Range, Activity, and Habitat of Large, Free‐Ranging Brown Trout in a Michigan Stream

Abstract

Little is known about behavior patterns and habitat use of large (400+ mm total length) brown trout Salmo trutta. We used radio telemetry to monitor the movements of eight large (437–635 mm), free‐ranging brown trout for up to 346 d. Total range of movement upstream and downstream varied from 370 m to 33,420 m. At some time during the year, six of the fish moved out of an area protected with no‐kill fishing regulations, even though none of them were tagged and released closer than 2 km from its boundaries. However, four of five fish tracked during the height of the fishing season spent 87% of their time in the protected area. The fish appeared to have separate winter and summer ranges. Five of six fish tracked during autumn–winter moved upstream about 10 km to slower, deeper parts of the river between August and November, and remained there at least through the following April. The part of the river used as autumn–winter range was considered only marginal habitat for brown trout during summer due to warm water temperatures. Individuals used as many as four specific home sites within their spring–summer range; average separation between home sites was 386 m. The fish typically rotated among sites, spending 2–3 d at one site before moving, usually at night, to another section of river. Activity was divided into two categories: active displacement – long‐range travel from one part of the range to another; and foraging – short‐range movements that might occur when searching for food. Active displacement appeared to be correlated with stream discharge, Fish displaced themselves an average of 239 m between telemetry observations in spring–summer and 3,103 m in autumn–winter; only 11% of this variation was attributable to variations in the length of time between telemetry observations in spring–summer and autumn–winter. Daily foraging activity varied by month and appeared to be related to light level, food availability, and water temperature, In spring–summer, fish typically moved to midstream to forage at night, then returned to cover at specific home sites during the day. The sites they selected in daytime were characterized by deep (>30 cm), slow (< 10 cm/s) water with heavy log cover and silt substrate.