THE EFFECT OF THE INTRODUCTION OF CUTTHROAT TROUT ON THE BENTHIC COMMUNITY OF LAKE LENORE, WASHINGTON (PREDATION, VERTICAL MIGRATION, CHAOBORUS, FISH)

In 1979, the Washington State Department of Game began stocking cutthroat trout (Salmo clarki henshawi) into Lake Lenore, a moderately alkaline lake that previously contained no resident populations of fish. In this dissertation, I examined effects of the introduction of cutthroat trout on populations of planktonic and benthic invertebrates in Lake Lenore.;Analysis of stomach contents of cutthroat trout collected in gill nets revealed that mature trout fed selectively on several prey species including: Chaoborus flavicans, Hyalella azteca and Callibaetis sp. Although the densities of these invertebrates have not declined dramatically since trout were introduced, subtle changes in the spatial and temporal distribution of these prey have occurred. Formerly, Hyalella and Callibaetis were present in both littoral and profundal areas of the lake. Since 1979, Callibaetis has disappeared and abundance of Hyalella has declined in samples from the profundal zone. Also, a change in the pattern of diel vertical migration of Chaoborus flavicans has been observed. Formerly, third and fourth instar larvae and pupae resided in the water-column during day and night. Since the introduction of trout, individuals of these developmental stages have migrated into the sediments during the day to avoid consumption by trout.;A population budget for Chaoborus flavicans indicated the cutthroat trout consumed 68% of pupae during the spring emergence of 1984. These high rates of predation are predicted to result in declines in density of C. flavicans if abundance of trout continues to increase. These predicted decreases in Chaoborus density would affect the planktonic community. The appearance of C. flavicans in 1974 probably resulted in the disappearance of Diaptomus nevadensis and declines in abundance of Diaptomus sicilis, as well as the appearance of spines on the nape of juvenile Daphnia pulex. I predict that continued stocking of cutthroat trout into Lake Lenore will lead to reductions of C. flavicans and a return of the planktonic community to conditions similar to those which occurred before establishment of Chaoborus in 1974.;In laboratory feeding experiments, juvenile trout smaller than 6.5 cm standard length (SL) fed primarily on planktonic prey, whereas larger juveniles fed more frequently on benthic prey. Results of stomach content analysis of trout reared in an enclosure in Lake Lenore also indicated that the diet of trout shifted from planktonic prey to a diet composed primarily of benthic invertebrates at about 7 cm SL. A change in the allometry of mouth gape to SL coincided with these changes in composition of diet.