Masters Thesis

The effects of introduced trout on native macroinvertebrates from lakes in the Trinity Alps Wilderness in northern California

I examined differences in native macroinvertebrates among four lake management categories (fish stocked, temporary stocking suspension, fish removal lakes, and historically fishless lakes) and among three habitats (rock, organic/silt substrate, and emergent vegetation) from 16 different lake basins in a four-year study (2003-2006) in the Trinity Alps Wilderness in northeastern California. This study showed that introduced insectivorous fish reduce the diversity of native aquatic insects. Chironomid midges were more abundant and in greater proportion in fish lakes than in fishless lakes. Additionally, more taxa were sampled each subsequent year following fish removals and more taxa were sampled from Hidden Lake, a stocking suspension lake that did not maintain a fish population, than in the other three stocking suspension lakes that did sustain viable fish populations. The reduction in insect diversity due to fish was further exemplified in Hidden Lake alone, where more taxa were recorded each subsequent year of the study. Libellula, a stout-bodied predaceous dragonfly, was most common in fish stocked lakes. The life history and morphology of Libellula (Odonata: Libellulidae) seems to give them an advantage over other invertebrate predators in fish lakes. Fish create top-down effects that are illustrated by the apparent ability of Libellula to regulate other insect abundances and proportions. This study also demonstrated how large-bodied insects are more commonly found in complex habitats, which may be attributable to increased habitat availability and resources, to the invertebrates seeking refugia from insectivorous fish, or to a combination of both.