Racial Identification Of Chinook Salmon Oncorhynchus Tshawytscha And Juvenile Steelhead Trout Salmo Gairdneri PDF Download

Efforts to manage stocks of salmonids in Pacific Northwest stream systems are complicated by the occurrence of several runs of a species thought to represent races rearing sympatrically as juveniles. In order to collect the population statistics needed to properly manage these stocks, managers need a method of identifying juvenile salmonids by race. In an effort to determine if taxonomic or morphological differences exist between these races, wild juvenile summer and winter steelhead trout (Salmo gairdneri) and wild juvenile spring and fall chinook salmon (Oncorhynchus tshawytscha) from the Rogue River (Oregon) were studied. The goal of the study was to determine if differences in taxonomic characters would allow racial identification of individual fish with 90% accuracy. The wild juvenile summer steelhead trout had significantly fewer vertebrae and larger nuclei, greater average intracircular spacing, and a larger length at first annulus formation of scales than winter steelhead trout. No differences between juvenile summer and winter steelhead trout were found in otolith nuclear diameter, total lipids or fatty acid composition of the muscle tissue. Wild juvenile spring chinook salmon exhibited significantly lower vertebral counts, larger otolith nuclear diameter, greater average intracircular spacing, larger scale nuclei, and a larger first and second band of five intracircular spaces than juvenile fall chinook salmon. Mesentary fat deposition was highly variable between fish from the same streams and was not useful in separating juveniles by race. Although significant differences between races were found for both species, none of these differences were sufficient to allow the racial identification of individual fish with 90% accuracy. It is not known if the differences found were caused by genetic or environmental effects or both. One experiment showed that summer and winter steelhead showed no difference in vertebrae or otolith dimensions when incubated under the same conditions, suggesting that differences found in wild steelhead trout were caused by environmental effects. The lack of distinct phenotypic differences between individual fish of different races may be caused by extensive interbreeding between races of salmonids in the Rogue Basin due to large environmental variability during the spawning season for chinook salmon and steelhead trout. It is also possible that environmental variation in the early life history masked genetic differences between the races.