Active management is required to turn the tide for depleted Ostrea edulis stocks from the effects of overfishing, disease and invasive species

• Published in: PeerJ
• Main Authors: Luke Helmer, Paul Farrell, Ian Hendy, Simon Harding, Morven Robertson, Joanne Preston
• Format: Article in Journal/Newspaper
• Language: English
• Published: PeerJ Inc. 2019
• Subjects: Ostrea edulis; Crepidula fornicata; Invasive species; Population structure; Ecological niche; Oyster restoration; Medicine; R; Biology (General); QH301-705.5; Pacific; Pacific oyster
• Online Access: https://doi.org/10.7717/peerj.6431; https://doaj.org/article/7709798e9f9f4662958877797ad9c029

The decline of the European oyster Ostrea edulis across its biogeographic range has been driven largely by over-fishing and anthropogenic habitat destruction, often to the point of functional extinction. However, other negatively interacting factors attributing to this catastrophic decline include disease, invasive species and pollution. In addition, a relatively complex life history characterized by sporadic spawning renders O. edulis biologically vulnerable to overexploitation. As a viviparous species, successful reproduction in O. edulis populations is density dependent to a greater degree than broadcast spawning oviparous species such as the Pacific oyster Crassostrea (Magallana) gigas. Here, we report on the benthic assemblage of O. edulis and the invasive gastropod Crepidula fornicata across three actively managed South coast harbors in one of the few remaining O. edulis fisheries in the UK. Long-term data reveals that numbers of O. edulis sampled within Chichester Harbour have decreased by 96%, in contrast numbers of C. fornicata sampled have increased by 441% over a 19-year period. The recent survey data also recorded extremely low densities of O. edulis, and extremely high densities of C. fornicata, within Portsmouth and Langstone Harbours. The native oyster’s failure to recover, despite fishery closures, suggests competitive exclusion by C. fornicata is preventing recovery of O. edulis, which is thought to be due to a lack of habitat heterogeneity or suitable settlement substrate. Large scale population data reveals that mean O. edulis shell length and width has decreased significantly across all years and site groups from 2015 to 2017, with a narrowing demographic structure. An absence of juveniles and lack of multiple cohorts in the remaining population suggests that the limited fishing effort exceeds biological output and recruitment is poor. In the Langstone & Chichester 2017 sample 98% of the population is assigned to a single cohort (modal mean 71.20 ± 8.78 mm, maximum length). There is ...