Suppression of reproduction in Atlantic cod Gadus morhua

We explored the frequency of suppressed reproduction in Atlantic cod Gadus morhua in Smith Sound, Trinity Bay (NAFO Area 3L), Newfoundland, Canada, the largest remaining spawning aggregation of the so-called 'northern cod'. In the years 1999 to 2004, suppression of reproduction via mass atresia of vitellogenic oocytes occurred in 8.4 to 55.6 % of potential female spawners. Suppression of reproduction was particularly common in the 40 to 49 cm size class, perhaps representing females aborting their first attempt at spawning, but was also common in all larger size classes, suggesting that spawning was not annual for all fish. The fact that non-reproductive fish were generally in poorer condition than those that were successfully ripening suggests a nutritional control to spawning. Stable isotope (delta C-13) signatures suggest that small to medium sized cod (40 to 60 cm) were more likely to spawn when feeding on pelagic (i.e. capelin) than benthic (i.e. shrimp) prey. Small to medium sized cod in Smith Sound were generally in poorer nutritional condition than those in nearby Placentia Bay. Stomach content analysis suggested that Placentia Bay cod fed more heavily on capelin than those in Smith Sound and that they fed year-round, whereas Smith Sound cod ceased feeding during winter. Years with the highest degree of reproductive suppression in Smith Sound were also the years with the highest proportion of female fish in the 40 to 49 cm size class, suggesting that yearly variability in the number of non-reproductive fish was related to population size structure. Failure to account for the non-reproductive portion of the population resulted in overestimation of egg production by 8 to 41 % annually for the Smith Sound aggregation. The results emphasise the need to adjust spawning stock biomass accordingly in order to accurately estimate reproductive potential in marine groundfish and develop effective management strategies.