Natural replacement of invasive brown trout by brook charr in an upper Midwestern United States stream

Competition with invasive species and a warming climate have threatened brook charr (Salvelinus fontinalis) populations throughout their native range. In particular, brown trout (Salmo trutta) often displace brook charr when the two are sympatric. Brook charr and brown trout populations were monitored annually in a Minnesota stream from 1981 to 2017. Both species increased in annual production during the first 18 years of monitoring. During the last 15 years, brown trout production steadily decreased, while brook charr continued to increase. Adult brook charr production increased from a low of 13.5 kg/ha/year in 1983 to a high of 357.7 kg/ha/year in 2013. Adult brown trout production increased from 40.1 kg/ha/year in 1982 to a high of 256.9 kg/ha/year in 2000 before decreasing to zero in 2011. Contrary to previous observations and predictions, brook charr displaced brown trout during our long-term monitoring. During this period, both air temperature and brown trout abundance increased in the region. However, brook charr may be outcompeting brown trout in this particular reach due to decreasing water temperature brought about by increasing base flow discharge. This study provides a rare example of a native charr species displacing an invader without direct management intervention.