A Study of Brown Trout and Coho Salmon in a Lake Superior Tributary in Wisconsin

Abstract

I used a fish trap, seining, and a creel survey to determine population statistics of anadromous brown trout (Salmo trutta) and coho salmon (Oncorhynchus kisutch) in the Sioux River, a Lake Superior tributary in Bayfield County, Wisconsin, in 1987 and 1988. Brown trout populations exhibited some evidence of stability: population estimates for 1987 and 1988 were 3231 and 2981. Coho salmon runs were smaller and more variable: population estimates for 1987 and 1988 were 1537 and 2782. The sex composition of the brown trout run changed as the_ run progressed, with the number of females increasing at the end of the 1987 run. Females predominated throughout the 1987 coho salmon run. Brown trout grew to lengths exceeding 60 cm, and growth was influenced by the time that fish spent in the river before smolting. The majority (63 percent) of brown trout spent 2 summers in the river before smolting, whereas all coho salmon spent 1 year in the river before smolting. No coho salmon was found older than age 2. Repeat spawning of brown trout was low, 4 percent. My estimates of exploitation of ascending adults, 4 to 10 percent for brown trout and 31 to 60 percent for coho salmon, indicate that at least 90 percent or more of the brown trout, and 40 to 69 percent of the coho salmon run escape the stream sport fishery. Estimated total annual mortality of adult brown trout was 89 percent; hence, the stock suffered substantial mortality from sources other than stream anglers.