Export of invertebrate drift from fishless headwater streams

An understanding of ecological linkages between headwater systems and downstream habitats is needed to enhance management practices for aquatic conservation. I quantified and described the export of invertebrate drift from fishless headwater streams to assess its potential importance to downstream populations of coastal cutthroat trout (Oncorhynchus clarki clarki) in the lower Klamath River in northern California. From June 2015 through April 2016, I sampled invertebrate drift in six fishless headwaters in the sub-basins of Tectah, Ah Pah, and Tarup creeks. Concurrently, I collected invertebrate drift and trout diet samples from adjoining fish-bearing streams. Drift export rates were lowest in October and greatest in April, and ranged from 98 - 1331 mg dry mass stream-1 d-1. Invertebrate taxa of aquatic origin dominated drift biomass in fishless streams on all four sampling occasions; the proportion of terrestrially-derived biomass was highest in October. Estimates of daily drift flux in fish-bearing streams exceeded the delivery of drift mass from fishless streams throughout the year. Trout diet samples demonstrated tremendous variability in invertebrate biomass among individuals within seasons. However, average biomass per diet sample differed strongly among sampling occasions, and was greatest in April. Terrestrial taxa dominated the biomass of trout diets in June and October. Both drift and diet samples were taxonomically rich, but exhibited little similarity to each other. I estimated drift exports from fishless headwaters could support a maximum of 37 g dry mass stream-1 year-1 of trout, theoretically accounting for one-tenth to one-quarter of the annual production of over-yearling trout in the study streams