Nonrandom mating in a broadcast spawner: Mate size influences reproductive success in Atlantic cod (Gadus morhua)

We tested the hypothesis that reproductive success in Atlantic cod (Gadus morhua) is random with respect to similarity in body size between mates. Immediately prior to their natural breeding periods, groups of 52-93 cod from three Northwest Atlantic populations were transported to a large (15 m diameter, 4 m deep) tank where they spawned undisturbed at densities similar to those in nature. Based on microsatellite DNA-parentage assignment of 8913 offspring from four spawning groups, females and males achieved their highest reproductive success when breeding with mates that were larger than themselves. Our observations are consistent with the hypothesis that some form of intrasexual competition or mate choice is a constituent of the mating system of this species and that this can have an important influence on individual fitness. Our results further suggest that reductions in the mean and variance in body size of commercially exploited marine fishes concomitant with size-selective harvesting may have greater negative consequences for population recovery than previously thought.