Inventory of spawning redd locations and habitat partitioning among five co-occurring salmonids in the Bois Brule River, Wisconsin

Abstract

The Bois Brule River is a renowned, spring-fed, western Lake Superior tributary that supports five naturally reproducing populations of salmonids including native brook trout Salvelinus fontinalis and introduced brown trout Salmo trutta, rainbow trout Oncorhynchus mykiss, coho salmon Oncorhynchus kisutch, and chinook salmon Oncorhynchus tshawytscha. This sympatric mix of native and introduced salmonids that includes potamodromous and stream resident life histories may result in overlap of natural reproduction strategies and spawning habitat requirements. With increases in recreational angler use, combined with predicted changes to trout stream habitat in Wisconsin from a warming climate, a better understanding of species interactions during spawning will become increasingly important to guide future management of these sportfish populations. My objective was to map species-specific spawning redd locations and evaluate physical, flow, and thermal conditions in these habitats of the Bois Brule River, Wisconsin during 2021-2022. I conducted redd surveys by canoe over a 9.5-river mile section that encompassed historically important spawning areas. My results indicated that spring spawning rainbow trout and fall spawning pacific salmon species were using the same spawning locations on larger gravel reefs in the center of the channel, downstream of riffle sections. Native brook trout were found spawning on smaller substrates with slower streamflow on the edges of the stream channel, with large congregations of spawning activity occurring in shoreline areas of lentic habitats of the river. Results from my study provide valuable spawning habitat information and increase understanding of species interactions for stream habitat management of the Bois Brule River in the future and may provide insight into management of other Great Lakes tributaries with similar sympatric, naturally reproducing salmonid species.