Health Assessment, Associated Metrics, and Nematode Parasitism of Rainbow Trout in the Colorado River below Glen Canyon Dam, Arizona

Abstract

We captured rainbow trout Oncorhynchus mykiss in the Colorado River below Glen Canyon Dam, Arizona, between 1995 and 1997 to develop baseline data for a modified health assessment index (mHAI) and to determine relationships among mHAI, parasite burden, relative condition, mesenteric fat index, and diet. We also examined effects of a simulated flood on these variables during 1996. There were no significant effects of the flood on variables measured in the fish. Total stomach volume increased during spring and summer following the flood and reflected an expected trend. Health of fish, based on the mHAI, remained good throughout the study but declined with increasing burden of the nematode Truttaedacnitis truttae and with trout size. Increasing relative condition and greater mesenteric fat levels and stomach volumes of macroinvertebrates were associated with reduced mHAI. Larger trout also ate more Cladophora glomerata than did smaller fish, and nematode burden was related negatively to volumes of the alga and macroinvertebrates in stomachs, suggesting relationships among fish age, parasite burden, and diet. We suggest that health assessments for the tailwater rainbow trout fishery at Glen Canyon Dam should include relative condition and indices of mesenteric fat levels, nematode burden, and diet as metrics. Moreover, evaluations of health should address length‐specific categories for long‐term monitoring of rainbow trout in the tailwater and fish populations in general.