Does Fish Distribution Reflect the Fitness Value of Habitat for Stream Salmonids or Predict Population Responses to Changes in Habitat Availability?

The assumptions that fish distribution reflects the value of particular habitats and that populations will respond positively to increases in the availability of heavily used habitat have been highly influential in fisheries management. These assumptions are difficult to test in the field; we did so within a spatially explicit, individual-based modeling (IBM) framework formulated for stream salmonids.  Trout in the IBM select habitat to maximize their potential fitness, which is a function of growth potential (including food competition) and survival of a suite of risks including predation. We know each habitat cell’s intrinsic habitat quality, the fitness potential a trout in the cell would experience in the absence of competition. There was no strong relation between fitness potential and the density of fish in the IBM: cells where fitness potential was high but density low were common for all age classes, and fitness potential was not proportional to density. We also compared the ability of a model of habitat use to predict fish survival across a range of streamflows. While predicted density was related to survival at streamflows close to the one used to build the model of habitat use, the relation broke down at higher streamflows.