Multi-scale variation in abundance and distribution of juvenile salmonids in neighboring Lake Superior tributaries: Potential effects of fine sediment aggradation and species interactions

Background/Question/Methods

The Salmon Trout River (STR) in Marquette County, MI hosts the last verified remnant and reproducing population of adfluvial coaster brook trout (Salvelinus fontinalis) in the south central shore of Lake Superior.  Restoration of migratory coasters and all the various life history forms of brook trout is a Lake Superior basin wide goal.  Beginning in 2003, we have conducted annual multi-pass electrofishing surveys of fish populations in six fixed study sites in the STR and one site each in a set of neighboring rivers. Along with fish surveys, we have quantified substrate conditions (e.g., percent fine composition of sediments, sand depth, and embeddedness) in each site to assess multi-scale relationships between brook trout juveniles and the biotic and abiotic components of the river habitat. These long-term surveys reveal juvenile salmonid densities in the STR are substantially lower than densities in neighboring rivers. At the reach scale within the STR we detect species-level longitudinal variation in distributions. We hypothesize that total and species-specific juvenile salmonid densities relate to river habitat conditions (e.g., aggradation of fine substrates) at some scale through effects on system production or habitat selection, yet species interactions likely influence how individual species are distributed. 

Results/Conclusions

At the regional scale (i.e., across rivers), juvenile salmonid (brook trout, coho salmon, and rainbow trout) densities varied by a factor of 3 across the tributaries, and sites with greater streambed aggradation of fine sediments had fewer juvenile salmonids.  The composition of juveniles varied among rivers with low fine sediment aggradation and ranged from a brook trout only system above a barrier, to rivers where brook trout are absent and likely excluded by nonnative salmonids.  The STR study sites ranged from higher gradient sites upstream to relatively low gradient sites downstream.  Longitudinal surveys along this gradient revealed greater abundances of brook trout in lower reaches where substrates were more embedded with sand. Greater abundances of potential competitors (juvenile coho salmon and rainbow trout) were observed in upper reaches that appear to have better habitat quality.  However, associated research in the STR indicates brook trout prefer less sandy habitats.  These results support the hypotheses that brook trout density may be limited by biotic interactions with non-native salmonids in higher quality habitat, brook trout find competitive refuge in more degraded sites, and may be relatively excluded from some of their native rivers that are less degraded due to aggraded fine sediments.