Abstract
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.
Highlights
The Firth of Clyde is a large estuary in the west coast of Scotland that once supported substantial demersal fisheries for a range of species such as cod, haddock, saithe, whiting and hake (Ross et al, 2009)
The positioning of the Probabilistic maturation reaction norms (PMRNs) were slightly different for female cod; the negative slopes were steeper, Lp50 was greater than the lengths of one year old fish – which were immature – and tended to intersect the smaller two year old fish
Haddock, whiting and female cod in the Scottish west coast have been maturing at progressively smaller lengths and younger ages since 1986, and this has occurred most rapidly in the Clyde populations of haddock and whiting
Summary
The Firth of Clyde is a large estuary in the west coast of Scotland that once supported substantial demersal fisheries for a range of species such as cod, haddock, saithe, whiting and hake (Ross et al, 2009). Biomass estimates of the six main commercial species; cod, hake, haddock, plaice, saithe and whiting indicate that the community biomass in the late 2000s was approximately double what is was pre-1960. This biomass consists almost entirely of small fish that are less than 40 cm in length, and small species – predominately whiting – dominate. Trends in maturation schedules may be related to some of the observed changes in length structure and species complement
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