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.