Adaptive phenotypic plasticity in brook trout, Salvelinus fontinalis, life histories

I analyzed life history data for three unexploited populations of brook trout, Salvelinus fontinalis, to explore how environmental variation in juvenile (pre-reproductive) growth rate can generate variation in fitness (r) and to quantify the fitness benefits associated with phenotypic plasticity in age at maturity. Within populations, differences in growth rate to maturity are associated with significant variation in fitness through size-dependent effects on life history traits. Among populations, variation in size at maturity effected comparatively little variation in egg size and reproductive allotment (gonad weight/body weight) but was associated with high variability in fecundity and post-reproductive, overwinter survival. The relationship between fitness and the oft-reported negative association between growth rate and age at maturity in indeterminately growing organisms is supported empirically and is shown to vary among populations. Fitness functions are consistent with the prediction that early maturity is favoured by fast growth and delayed reproduction by slow growth. However, the fitness benefits to slow growing individuals of delaying reproduction decrease with reductions in adult survival. Fitness variation within brook trout populations can be linked empirically to size-dependent differences in life history traits through environmentally-induced variation in juvenile growth rate. The results illustrate how environmental variation can favour the evolution of adaptive phenotypic plasticity in life history through growth-dependent effects on fitness.