Masters Thesis

Predicting prey availability for stream salmonids

In this study I evaluated the effectiveness of the percentage by mass of behaviorally drifting invertebrates within drift samples to serve as an index of a stream’s capacity to support salmonid growth. I tested hypotheses over seasonal timeframes that: 1) the taxonomic composition of stream salmonid diets corresponds more closely with the composition of the drift than of the benthos; 2) the percentage by mass of behaviorally drifting invertebrates in salmonid diets corresponds with the percentage of behaviorally drifting invertebrates in drift samples; and 3) salmonid growth is positively correlated with the percentage by mass of behavioral drift. The study was conducted in two 100-m reaches in each of six 2nd - 3rd order streams within the Smith and Klamath River basins in coastal northern California. Stream reaches received experimental manipulations of canopy opening and salmon carcass addition that produced sites differing in salmonid production. Within each reach, invertebrate benthos, drift, and diets from resident cutthroat trout (Oncorhynchus clarki), and rainbow trout (Oncorhynchus mykiss) were sampled three times each during summer and winter base flow conditions and seasonal measurements of salmonid growth from PIT-tagged individuals were obtained. Taxonomic composition of the salmonid stomach samples was not substantially similar to either the drift samples or the benthic samples. Salmonids did not appear to be actively selecting for behaviorally drifting invertebrates and no relationship was observed between the growth of juvenile salmonids and the percentage of behaviorally drifting invertebrates in the drift. As such, the relative percent of behavioral drift was found to be insufficient as an index of prey availability. Contributing to the difficulty of assessing prey availability was stream flow variability, seasonal and diel abundance of other prey sources, incomplete taxonomic classification, and the opportunistic feeding behavior of juvenile salmonids.

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