Fishery-induced changes to age and length dependent maturation schedules of three demersal fish species in the Firth of Clyde

Probabilistic maturation reaction norms (PMRNs) were used to investigate the maturation schedules of cod, haddock and whiting in the Firth of Clyde to determine if typical lengths at maturation have changed significantly since 1986. Some potential sources of growth-independent plasticity were accounted for by including sea-surface temperature and abundance variables in the analysis. The PMRNs of the Clyde populations were compared with those from the wider west coast, in conjunction with regional differences in the fishery, to assess whether fishing may have been driving the observed trends of decreasing lengths at maturation. The lengths at which haddock, whiting and female cod were likely to mature decreased significantly during 1986–2009, with rates of change being particularly rapid in the Clyde. It was not possible to estimate PMRNs for male cod due to limited data. Trends in temperature and abundance were shown to have only marginal affects upon PMRN positions, so temporal trends in maturation schedules appear to have been due to a combination of plastic responses to other environmental variables and/or fishing. Regional differences in fishing intensity and the size-selectivity of the fisheries suggest that the decreases in lengths at maturation have been at least partially due to fishing. The importance and scale of the Clyde Nephrops fishery increased as demersal landings declined, and the majority of demersal fish landings have come from Nephrops bycatch since about 2005 when the demersal fishery ceased. Since it appears as though fishing may have caused increasingly early maturation, and a substantial Nephrops fishery continues to operate in the Clyde, reversal of these changes is likely to take a long time – particularly if there is an evolutionary component to the trends. If size-selective fishing has contributed to the lowered abundance of large fish by encouraging maturation at increasingly small lengths, then large fish may remain uncommon in the Clyde until the observed trends in maturation lengths reverse.