Temperature, trout, and trophic responses: Interactions in Michigan trout streams.

Abstract

The central problem of this dissertation was to determine the relative importance of temperature and macroinvertebrate abundance on growth of trout. I combined a field study with a growth experiment to address the direct and indirect effects of temperature and macroinvertebrate standing stock on growth of juvenile trout. Temperature and trout were monitored at 17 sites, and macroinvertebrates at 12 sites, from 1993 to 1996. I used principal components analysis to describe the overall thermal character of the streams. Two principal components (PC), representing cool summer temperature and low thermal variation, explained 74.8% of the variation in 25 temperature summary statistics. These two PCs explained over 60% of the variation in growth of trout observed in the field. Simple linear regression models of July temperature summaries explained about 50% of the variation in growth of brook trout (mean daily fluctuation, R<super> 2</super> = 0.482) and brown trout (daily mean, R<super>2</super> = 0.531). Macroinvertebrate standing stocks were significant predictors of growth of trout and explain an additional 5 to 10% of the variation in growth of brook trout when combined with July temperature fluctuation. Causal path models were developed for trout growth and parameterized with data from the field study and growth experiment. Temperature was the strongest factor affecting variation in growth of trout having a 50% greater effect than the combined effect of invertebrate standing stocks. However, indirect effects of temperature acting through the invertebrate variables accounted for about half of the total temperature effect. These models suggest that temperature effects growth of trout directly through physiological processes and indirectly through prey availability. I used replicated large-scale perturbations in the benthic community to address the effects of variation in prey standing stock to predators of macroinvertebrates. Growth rates of juvenile brook trout declined in response to the reduction of a dominant periphyton grazer (<italic>Glossosoma nigrior </italic>). A causal path model of the biotic components affecting trout growth suggests that <italic>Glossosoma</italic> populations influence growth by increasing the vulnerability of other invertebrates to trout. This study is the first to document the impact of a pathogen-induced trophic cascade on lotic macroinvertebrate predator and trout populations.