We conducted a study of brook trout, Salvelinus fontinalis, movement and habitat use in an unnamed tributary (hereafter referred to as Emerson Brook)in the Nash Stream watershed, Coos County, NH in 2010.We measured movement and dispersal in this small, fragmented mountain stream population of brook trout using Passive Integrated Transponder (PIT) tags to track movement from May to November. Emerson Brook is a small, mountain stream that flows into Nash stream on its western side that is divided into three discrete reaches by two waterfalls. We hypothesized that the geomorphology of these two waterfalls would prevent any upstream movement between reaches, therefore decreasing genetic mixing. Through instream monitoring of fish, we discovered that the upper waterfall was impassable to PIT-tagged brook trout and the lower waterfall was navigated by five PIT-tagged fish that moved upstream through the water fall in 2010. These findings were corroborated by genetic analysis based on microsatellite-based markers showing that little introgression occurred across these geomorphological features.
We collected data over the course of the 2010 season to address the following objectives: 1) to describe habitat and instream wood use by brook trout, 2) to quantify brook trout movement, and 3) to determine the relationship between fish movement and habitat use. Brook trout in Emerson Brook (on average 107 mm in total length) moved continually throughout the season with an average total movement of 98 m over the course of the season in 2010. Habitat use by brook trout in Emerson Brook was similar to that reported for other salmonids in that they were keying into pools and habitat with instream wood nearby. Juvenile brook trout preferred pool habitat (Pearson Chi-Square = 21.836, df = 2, p < 0.001) and habitat with wood jams close by (Pearson Chi-Square = 10.880 df = 3, p < 0.001). Adults also preferred pools with both logs and wood jams (compared to pools without wood) and riffles without wood (Pearson Chi-Square = 121.406, df = 4, p < 0.001). We found positive significant relationships between movement and fish total length (df = 134, R2= 0.148, p < 0.001) and movement of fish and age (df = 13, R2= 0.316, p = 0.036). Fish in the upper reaches of Emerson Brook moved significantly less than those by its confluence with Nash Stream (df = 112, R2= 0.037, p = 0.042). Also, brook trout in Emerson Brook moved more to access pool habitat than riffle habitat (Paired T-test, T = 4.22, p < 0.001). The findings of this study support that brook trout in small, mountain streams need habitat diversity and specifically prefer pool habitat and instream wood.
Mountain streams that have fragmented subpopulations, as seems likely for Emerson Brook, should be carefully considered in watershed-wide management. Tributaries like Emerson Brook provide cold water input into Nash Stream, constitute a refuge for mainstem brook trout when temperatures increase, and offer an opportunity for genetic mixing when mainstem brook trout enter the lower, easily accessible reaches of the tributary to spawn.