In exploited fish stocks, long-term trends towards earlier maturation have been interpreted as an evolutionary response to sustained, high fishing mortality. The evidence used to support this diagnosis consists of directional shifts in probabilistic maturation reaction norms (PMRNs) that are consistent with the expectation that high fishing mortality favours the genotype for early maturation. Most PMRNs describe the probability of becoming mature solely as a function of age and length. Because they do not fully account for several physiological aspects of maturation (including growth effects on maturation, critical time windows for maturation decisions and developmental thresholds for maturation), it is possible that the observed shifts in PMRNs that are currently being attributed to changes in genotype actually reflect environmental effects on maturation. In this study, a comparative approach was used to interrogate the historical database for Northeast Arctic cod Gadus morhua in relation to 2 contrasting but not mutually exclusive hypotheses: (1) that there is a significant effect of food availability on the probability of being mature, using condition as a proxy for food availability; and (2) that there has been a long-term shift in the PMRN for maturation in a direction that is consistent with a diagnosis of fisheries-induced evolution. The results show that the maturation trends in Northeast Arctic cod could be variously interpreted as showing a strong environmental effect, no genetic effect, or a strong genetic effect. If the scope of the analysis had been restricted to testing a single hypothesis related to either an environmental effect or a genetic effect, then the study could very easily have made a Type I error of inference. A more integrated view of maturation, incorporating key aspects of the physiological processes that culminate in maturation, is therefore required to avoid incorrect inferences about the underlying causes of earlier maturation.
- reaction norms
- Atlantic salmon