Regional Synchrony of Brown Trout and Brook Trout Population Dynamics among Michigan Rivers

Abstract

The ability to describe regional patterns in trout density would be useful for biologists concerned with population status across large regions as well as managers of rivers at the local scale. Noting the importance of flow conditions at the time of emergence to trout year-class strength in Michigan streams and the influence of age-0 trout abundance on subsequent abundance of older age-classes, we assessed the potential for regional synchrony in the population dynamics of brown trout Salmo trutta and brook trout Salvelinus fontinalis among Michigan rivers. We used correlation analyses to look for regional synchrony in May stream discharge (approximating the time of brown trout fry emergence) and fall trout density among many Michigan trout streams. We found a high degree of synchrony in average May discharge among streams, particularly those in the northern portion of Michigan's Lower Peninsula. There were significant correlations in the long-term densities of brown trout and brook trout year-classes among several rivers in this area, including sites up to 140 km apart and rivers draining into different Great Lakes. Predicted numbers of days to 50% swim-up of brown trout fry were similar among four streams and synchronous, further supporting the hypothesis of synchrony in trout population dynamics in Michigan streams at the regional scale. Long-term trout population estimates and streamflow data collected from a network of long-term index (fixed) sites throughout Michigan will aid in further description of the spatial extent of synchrony in trout populations.