Juvenile Steelhead Growth and Consumption in a Hydrologically Altered Watershed

As increasing urbanization and changing climates contribute to altered hydrologic regimes, there is increasing need to understand how stream water supply and temperature influences the energy requirements for growth of fishes.  In hot and dry summer climates the scope of growth can be exceeded for cool- and coldwater species such as steelhead (Oncorhynchus mykiss). Water storage reservoirs and reduced streamflow can elevate streamwater temperatures and reduce prey delivery rates, hence exacerbating the temperature effects on growth. We employed a spatially explicit bioenergetic modeling approach calibrated for juvenile steelhead to 1) assess the spatial and temporal distribution of energetically favorable habitat, 2) relate achieved growth in individually tagged fish to their unique diets, consumption rates, and habitat, and 3) relate these patterns to hydrologic alterations in the Lapwai watershed of north-central Idaho, USA.

We found that the difference in the scope of growth was best described by changes in elevation, with lower elevations providing close to optimal temperatures for a longer period of the summer, but also exceeding the temperatures that allow positive growth (22 ^OC) most frequently. Growth rates were tightly associated with both the proportion of maximum consumption, P, (R² = 0.88) and actual consumption (R² = 0.89) in age 0 fish across the basin, and conversion efficiencies into somatic growth were in the order of 83 % (R² = 0.91). Deviations between consumption rates and growth rates were explained by the variation in the thermal environment, streamflow, population densities, and physical habitat characteristics. The relatively low consumption rates in higher elevation sites (< 28 % of maximum for a given fish size and temperature) suggest food limitation whereas consumption rates in the lower elevation sites were sufficiently high to produce rapid growth even with higher thermally induced metabolic demands. We discuss the implications of the altered hydrograph in two of the streams on juvenile steelhead diet, growth rates, and consumption rates. Because bioenergetic models provide a mechanistic linkage between fish and their environment we conclude that it is a suitable approach in describing growth opportunities in fish populations in areas where water temperatures approach, and even exceed, their tolerances.