Phenotypic Plasticity in Age and Size at Maturation and Its Implications to PMRN Approach

Decreases in the age and size at maturation have been documented in several intensively exploited fish stocks. Analyses of probabilistic maturation reaction norms (PMRNs) incorporate the argument that these changes cannot be attributed to parallel changes in growth. Based on the assumption that environmental variability is generally reflected by growth, most PMRN analyses have been interpreted to suggest that temporal shifts in the maturation schedule are representative of evolutionary responses to fishing. Here, I evaluate the underlying assumption of PMRN analyses, based on the results of two rearing experiments. The first, conducted on nine-spined sticklebacks, shows that temperature-induced variation in the age at maturation is not fully reflected by growth trajectory. Consequently, even after controlling for variability in growth, a temperature-related shift could be seen in the maturation schedule. A second experiment illustrates how PMRNs estimated for zebrafish show shifts in the maturation schedule that are induced by varying food supply. Taken together, this experimental approach draws attention to the caveat that growth-independent shifts in age and size at maturation can result from plastic phenotypic responses to environmental drivers, such as temperature or food, rather than evolutionary change alone.