Analysis of the parental contribution to a group of fry from a single day of spawning from a commercial Atlantic cod (Gadus morhua) breeding tank

The parentage of 300 Atlantic cod (Gadus morhua) fry from eggs collected on a single day in a commercial mass spawning breeding tank was analysed by DNA profiling. Employing eight polymorphic microsatellite DNA markers, exclusion-based parentage analysis unambiguously assigned 81% of fry to a single pair of parents each (or 84.6% of the fry successfully genotyped). Of 99 adult fish present in the broodstock tank, 26 were found to have contributed to the assigned offspring. The spawning pattern was highly skewed. One parental pair contributed to 25% of the sample, while nine parents contributed to 90% of the assigned individuals. Among these highly represented individuals, both multiple paternity and multiple maternity was common. There was no evidence for size assortative mating being a factor in the spawning outcomes, though the dataset was not ideal for such an analysis. The genetic data were backed up by observations of cod mating behaviour, in a breeding tank, using infrared cameras. The video recordings gathered showed the occurrence of ventral mountings and suggested a mechanism for multiple paternity of egg batches.