Quantitative ecology of a stream Brook Trout population over a 58-year period and lessons for resilience

Objective Long-term data sets for stream trout populations provide unique opportunities to examine the effects of factors thought to structure populations and influence population dynamics. We examined the effects of sportfishing, stock–recruitment relationships, and density dependence on a stream trout population in Michigan over a 58-year period, providing the data set for future research. Methods We compared age-specific densities of Brook Trout Salvelinus fontinalis between periods when the stream reach was open to fishing (Fishing period: 1949–1965) versus closed to fishing (Unexploited period: 1966–2006). We examined relationships between estimated densities of mature female Brook Trout or egg deposition in fall and age-0 fish densities in the following fall. We examined carryover of age-group densities to subsequent years and used quantile regression to examine density-dependent effects of age-group density on survival and growth from spring to fall and from fall to the following fall. Results Fall densities of Brook Trout at age 0, age 2, age 3, and ages 4–6 combined were significantly higher during the Unexploited period, with an increased prevalence of older age-groups rarely detected during the Fishing period. Mean annual survival doubled for age-1 and age-2 fish during the Unexploited period. Significant relationships occurred between mature female density, egg density, and age-0 trout density in the following fall, with the stronger relationship between egg density and subsequent age-0 Brook Trout density. Stock–recruitment models were fit using egg density and mature female density in the previous fall as predictors, with Beverton–Holt parameterizations identified as the best models. Significant carryover of Brook Trout year-classes was most notable among the age-0, age-1, and age-2 groups and when data for the entire 58-year study period were examined. Density-dependent effects on age-group-specific survival were most noticeable between spring and fall surveys, and no significant negative effects on growth were observed. No significant effects of fall density on survival to the following fall were detected. Conclusions Collectively, these relationships provide insight into potential effects of fishing harvest regulations on population abundance and age structure, suggesting that measures to protect mature Brook Trout in streams can potentially add resiliency to populations. The capacity for such actions to achieve their desired effects may be influenced by other factors, such as hooking mortality or the occurrence of predators or competitors.