A Comparative Analysis of Juvenile Steelhead Demographics in a Hydrologically Altered Watershed

Stream diversion projects are globally widespread and can have detrimental effects on salmonid rearing conditions. Withdrawals alter stream habitat conditions including temperature, hydrographs, velocities, connectivity, and habitat area. These alterations can then impact growth and survival of rearing salmonids. For anadromous populations, growth and survival in headwater streams is important not only to freshwater life history phases, but also for the successful migration of smolts to the ocean and return of spawning adults. In Lapwai watershed, northern Idaho, we compared two sites with reduced instream flows against two sites with relatively natural flow regimes to address the impacts of water diversion on growth, survival, and outmigration behavior of a population of federally threatened steelhead (Oncorhynchus mykiss). During repeated summer sampling visits we PIT tagged age 1+ steelhead and in subsequent months detected outmigrating individuals using instream PIT arrays. We used Pollock’s robust design mark-recapture models to estimate survival of tagged fish to the start of the migration season and multi-strata mark-recapture models to quantify outmigration patterns. Sites downstream of diversion dams had low densities of steelhead and consequently low levels of intraspecific competition. As a result growth and survival of fish in these sites was higher compared to fish in unimpacted habitats. Additionally, larger individuals within a site had lower survival. Such differences in density, growth, and survival can alter the outmigration patterns of fish at both the site and individual level. We found that the proportions of outmigrating age 1+ steelhead were higher from sites impacted by water diversions as well as from sites with higher density. We also found that outmigration was associated with both survival and body size, whereby individuals with larger body size or lower survival were more likely to leave the watershed. Moreover, an individual’s body size relative to its conspecifics within a site was a better predictor than was body size relative to all individuals from the four sites. These results suggest an interaction among density, growth, survival, and body size that manifests itself at the population level in terms of the number of outmigrating smolts and the condition of those smolts. Such differences can have large impacts on the number of smolts that survive to spawning age and subsequently on viability of local populations.