Disentangling the causes of maturation trends in exploited fish populations

Evolutionary theory predicts that selective harvesting will shift the frequency of genes for heritable traits towards those that express well-adapted phenotypes (Law 2007, this Theme Section). Applying this logic, commercial fisheries that select on the basis of size could reasonably be expected to favor the genotype for maturation at smaller sizes (and/or younger ages), given that rapidly maturing individuals will be more likely to reproduce prior to capture. This form of fisheries-induced evolutionary response is a parsimonious explanation for the observed longterm trend towards earlier maturation that has been observed in several commercially exploited fish stocks (Rijnsdorp 1993, Trippel 1995). Unfortunately, it is not possible to test this hypothesis directly, because the genes associated with maturation have not been isolated. Probabilistic maturation reaction norms (PMRNs) (Heino et al. 2002) have been proposed as a statistical tool for helping to disentangle genetic from plastic effects on maturation (reviewed by Dieckmann & Heino 2007, this Theme Section). For a given cohort, the PMRN describes the maturation probability of individuals that have survived to reach a given age and size. A midpoint curve, illustrating the age and size at which the maturation probability is 50%, is often used to summarise the PMRN. A shift in cohort-specific PMRNs over time is interpreted as being consistent with (although not direct evidence of) a genetic change at the population level. For example, a shift through