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

We establish that age and length at 50% proportion mature decreased from the 70’s to the 2000’s 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, unlikely for Norway pout, but not needed to 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.