Unraveling multifactorial risks in Pacific oyster mortality: a four-year study of environmental and age-related impacts

The mortality of Pacific oysters Crassostrea gigas remains a persistent challenge for aquaculture, driven by complex interactions between environmental conditions, pathogen dynamics, and host factors. We monitored 96 sentinel oyster cohorts across 8 French sites over 4 yr (2014-2018) to assess age-specific mortality risks and environmental influences. Survival analyses and Cox hazard models confirmed that spat exhibit the highest mortality, while juveniles and adults demonstrate increased survival, reflecting age-related physiological resilience. Seawater temperature emerged as the strongest predictor of mortality, with risks increasing significantly between 16 and 24°C, highlighting a critical temperature threshold. Wind speed and relative humidity also modulated survival, likely influencing pathogen dispersal and physiological stress. Importantly, the impact of these factors was neither constant nor always significant over time, emphasizing the need for non-proportional risk functions to accurately capture mortality dynamics. Over the monitoring period, no significant increase in spat survival was observed, suggesting that resistance to infectious agents in farmed oysters has not markedly improved under natural environmental conditions. In response, oyster farming practices have evolved to integrate mortality risks, notably by increasing spat input and adjusting rearing conditions. This study underscores the necessity of incorporating environmental and life-history parameters into predictive models for improved risk assessment. By providing long-term insights into mortality patterns, our findings support the development of sustainable management strategies to enhance oyster resilience in the face of climate change and evolving pathogen threats.