Temporal and spatial variation in reproductive investment of Atlantic cod Gadus morhua in the northern North Sea and Scottish west coast

The recent decline in many fish stocks has been accompanied by marked changes in age at maturity. However, there is little information on whether other components of reproductive investment have changed and the relative roles of phenotypic plasticity or genetic selection in such change. This study explored how reproductive investment in cod Gadus morhua from different regions around Scotland has changed between a period of high (1969, 1970) and low (2002, 2003) spawning-stock size. Maturity ogives indicated that inshore North Sea cod now mature at a smaller size and younger age than they did around 1970. The relationship between potential fecundity and size indicated that cod in the inshore region are now more fecund than 30 yr ago. Cod from the inshore region of the North Sea now have a higher fecundity at a given size and age than cod from the offshore region and the west of Scotland. There was no such spatial trend around 1970. Similarly, whilst the relative fecundity of cod in recent times is positively correlated with age, no such relationship was evident in 1969 and 1970. Nutritional influences on maturity and fecundity, particularly liver condition, were also evident from the recent study. However, the temporal and spatial differences in fecundity-size relationships were not consistent with a change in growth conditions, as there has been a significant decrease in body condition in recent times. Our findings regarding the temporal change in reproductive investment in the inshore North Sea region appear consistent with genotypic changes in life-history traits predicted to arise from intense periods of size-selective mortality.