Temperature and Streamflow Effects on Growth and Survival of Headwater Cutthroat Trout Populations in the Greater Yellowstone Area

The sensitivity of salmonids to stream temperatures and hydrologic regimes is undisputed, rendering concerns for how climate change will affect salmonid life-history patterns, interspecific interactions, and distribution and abundance.  Here, we examined the influences of temperature, streamflow, and food availability on summer growth of individual Yellowstone cutthroat trout in tributary streams within the Greater Yellowstone Area.  We employed capture-recapture techniques with multiple sampling events across years to quantify individual measures of fish growth and survival.  As expected, our results suggest temperature and discharge had strong effects on growth, which had implications for body condition.  More specifically, we found that during the summer months juvenile and adult trout invested more in structural growth (length) than in accumulation of reserve tissue (weight); this pattern was largely driven by stream discharge.  Moreover, body condition of most fish decreased over the summer; suggesting the importance of early spring in obtaining energetic requirements for reproductive efforts. Surprisingly, we found decreased survival with size, indicating selection for smaller-bodied trout under in headwater streams.  Slower growth and reduced survival for larger fish suggests headwater populations are likely to become dominated by smaller-bodied, low-fecundity fish, which may dramatically influence population persistence and foodweb linkages in montane ecosystems.