Numerical changes in stream-resident brown trout (Salmo trutta):: uncovering the roles of density-dependent and density-independent factors across space and time

The objective was to uncover the roles of environmental, density-independent, and intrinsic, density-dependent factors as drivers of population size in stream-resident brown trout (Salmo trutta) across space and time. The study was based on census data and environmental variables collected every fourth month over 1986-2006 at 12 sites selected along four Rio Esva tributaries (northwestern Spain). Density-dependent hypotheses involving the operation of current and lagged densities on recruitment and subsequent spawner density were analyzed at three increasingly larger geographical scales (site, stream, and river drainage). No evidence supported these hypotheses, suggesting an overall lack of negative feedbacks among year classes. Stream discharge in March when trout emerge and site depth explained substantial amounts of spatiotemporal variation in recruitment. The effects of these two factors on recruitment were sufficiently strong to be detected in year-class mortality and subsequent spawner density. Discharge and site depth determine the amount of spatial habitat suitable for sized individuals during their lifetime and, hence, the maximum number of individuals across space and time. Recruitment dependence, as a delayed density-dependent factor, adjusts the numbers in excess to the numbers established by spatial habitat. These findings differ markedly from the prediction of the most recent theories of population dynamics and are discussed in this context.