Disentangling genetic change from phenotypic response in reproductive parameters of Flemish Cap cod Gadus morhua

Abstract

Flemish Cap cod Gadus morhua, as with many other fish populations subjected to high fishing rates, has experienced variation in size and age at maturation. Following intense fishing activity and concurrent with a substantial decline in population biomass, cohorts in the early 1990s exhibited a reduction in the length and age at which there is a 50% chance of being mature (L50 and A50 respectively) with values lower than ever estimated before. A further decline in A50 (not in L50) was found after the cod fishing moratoria, in late 1990s and early 2000s cohorts. At the same time, the water temperature showed substantial changes and the abundance of the main prey species of juvenile cod experienced large variation. Shrimp replaced hyperiids as the main prey item since the late 1990s, which was followed by large changes in feeding habits, fish condition and mean size at age. The analysis of maturation reaction norms and countertrend variations in growth rate, water temperature and condition support the hypothesis of genetic change as the main explanation for the decline in size and age at maturation of the cohorts of the late 1980s and early 1990s. However the change in A50 in the late 1990s and early 2000s cohorts is suggested to be caused mainly by a phenotypic response as a consequence of variation in feeding opportunities, water temperature and density-dependent changes in growth rate. If food availability decreases and there is a decrease in water temperature, a decrease in the reproductive potential and survivorship may be expected.