met by these two fish species. C. violaceus and X. mucosus increased their consumption of energy-rich annual macrophytes during periods (summer and fall) of high food abundance, but nevertheless continued to take a mixed diet. 2) The prediction that abundance of lower-valued foods does not determine their inclusion in the diet was largely upheld by the feeding habits of these two intertidal fishes. The probability of an item being consumed apparently depends upon its abundance as well as its chemical composition. 3) The prediction that foragers will generalize as food abundance declines was largely met by the two fishes since their diets broadened considerably during periods (e.g. winter) of reduced food supply. Furthermore, diets of the two species converged during periods of high food abundance and diverged during months of low food abundance. Whether this seasonal pattern of overlap was due to interspecific competition or other factors is unknown. This study, in agreement with other recent investigations, indicates that optimal diet models cannot be based solely on energy maximization but should also include nutrient constraints in order to more accurately predict the seasonally fluctuating mixed diets of these fishes and other generalist herbivores.