The value of headwaters to brook trout (Salvelinus fontinalis) in The Ford River, Michigan, USA

The Ford River, Michigan, is classified as a "blue-ribbon" trout stream and contains a self-supporting population of brook trout (Salvelinus fontinalis). During the summer, however, water temperature in the mainstem of the Ford River exceeds the optimal growth range, and approaches the lethal temperature range for brook trout. To offset high summer temperatures in the mainstem, adult brook trout migrate into Two Mile Creek, a cold-water headwater tributary, where water temperature rarely exceeds 20 degrees C. Movement into the headwaters appears to be triggered by rising water temperature. By migrating to headwater streams, brook trout are able to ameliorate the impact of high summer temperature on their growth. While headwaters provide a thermal refuge, our analyses showed that growth was negatively correlated to the mean summer temperature and the number of days that temperature in the mainstem exceeded 20 degrees C. This was a function of the water temperature in the headwaters exceeding the optimal growth range for brook trout. However, had these fish been subjected to the higher temperature in the mainstem, they would have experienced lower growth and survival rates. To integrate the population-level value of headwater streams, we develop a conceptual model relating how headwater habitats augment the carrying capacity and surplus production of brook trout as mediated by impacts on adult growth, survival, and reproductive success. This model highlights the importance of headwater habitats to brook trout production in the Ford River, and emphasizes the need to minimize obstructions to fish movement and protect the integrity of headwater streams.