Use of long‐term (40+ year) trend data to evaluate management actions on brown trout, Salmo trutta, populations in groundwater‐fed streams

A 40+ year programme to monitor brown trout, Salmo trutta L. (populations at 25 groundwater‐fed stream sites in southeast Minnesota, USA, was initiated to identify population trends, evaluate management actions and test ecological theories regulating populations. Significant increases between 1970 and 2018 were found for total biomass (an average of 5% annually) and abundance of juveniles (7%), all adults (7%) and adults ≥305 mm (3%). Sites managed with instream habitat projects had an additional 30% higher abundance for trout ≥305 mm and 57% higher for trout ≥355 mm. Sites managed with a catch‐and‐release regulation had nearly 130% higher abundance of trout ≥305 mm and 100% higher for trout ≥355 mm. Trout recruitment was temporally synchronous over the 9,200 km2 area but not associated with spatial distance. There was little support for stock–recruitment relationships, and density‐dependent growth was only detected at half of the study sites. Increasing abundance trends represent a fisheries management success and suggest that these populations were largely regulated by a coupling of abiotic factors managed at two spatial scales: watershed (improved land use practices facilitating greater water infiltration) and instream (habitat improvement projects).