Estimating age at first maturity in fish from change-points in growth rate

Recent studies have drawn attention to the potential for evolutionary changes in life-history traits as a consequence of the size-selective process of fishing and evidence of so called fisheries induced evolution has been reported for a number of different species. Most studies of fisheries induced evolution have focused on changes in sexual maturation, using the probabilistic maturation reaction norm (PMRN) method that requires specific information on the age at which maturation occurs, often derived from macroscopic examination of the gonads. In the absence of sufficiently detailed measurements of maturity it is necessary to derive estimates of the age at which maturation occurs from alternative sources of information, for example from length at age data. We apply a relatively simple segmented regression model to length at age data, for plaice in the Irish Sea, to identify the change-point between two specific growth schedules that may be used as a proxy for age at first maturity in individual cohorts. We use a Bayesian approach for model fitting and map the resulting distribution of changepoints using gaussian mixture models to show that the age at which the change-point occurred in individual cohorts of both male and female plaice in the Irish Sea declined progressively over an 18 year period between 1988 and 2005.JRC.G.4-Maritime affair