Seasonal patterns of terrestrial and aquatic prey abundance and use by Oncorhynchus mykiss in a California coastal basin with a Mediterranean climate

Terrestrial invertebrates are a major source of prey for salmonids in many streams. Their importance as prey appears to be related to (1) the seasonal timing of terrestrial inputs relative to the abundance of aquatic prey and (2) water temperature, which affects food demand by fish. Most studies of seasonal patterns of terrestrial inputs have come from temperate systems, and patterns in most other systems are unknown. We measured monthly biomass of aquatic invertebrates, input of terrestrial invertebrates, and diets of Oncorhynchus mykiss (non-anadromous and juvenile anadromous life history forms) for 15 months in two streams in a basin with a Mediterranean-type climate on the Big Sur coast of California. Biomass of aquatic invertebrates and terrestrial inputs followed a similar seasonal pattern; highest levels occurred in summer and early autumn and were highly correlated with water temperature. Total annual input of terrestrial invertebrates was 8.7 g(.)m(-2.)year(-1), and terrestrial inputs provided about half of the prey biomass and energy consumed by O. mykiss during the study. Normative terrestrial isopods, primarily Armadillidium vulgare, contributed 30-40% of the biomass and 20-30% of the energy consumed-the highest proportions among all taxa. The annual input, seasonal pattern, and contribution of terrestrial invertebrates to salmonids in this coastal Mediterranean-type basin were similar to published values from temperate forested streams. However, the magnitude of seasonal fluctuations of inputs was less pronounced than that in most temperate streams and appears to reflect the lower intra-annual temperature variation and longer leaf-out period in this system. Unlike many temperate streams, where terrestrial inputs provide an alternate prey source when aquatic invertebrate abundance is low, terrestrial inputs to these two coastal streams apparently provide a year-round additional source of prey that (like aquatic prey) peaks when water temperature is warmest and hence when fish growth potential is high.