Spatial distribution in a temperate coastal ecosystem of the wild stock of the farmed oyster Crassostrea gigas (Thunberg)

Abstract

The Pacific oyster, Crassostrea gigas, well known throughout the world because of its ability to adapt to a wide range of environmental conditions, was introduced for cultivation into France on a massive scale in the 1970s. With global warming, the reproductive population, confined at the beginning to the south of the French Atlantic coast, became established at more northern latitudes (above 45° 58′ N), and wild C. gigas began to colonize coastal areas such as our study site, Bourgneuf Bay (1°-2° W, 46°-47° N), an oyster-farming site. An original approach, based on orthophotograph analysis and in situ biomass sampling, revealed that, in the northern part of this bay, more than 70% of the total C. gigas biomass was composed of wild oysters (i.e. C. gigas not bred by oystermen). The analysis of the spatial distribution of wild oysters indicated that 75% of the stock consisted of wild oysters in natural beds (rocky areas) and on low retaining walls of former fisheries. Wild C. gigas also colonized oysterfarming structures with lower biomasses (21% of the stock composed of wild oysters), but locally they could reach densities of up to 55 kg.m− 1 i.e. 2.5 times the mean biomass of cultivated oysters. The economic and ecological consequences of this colonization by C. gigas of an oyster culture site are discussed. Wild oyster seems to be the principal trophic competitor of cultivated oysters in Bourgneuf Bay. This may partly explain the decrease in growth of cultivated oysters observed in this bay during the last decade. Moreover, the trophic and spatial competition exerted by wild oysters may also affect the native biota and, in particular, the honeycomb worm Sabellaria alveolata. The results obtained in this study have led oyster farmers and regional authorities to modify oysterfarming practices and to destroy wild oyster stocks in concession areas. Résumé

Benthic assemblages associated with native and non-native oysters are similar

“…Climate-driven ocean warming, however, may lead to a more detrimental impact of C. gigas on species diversity by establishing high density populations, thus reducing associated species diversity (Cognie et al, 2006;Syvret et al, 2008). …”

Satellite remote sensing reveals a positive impact of living oyster reefs on microalgal biofilm development

“…During recent decades, C. gigas benefited from coastal eutrophication and rising sea temperature (Thomas et al, 2016), resulting in a poleward expansion of its distribution (Dutertre et al, 2010) and the formation of dense reefs along many coastal areas (Diederich, 2006;Brandt et al, 2008;Le Bris et al, 2016). In some ecosystems, wild C. gigas is now considered as a trophic competitor of its cultivated conspecifics (Cognie et al, 2006). Newly established oyster populations also impact biogeochemical fluxes and ecosystem processes, 906 C. Echappé et al: RS reveals oyster reefs' positive impact on MPB development leading to both positive (e.g.…”

Multi-decadal changes in two co-occurring ophiuroid populations

“…Simultaneously, rocky western coasts of France (including the Bay of Brest) have been heavily colonized by wild oysters Crassostrea gigas in the last decade (Cognie et al 2006). Lejart & Hily (2011) investigated the consequences for the Bay of Brest and reported that mass biodeposits from the invasive oyster reefs were washed away from the reefs and sedimented in deeper areas of the bay.…”