Effect of Age on the Seasonality of Maturation and Spawning of Atlantic Cod, Gadus morhua, in the Northwest Atlantic

Analysis of 46 yr of research trawl survey data from seven regions in the Northwest Atlantic (Newfoundland and Labrador) indicated that older individuals of Atlantic cod, Gadus morhua, of both sexes initiated and completed spawning later, and spawned for a greater length of time, than younger individuals. Within regions, males were in spawning condition for longer periods than females. Among regions, spawning duration of males (age = 11 yr) averaged 43.5 ± 13.6 d and was significantly longer than that of females (age = 11 yr; 25.2 ± 8.4 d). The strong association between age and time of spawning provides empirical evidence of size-specific, assortative mating in cod. The decline in recruitment of cod in NAFO Division 2J3KL since 1962 is associated with a dramatic reduction in the fecundity contribution of older (15–20 yr) relative to younger (7–9 yr) individuals. Our results indicate that this increase in proportional abundance of younger individuals should be concomitant with a decline in the duration of spawning time, reducing the probability that larval emergence will match peak abundances of zooplankton. Thus, age dependence of maturation and spawning times provides a mechanism by which size-selective mortality against larger, older individuals can increase variability in recruitment in Atlantic cod.