Skewed sex ratios in spawning shoals of Atlantic cod (Gadus morhua)

Research vessel survey data from the northern Grand Bank and southern Scotian Shelf/Bay of Fundy were analysed for evidence of the occurrence of shoals of Atlantic cod (Gadus morhua) with unequal sex ratios during the spawning season. Cod were found to form both male dominated and female dominated shoals during spawning. In both areas, female dominated shoals were found in significantly deeper waters. A higher proportion of both males and females were in spawning condition in the male dominated fishing sets and a higher proportion of females had completed spawning in the female dominated fishing sets. Males may arrive at the spawning area first, with females moving into the area when ready to spawn and then returning to deeper, warmer water once they have completed spawning.