Summer physical habitat associations and movement of sympatric juvenile rainbow trout (Oncorhynchus mykiss) and Dolly Varden char (Salvelinus malma) in coastal streams of British Columbia

We investigated the physical habitat associations and movement of sympatric rainbow trout (Oncorhynchus mykiss) and Dolly Varden char (Salvelinus malma) during summer in five tributary streams of the Upper Nahmint River, British Columbia, Canada. The ecology of juvenile salmonids is well-studied, but studies involving these species in sympatry are not common. We surveyed approximately 200 m of five tributary streams of varying stream widths (5.1 to 10.6 m) for fish habitat and conducted mark-recapture fish sampling using minnow traps and passive integrated transponder tags to estimate, for fish > 60 mm (age 1 +), population sizes and densities, establish habitat associations, and characterize movements. Rainbow trout population estimates ranged among streams from 21 to 181, while Dolly Varden population estimates ranged from 14 to 202 individuals. Both species were negatively associated with habitat unit gradient (P < 0.001 and P < 0.006, respectively). The density of both species was significantly higher in pool habitats compared to other habitat unit types across all tributary streams. Sixty-five percent of rainbow trout did not move from original trap locations, but of the movers, distances ranged from 25 m downstream to 30 m upstream, with no direction preference. Fifty-five percent of Dolly Varden moved, ranging from 40 m downstream to 25 m upstream; for both species, there was no clear directionality for those which moved. Overall, our results highlight that to conserve these species in sympatry in small coastal streams, particularly in an era of changing hydrological and thermal regimes, it will be important to ensure pool habitats are protected, or if needed, enhanced.

Automated summary

This study investigated the habitat associations and movement of sympatric rainbow trout and Dolly Varden char in five tributary streams of the Upper Nahmint River in Canada. The results showed that both species preferred pool habitats and had no clear directionality in their movements. Overall, this study highlights the importance of protecting pool habitats in small coastal streams for the conservation of these species in the face of changing climatic and hydrological conditions.