Ecology of stream fish: insights gained from an individual-based approach to juvenile Atlantic salmon

Using data from an ongoing study of juvenile Atlantic salmon growth and sur vival in tributaries of the Connecticut River, USA, we compare standard pop ulation-level approaches to those focusing on individuals. We highlight the potential benefits of resampling individually tagged stream fish as compar ed to standard approaches. Specifically we focus on growth, survival, movem ents and population estimation. The advantages of estimating sizes and grow th rates from individual size trajectories include obtaining growth histori es and the ability to perform retrospective analysis of the consequences of different life-history strategies. An example might be the patterns of gro wth leading to either early maturity or migration. Resampling known individ uals is the only way we know to chart both short-term and long-term movemen ts and to assign growth and mortality consequences to such movements. Final ly individual-level data permit robust estimation of survival and density/a bundance using methods such as Cormack/Jolly-Seber. The results indicate th at population estimates were about 10% lower using individual data than usi ng population data, that survival from sample to sample was typically >90%, that the majority of recaptured fish did not move during the summer, that growth was rapid during spring and most fish lost mass during the summer an d that growth trajectories for maturing and nonmaturing fish showed substan tially different patterns. An individual-based approach to stream fish ecol ogy provides the opportunity to explore the mechanisms responsible for popu lation-level patterns but comes at the cost of significant field effort. Tr adeoffs between increased data resolution and the effort required to obtain the data must be considered before undertaking individual-based field stud ies of stream fishes.