Flat oyster fishery management during a time with fluctuating population size

Wild populations of the native European flat oyster (Ostrea edulis) are nowadays rare. Worldwide, flat oyster populations have declined due to overexploitation, diseases and pollution. Yet, in the Limfjorden, Denmark, a small but persistent population of wild flat oyster have endured for more than 165 years, which has sustained a unique fishery on wild oyster beds. Over time, fluctuations in the wild population size has had implications for the fishery, alternating between a large population with high yields, and too small to withstand a fishery. The flat oyster fishery has been alleged to cause these unpredictable fluctuations in the population. However, the combined effect of natural variation (e.g. water temperature) and the fishery has not been investigated. Here we indicate that summer water temperature is an important factor causing fluctuations in the flat oyster population, thought to be due to its influence on recruitment success. Whereas the fishery often constitutes a minor part of the net oyster mortality. We found a positive correlation between summer water temperature and increased oyster landings ≥5 years later. Our results demonstrate the importance of adaptive management for the oyster fishery in the Limfjorden, which has ensured the survival of the flat oyster population, even when the oyster population has been low. These results highlight that management based on annual stock assessments, closed areas with brood stock, and dynamic annual total allowable catch, can ensure the persistence of an endangered oyster species and support a sustainable fishery.