The efficacy of suspended broodstock cages as a restoration strategy for the European flat oyster Ostrea edulis Linnaeus, 1758

: a case study in the Solent, UK

Abstract

The European flat oyster Ostrea edulis has been subject to multiple stressors that have resulted in the functional extinction of many populations, those that remain are fragmented and require active intervention to accommodate their recovery. One of the last self-sustaining populations in Europe was, until recently, present within the Solent, but extraction pressure, disease and sporadic recruitment resulted in its recent collapse. This study aimed to determine the abundance of the remaining oyster populations in relation to the invasive American slipper limpet Crepidula fornicata, suspected to occupy the ecological niche made available by the removal of O. edulis. Oyster populations were minimal within the three harbours surveyed, < 0.2 ±0.2 individuals / m² (Portsmouth, Langstone and Chichester), whereas, C. fornicata was extremely abundant, present in densities of up to 4043 ±2374 individuals / m². In an attempt to increase reproduction and larval output, therefore seabed recruitment, mature oysters were purchased from the local fishery and placed into broodstock cages at high stocking densities across the Solent. Monitored across two years, the effect of stocking density, pressure-washing and environmental conditions on mortality, larval production and disease prevalence were observed. Mortality was seasonal peaking after an increase in brooding activity and spawning. Brooding occurred within the expected temporal period, peaking in June, and larval brood sizes were significantly different between oysters in different marinas, as well as between full- and half-density populations, despite there being no significant difference in the proportion of brooding adults. Disease status was monitored and geographical variation in Bonamia ostreae prevalence was observed. The detection of Bonamia exitiosa in the region increases the described range of the parasitic species. With design and operational modifications, the broodstock cage system can be used as an effective management tool to assist with increasing initial densities of larvae in recruitment limited areas in the early stages of restoration

Date of Award	Sept 2019
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Joanne Preston (Supervisor), Ian Hendy (Supervisor) & Chris Hauton (Supervisor)