Movement Patterns of Large Brown Trout in the Mainstream Au Sable River, Michigan

Abstract

We used radiotelemetry to monitor spring and summer movements of 11 brown trout Salmo trutta (442–584 mm) for up to 904 d in a Michigan stream. Individual brown trout used a few specific locations near cover (referred to as home sites) as resting locations during the day, moved across various distances at night, and generally returned to the same home site the next morning. Home sites were predominantly artificial cover (88%) rather than natural sites, as natural cover was very limited in the study area. Some fish used multiple home sites, and the average separation between multiple home sites for individual fish was over 500 m. Fish tracked for more than 1 year used the same home sites each summer and generally exhibited similar behavior each year. Fish belonged to two general categories of daily movement behavior: mobile or stationary. Mobile fish tended to move frequently and were found within their home sites only 43% of the time at night. Stationary fish did not move far from home sites, even at night. There was a negative correlation between the average gradient and the maximum distance fish moved from their home sites during nocturnal periods. Stationary fish resided in areas of steeper gradient (usually about 0.20%) and moved less often nocturnally than did mobile fish. Three fish were tracked extensively over 36 d to quantify diel activity patterns. The hourly activity of fish increased dramatically at dusk, continued at a lower level overnight, and then increased again at dawn before declining to near zero during the day. This behavior pattern was similar among all individuals tracked and also between the months of June, July, and August for an individual fish. Nocturnal movements involved significantly greater distances than diurnal movements for these fish. The relationship between movement and gradient may indicate energetic tradeoffs between the cost of moving against a current and the energy gained during active foraging. Also, the dominant use of artificial home sites has implications for the value of habitat improvements meant to increase abundance of large brown trout.