Abstract
Reaction norms are a valuable tool in evolutionary biology. Lately, the probabilistic maturation reaction norm approach, describing probabilities of maturing at combinations of age and body size, has been much applied for testing whether phenotypic changes in exploited populations of fish are mainly plastic or involving an evolutionary component. However, due to typical field data limitations, with imperfect knowledge about individual life histories, this demographic method still needs to be assessed. Using 13 years of direct mark–recapture observations on individual growth and maturation in an intensively sampled population of brown trout (Salmo trutta), we show that the probabilistic maturation reaction norm approach may perform well even if the assumption of equal survival of juvenile and maturing fish does not hold. Earlier studies have pointed out that growth effects may confound the interpretation of shifts in maturation reaction norms, because this method in its basic form deals with body size rather than growth. In our case, however, we found that juvenile body size, rather than annual growth, was more strongly associated with maturation. Viewed against earlier studies, our results also underscore the challenges of generalizing life-history patterns among species and populations.
Highlights
The age and body size at which organisms reach sexual maturity are key life-history traits potentially shaped by plastic responses to environmental conditions, as well as evolutionary responses to natural and anthropogenic selection (Reznick et al 1990; Ernande et al 2004; Gamelon et al 2011)
By direct observations of growth and maturation of wild fish in their natural habitat, this study evaluates the probabilistic maturation reaction norm approach as a tool in evolutionary biology
Our study showed that body size measured at the previous age is a more important determinant of maturation than body size measured at current age
Summary
The age and body size at which organisms reach sexual maturity are key life-history traits potentially shaped by plastic responses to environmental conditions, as well as evolutionary responses to natural and anthropogenic selection (Reznick et al 1990; Ernande et al 2004; Gamelon et al 2011). From both an evolutionary and a conservation perspective, there is a need to understand the underlying causes of phenotypic variation in maturation. This probabilistic maturation reaction norm approach has been applied to time series on several exploited fish species, exploring to what extent temporal changes in maturation patterns may reflect fisheries-induced evolution versus phenotypic plasticity (Grift et al 2003; Engelhard and Heino 2004; Olsen et al 2004; Barot et al 2005; Mollet et al 2007)
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