Masters Thesis

Seasonal ecology of fish and macroinvertebrates inhabiting Fourmile Creek, Upper Klamath Lake, Oregon

I investigated the species assemblage and abundances of fish and macroinvertebrates inhabiting the lower three km of Fourmile Creek, an altered and largely unstudied intermittent tributary to Upper Klamath Lake, Oregon. Seasonal reproductive state, body condition and diet of the four most abundant fish (speckled dace, Rhinichthys osculus, fathead minnow, Pimephales promelas, slender sculpin, Cottus tenuis, and yellow perch, Perca flavescens) were also examined. Nine native and seven non-native fish species from eight families were captured March – October, 2005 and 2006. Native species, including speckled dace and slender sculpin, tended to dominate the fish catch from March – August; non-native species, including fathead minnow and yellow perch, increased in abundance during fall. Both abundance and species richness were greatest at sites situated within the lower one km of Fourmile Creek where surface flow persisted longer and water quality (temperature, dissolved oxygen and pH) tended to be more stable than at more ephemeral upper creek sites. Gonadosomatic indices and the presence of larvae and juveniles show Fourmile Creek to be an important breeding habitat for speckled dace, slender sculpin and other native fish. Speckled dace and yellow perch spawn in spring, fathead minnow spawn in summer, and slender sculpin spawn during late summer/fall. Condition factors of sexually mature fish tended to decrease both during their reproductive season, and as surface waters disappeared. Especially poor body condition in slender sculpin likely resulted from the temporal overlap of reproduction with stream drying, followed by growth-limiting winter habitat conditions. Benthic insects and crustaceans were the most important foods for speckled dace and slender sculpin; detritus and crustaceans were the most important foods for fathead minnow; insects, crustaceans and fish were the most important foods for yellow perch. Dietary overlap between speckled dace and fathead minnow was not significant across seasons, even as the creek dried, suggesting minimal dietary competition between these two abundant cyprinid species. Crustaceans, insects and annelids were the dominant macroinvertebrates inhabiting Fourmile Creek. Cladocerans and copepods, the dominant crustacean taxa, peaked in abundance during spring/early summer and during the fall. Dipterans (principally chironimid larvae) dominated the insect community during most months, while hemipteran and coleopteran taxa increased in abundance as the creek dried. Other insects, including simuliid larvae, were only abundant until early summer, when water flow decreased.

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