A couple of years ago, the flat oyster (Ostrea edulis) population was thriving in the Limfjorden (Denmark), with a sustainably regulated fishery landing up to 320t/year. In late 2019, fishermen reported the first event of mass mortality in some fishing areas, probably due to the proliferation of parasitic pathogens.
Preserving and maximizing genetic diversity in putative conservation programs, including restocking, are therefore now of high importance. A new strategy was implemented in the sole flat oyster hatchery in Denmark for producing parasite-free spat and preserving high genetic diversity.
Hatchery procedures may affect the genetic variation among stocked spat in comparison with natural reproduction in the wild. We tested a cost-effective and easily applicable method to assess the genetic diversity in flat oyster production in the hatcheries. It consists in assigning parental contribution of broodstock to the offspring, reliably, in hatchery conditions.
To test our method, we studied the genetic composition at 17 microsatellite loci of hatchery-produced spat and compared it to that of the wild population, wild spat and adults from the broodstock source in the Limfjord. We found that swarming events were characterized by a single maternal and several paternal contributions, but also that only some potential parents contributed offspring per swarming event and that the number of successful parents varied between tanks in the hatchery. The work has provided a protocol that can give a robust estimation of the number of breeding pairs per swarming event (larval release) and it can provide input for potential conservation/restoration programs that utilize hatchery reared spat to support natural populations.
.