Habitat Selection Influences Sex Distribution, Morphology, Tissue Biochemistry, and Parasite Load of Juvenile Coho Salmon in the West Fork Smith River, Oregon

Abstract

Given the strong influence of water temperature on salmonid physiology and behavior, in the summers of 2004 and 2005 we studied juvenile male and female coho salmon Oncorhynchus kisutch in two reaches of Oregon's West Fork Smith River with different thermal profiles. Our goals were to compare the body morphology, tissue biochemistry, genetics, and parasite load and determine whether sex, tissue biochemistry, and infection with multiple parasite species influence swimming performance. Sex differences in habitat selection distribution were apparent; proportionately more females occupied the cooler, upper reach, and males predominated in the warmer, lower reach. Despite having similar genotypes, fish in the upper reach had deeper bodies and higher condition factors, regardless of sex. These fish also had higher blood lipids and elevated citrate synthase activity in epaxial white muscle, suggesting a greater potential for aerobic metabolism. Critical swimming speeds measured streamside at 18°C and endurance time were influenced by sex, females performing much better than males. Plasma lactate levels were inversely correlated with swimming performance, indicating that females relied more on aerobic metabolism for energy production. We also found a high prevalence of tissue-specific myxozoans (Myxobolus insidiosus and M. kistuchi) and larval trematodes (Nanophyetus salmincola and Apophallus sp.) in the muscle and “black spot” in the skin. The prevalence of these parasites was higher in the warmer, lower reach than in the upper reach and the lower-reach fish lower condition indices. The degree of parasitism was not correlated with the swimming ability or sex of fish.