Temporal trends in age and size at maturation of four North Sea gadid species: cod, haddock, whiting and Norway pout

Abstract

Younger ages and smaller sizes at maturation have been observed in commercial fish stocks over the last century. We establish that age and length at 50% proportion mature (i.e. the proportion of mature individuals in a population or the probability that an individual is mature) decreased from the 1970s to the 2000s in North Sea cod Gadus morhua, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus, but not in Norway pout Trisopterus esmarkii. The potential contributions of demography, phenotypic plasticity and evolution to these trends were assessed. First, maturation trends were extricated from demographic effects and growth-dependent plasticity by estimating probabilistic maturation reaction norms (PMRNs). PMRN midpoints have significantly shifted downwards at most ages for cod, haddock and whiting, but not for Norway pout. Second, increased temperature and food abundance, loosened trophic competition and relaxed social pressure may also trigger growth-independent plasticity in maturation. Principal component regression of PMRN midpoints on annual estimates of relevant environmental variables exhibiting a temporal trend suggest that, despite some evidence of environmental effects, PMRN trends were mostly independent of growth-independent plasticity in haddock, whiting and male cod, but not in female cod. According to these findings, evolution of maturation, potentially in response to fishing, is plausible in haddock, whiting and male cod, but unlikely for Norway pout, and does not explain trends in female cod maturation. In agreement with life-history theory, the maturation response was larger in fast-growing, late-and large-maturing species exhibiting moderate reproductive effort.

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

“…Temporal trends in L p50 are not unique to cod, haddock and whiting in the Scottish west coast, but have also been observed in the North Sea Marty et al, 2014), as well as in other species from several different regions (Grift et al, 2007;Jorgensen et al, 2007;Kuparinen and Merila, 2007;Mollet et al, 2007;Hutchings and Fraser, 2008;Sharpe and Hendry, 2009). We expected regional differences in the rate of change of L p50 to correspond with the regional difference in the fisheries such that fish in the Clyde would experience more rapid declines in L p50 .…”

Assessing reproductive resilience: an example with South Atlantic red snapper Lutjanus campechanus

“…However, due to a lack of historic maturity estimates for this stock, it cannot be confirmed whether the early maturation is a result of overfishing, as has been seen in other species (Jørgensen et al 2007, Audzijonyte et al 2013, Marty et al 2014), or simply a very resilient reproductive strategy. If this early maturation is due to 'fisheries-induced adaptive change' (Heino et al 2013), it could have important implications for fisheries management.…”

Reproductive resilience: a paradigm shift in understanding spawner‐recruit systems in exploited marine fish

“…; Marty et al . ). Traditionally, maturity has been assumed to be invariant, and stocks with earlier maturation assumed to be more resilient to fishing.…”

“…However, for many exploited species we do not yet track changes in maturity schedules to assess whether overfishing is causing earlier maturation (Fig. 7), as reported for multiple species Audzijonyte et al 2013;Marty et al 2014). Traditionally, maturity has been assumed to be invariant, and stocks with earlier maturation assumed to be more resilient to fishing.…”