Variation in brook and brown trout survival within and among seasons, species, and age classes

The Program MARK was used to generate and test a plausible set of survival models for brook trout Salvelinus fontinalis and brown trout Salmo trutta to determine whether survival differed by season, species or age class. Apparent survival varied with time and age, but not by species. For the older (1999) age class, survival was lowest during the autumn of their age 1+ year whereas survival was lowest for the younger (2000) age class during the early summer of their age 1+ year. Additionally, estimates of survival for the younger age class exceeded those of the older age class in all but one interval (early summer 2001) but significant differences were observed in only one of these intervals (autumn 2000). To determine whether the observed seasonal differences in survival were related to seasonal differences in movement rates, multistrata spatial models were used within Program MARK to determine the probability of moving for each interval. In-site movement rates were found to be low regardless of season (average for all cohorts combined was 5%). The ability of the multi-strata modelling approach to detect temporal variability in movement, however, was potentially limited by spatial scale of the study reach (c. 1 km). Differences in survival between different aged fishes could lead to selection acting on age at maturity or reproductive effort at a given age. (C) 2003 The Fisheries Society of the British Isles.