Fish food habits and their interrelationships in lower Redwood Creek, Humboldt County, California

Eleven species of fishes within the lower Redwood Creek watershed were examined for food habits from February to December, 1980. Seasonal and spatial intraspecific patterns were analyzed, as well as interspecific diet overlaps. The fishes of lower Redwood Creek relied almost entirely on on autochthonous production for their food resources. The food habits of these fishes suggested that most of the species were benthic predators. The exception was juvenile chinook salmon, which appeared to rely more heavily on drift organisms. The limited contribution to the food habits by terrestrial prey items was, at least partially, a result of the lack of riparian habitat along the main channel of lower Redwood Creek. Such productive areas were eliminated during levee construction. The food habits of most fish suggested opportunistic feeding strategies, except for the more specialized Humboldt sucker. The fishes that were generally considered freshwater species fed most heavily on aquatic dipterans, specifically chironomids, at least while residing in the main channel portions of the study site. The most notable exception to this was the heavy utilization, by some fishes, of the abundant Corophium resource that was present in the embayment in the fall. The euryhaline fish species relied more on benthic crustaceans throughout the study. In the sloughs, all fish ate primarily crustaceans. Most of the intraspecific food habit oomparisons showed significant temporal and spatial variations, although there were a few cases of significant diet overlaps between sites and between seasons. This variability was attributed to the diversity of the food habits of most of the fishes. The most notable exception to this variation in food habits was for the Humboldt sucker that resided in the main channel of Redwood Creek. For this species, the almost sole reliance on chironomid larvae was responsible for the extreme similarity of diets. All interspecific comparisons of diets were significantly different. Common utilization of certain food resources, mostly chironomid larvae or Corophium, by different species of fish resulted in some significant overlaps of food habits. Most of the overlaps were between nonsalmonid fish species. There were no cases of biologically significant diet overlaps between any of the salmonids. This might not have been the case if the berm had not been prematurely breached and the fish had been allowed to rear and compete in the embayment throughout the summer and fall. In any case, most of the biologically significant diet overlaps occurred during the summer when food was probably not a limiting factor. Fish predation on other fishes within lower Redwood Creek was insignificant for all species examined.