The importance of recruitment for the production dynamics of stream-dwelling brown trout (Salmo trutta)

The objective was to highlight the role of recruitment for production dynamics of stream brown trout (Salmo trutta). The analysis of 51 cohorts (hatched in 1986-1999) at four sites of Rio Chaballos (northwestern Spain) showed sinusoidal growth patterns with more intense growth in spring and summer and reduced growth in winter. Survivor abundance described two-phase trajectories over the lifetime (1000-1400 days after emergence). A first phase of negligible or no mortality was prolonged for 500-650 days. Severe mortality during the second phase differed among cohorts and among sites. Spatiotemporal variations in growth, mortality, density, spawner abundance, biomass, and production underlay variations in recruitment. Increased recruitment affected growth negatively (except at one site) and mortality positively. Cohort production decreased with increased growth at three sites but increased with increased growth at another site. Greater mortality typified the most productive cohorts. However, 90.8% and 83.6% of the variations in density and production, respectively, were explained by variations in recruitment. Thus, cohort size and production appeared to be determined by recruitment in which postrecruitment processes played a minor role. Recruitment reset the cohort's numerical and productive capacity, and increments in recruitment continued to increase cohort size and production over the recruitment magnitudes observed across sites and years.