Identifying common factors resulting in hatchery crashes during the production of Ostrea edulis for ecological restoration in Europe

The European flat oyster, Ostrea edulis , once formed extensive reefs along European coasts. These reef ecosystems are now functionally extinct, but support for their restoration is rapidly growing. Efforts are currently limited by a bottleneck in O. edulis supply. O. edulis is a challenging species to produce in a hatchery. Currently unknown causes of high mortality and hatchery crashes must be addressed to meet the increased demand for spat from the ecological restoration sector. Here we present the results of a collaborative effort between nine European hatcheries and marine research laboratories to share recent experiences, as well as production and protocol-related data. We found that crashes were widespread and suspected to be caused by diverse culprits, including Vibrio species, predatory zooplankton, and poor water quality. A Boruta regression analysis of production data identified eleven factors including water temperature in the larval tank, broodstock origin, and number of broodstock as being potentially important in explaining crashes during the larval growth phase. Eight factors including duration of larval growth stage and larval density at transfer to the settlement tank, were identified as potentially important in explaining crashes during the settlement phase. When applied to larval mortality data of batches that did not crash, the Boruta analysis identified 13 factors, including water temperature, broodstock and larval densities and broodstock origin in determining larval mortality during the larval growth stage and two factors, number of larvae in the initial larval release and broodstock origin, during the larval settlement phase. This research highlights the value of knowledge exchange between hatchery facilities in overcoming spat production problems, identifies factors that may contribute to increased larval mortality and the risk of larval crashes, as well as the importance of developing collaborative research programmes to provide for greater commonality in data collection for future coordination and production analysis.