Characteristics of Local Populations of Steelhead Trout in Three South-Central Lake Superior Streams

This study characterizes populations of steelhead trout (Oncorhynchus mykiss) in three streams in the South-Central Lake Superior watershed.  Steelhead trout were introduced to the Great Lakes in the 1880’s and are potential competitors with native brook trout (Salvelinus fontinalis) and other native species. Our laboratory is monitoring the effects of an exotic salmonid removal, including steelhead trout, on the brook trout population in Sevenmile Creek as part of a long term coaster brook trout rehabilitation project in Pictured Rocks National Lakeshore, Michigan.  The non-native salmonids of Lake Superior are little studied and it is currently unclear how their distinctive life history characteristics may be impacting local fish communities.  We examined steelhead trout life history parameters including length at age, condition at age, cohort and individual growth rates, habitat use, and distribution patterns.  Other aspects of the study included identifying average dates of upstream and downstream migration, and determining at what size and age these events occur.  The data suggests that there are two size classes, likely representing two age classes (YoY and Age 1), present in the streams at times other than spring; during spring, adults of multiple ages join the community.  Juvenile steelhead in our study streams typically smolt at age 2, usually migrating in late May through early July.  They return back to the streams to spawn in late March and early April, typically at age 4-5.  There is an evident difference in the in-stream distribution of steelhead between Mosquito River and Sevenmile Creek; steelhead are evenly spread throughout Mosquito River while in Sevenmile Creek they tend to dominate the downstream reaches.  We also assessed the effects of the physical removal of exotic salmonids on the remaining steelhead population.  It was expected that the steelhead trout population that persisted in Sevenmile Creek would be reduced in the total number of individuals, and the individuals would be, on average, younger and smaller in size, but have a higher condition and faster growth rates than the population before 2008 because of reduced intraspecific competition; early results support these hypotheses.  The average dates of upstream and downstream migration, and at what size and age these events occur, appear to be unchanged by the removal project in Sevenmile Creek.  This study will contribute to our understanding of the status of non-native salmonids in Lake Superior and the impact they have on local fish communities.