Fluctuating densities of juvenile brown trout (Salmo trutta) and Alpine bullhead (Cottus poecilopus) over a 25-year period in the Atna River, Norway

Long term monitoring data from pristine or relatively unimpacted watersheds represent a valuable ecological baseline in the 21st century. Here, we present a 25-year time series of brown trout (Salmo trutta) and Alpine bullhead (Cottus poecilopus) densities in the boreal-subalpine Atna River in Norway (62 °N) and investigate their relationships with discharge and water temperatures. There was an overall negative trend in densities of Alpine bullhead (3 out of 4 sites), a declining trend in densities of age 0 brown trout (2 of 4 sites), and an inconclusive trend for older brown trout (1 of 4 sites). There were substantial fluctuations in densities for both species, and the majority of the variance originated from year-to-year variation. For older brown trout, approximately 33 % of the variance could be attributed to site-level factors, i.e. systematic differences in densities between sites. Concurrently, there were changes in some of the discharge and temperature-related metrics. The lowest flow of the year became gradually higher throughout the period whereas mean discharge in spring decreased. The best supported models showed a negative relationship between fish densities and mean annual discharge (Alpine bullhead and older brown trout) and a negative relationship between density and minimum annual discharge for age-0 brown trout. The models explained only modest amounts of overall variance in the dataset (range 0 to 12 %), but did document a potentially limiting effect on fish densities on an annual basis. We discuss the likely mechanisms by which discharge and water temperature can influence the abundance of brown trout and Alpine bullhead in river systems, and highlight the value of long-term monitoring as a baseline when investigating broad-scale shifts in abundance.