Connectivities with shellfish farms and channel rivers are associated with mortality risk in oysters

Abstract

Oyster diseases have major consequences on fisheries and aquaculture. In France, young Pacific oysters Crassostrea gigas are severely hit by the ostreid herpesvirus, whereas adults suffer mortalities presumably caused by pathogenic bacteria. Here we investigated the origin and spread of mortalities that affect both young and adult oysters, and we identified and compared their risk factors. Mortality was monitored in 2 age classes of oysters deployed in early spring at 39 sites spread over a 37 km 2 surface area inside and outside of shellfish farms. Environmental data obtained from numerical modelling were used to investigate risk factors. Mortality of young oysters associated with ostreid herpesvirus occurred in the oyster farming area. Hydrodynamic connectivity with oyster farms was associated with higher mortality risk, whereas chlorophyll a concentration was associated with a lower risk. Adult oysters experienced 2 mortality events that were associated with different risk factors. The first event, which occurred after deployment and was probably caused by endogenous pathogens, was mainly associated with connectivity to channel rivers and salinity. The second mortality event observed at the end of the summer was mainly associated with connectivity to oyster farms, suggesting pathogen transmission. The risk factors involved in young and adult oyster mortalities were partly different, reflecting distinct origins. Connectivity with oyster farms is a mortality risk factor for both young and adult oysters; thus, disease management strategies that focus on oyster farming areas will impact overall disease risk.

Latitudinal drivers of oyster mortality: deciphering host, pathogen and environmental risk factors

“…that are not necessarily correlated with latitude. The risk factors involved in young and adult oyster mortalities were partly different, reflecting distinct pathogens 14 .…”

“…The infected host shed viral particles into the water column and dispersal to new hosts occurs via water currents 8,12,13 . Mortality starts in the intertidal farming areas and spread off-coast in a way that depends of food quantity and quality, turbidity, growth rate and energetic reserves of oysters [14][15][16] . Latent and asymptomatic OsHV-1 infections are able to persist in hosts 17 .…”

“…It increases, however, for Vibrio aestuarianus 19 , a pathogenic bacterium that cause major mortality in adult oysters 3,4 . The risk factors involved in young and adult oyster mortalities are partly different, reflecting differences in the interactions with these distinct pathogens 14 . For adult oysters, mortality is primarily associated with connectivity to channel rivers and salinity, and secondarily with connectivity to oyster farms 14 .…”

A 26-year time series of mortality and growth of the Pacific oyster C. gigas recorded along French coasts

“…In the late eighties, significant mortality events of C. gigas occurred in France 9 , one of the main producers of oysters in Europe 10 , and particular interests in biological monitoring of C. gigas emerged. The causes of mass mortality events seem to differ between spat and half-grown oysters 11 . Indeed, mortality events of spat are mainly caused by Ostreid herpesvirus type 1 (OsHV-1) 12 , whereas those of half-grown oysters are caused by strains of the Vibrio aestuarianus bacteria.…”

“…Indeed, mortality events of spat are mainly caused by Ostreid herpesvirus type 1 (OsHV-1) 12 , whereas those of half-grown oysters are caused by strains of the Vibrio aestuarianus bacteria. In the two diseases, pathogens interact with the host and environmental risk factors to determine the dynamic and the severity of the outbreak 11 , 13 – 17 . Considering the complexity of the different processes that influence mortality proportions, monitoring made it possible to disentangle the drivers of oyster outbreaks in the field 11 , 15 , 18 .…”