Reproductive investment in the North Sea haddock: temporal and spatial variation

Maturation and fecundity have been examined since the 1970s in 2 putative sub-populations of North Sea haddockMelanogrammus aeglefinus. Reproductive investment within the western North Sea haddock sub-population significantly increased between the 1970s and 2000s, as was evident from both a decline in maturation probability and an increase in fecundity with size. A decline in maturation probability was also evident in males from the east North Sea, although the length at 50% probability of maturing (Lp50) at the beginning and end of the study period was much higher than in the west North Sea. The changes in reproductive traits could not be explained as a compensatory response, since the trend for increasing reproductive investment was not reversed when the North Sea stock recovered over the past decade. Indeed increased reproductive investment was accompanied by a reduction in somatic growth rate. Temperature immediately prior to secondary gametogenesis could partially explain inter-annual variation in female maturation probability and may have influenced the difference in sub-population trends. However, declines in maturation probability with cohort were still highly significant after accounting for a temperature effect, and differences in potential fecundity could not be explained by changes in somatic condition. Overall, it appears that west North Sea haddock have increased their reproductive effort, following decades of high mortality, consistent with an evolutionary response.