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

Abstract

Maturation and fecundity have been examined since the 1970s in 2 putative sub-populations of North Sea haddock Melanogrammus 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.

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

“…Earlier studies found shifts towards lower A 50 and L 50 in North Sea cod and haddock stocks from 1983 to 1995 (Rochet 2000b) and in North Sea Norway pout from 1983 to 2006 (Lambert et al 2009). PMRN shifts have also been observed in similar direction for inshore haddock along the Scottish coast between the mid-1970s and the mid-1990s (Wright et al 2011a) and for 3 subpopulations of North Sea cod (Wright et al 2011b). …”

“…Wright et al (2011b) found a strong decline in L p50 in the southern and north-western subpopulations of North Sea cod, and a mild decrease in the north-eastern subpopulation, the difference being attributed to stronger selection for early maturing genotypes in the 2 former. Likewise, Wright et al (2011a) showed that the magnitude of changes in maturation probability was greater in western than eastern North Sea haddock.…”

“…Since cod, female whiting and haddock PMRNs have varied, demography and growth-dependent plasticity cannot fully explain temporal changes in their maturity ogives: changes in maturation propensity due to growth-independent plasticity in response to environmental conditions and/or fisheries-induced evolution in maturation are likely to have also played a role. Negative temporal trends in PMRN midpoints have already been observed for inshore haddock along the Scottish coast (Wright et al 2011a, but see Devine & Heino 2011) as well as for other cod stocks (Heino et al 2002b, Olsen et al 2004, 2005, Yoneda & Wright 2004, Pardoe et al 2009 Table 2. Regression coefficients and their standard errors (SE) for cod L p50 at age 3, haddock L p50 at age 2 and whiting L p50 at age 2, obtained from transforming back to the original set of axes the principal component regression coefficients and their SEs.…”

“…North Sea cod is widely distributed throughout the North Sea and is structured in many sub-populations (Hutchinson et al 2001, Wright et al 2006, Wright et al 2011b. North Sea haddock usually occurs north of the Dogger Bank (a sandbank located at 54.5°N), with major spawning areas in the northern North Sea and off the Scottish east coast, possibly corresponding to 2 different subpopulations (Wright et al 2011a). Adult whiting are widespread in the North Sea, whereas juveniles concentrate off the Scottish coast, in the south-eastern bight of the North Sea, and along the coast of the Netherlands.…”

“…Gadid stocks have been shown to be prone to fisheries-induced changes in their life history (Jørgensen 1990, Olsen et al 2004, Pardoe et al 2009, Devine & Heino 2011, Wright et al 2011a. North Sea gadids therefore provide an adequate study system for a comparative analysis of fisheries-induced changes in maturation.…”