POPULATION-SPECIFIC RESPONSES OF Ostrea edulis TO ESTUARINE VARIABILITY: IMPLICATIONS FOR AQUACULTURE AND RESTORATION
Introduction: Natural oyster reefs, once dominant in estuaries worldwide, have undergone severe global decline, with ~85% loss1. In Europe, the native species Ostrea edulis is now considered threatened due to overexploitation, habitat loss and disease outbreaks2, which has led to restoration and reintroduction efforts supported by policy initiatives and aquaculture development. However, these efforts face increasing challenges from climate change, particularly shifts in temperature and salinity regimes, alongside pollution and pathogen pressure, all of which affect growth, physiology, and survival of the oysters. This in situ field study evaluates survival and growth of O. edulis under natural estuarine variability, assessing population-specific responses and tolerance to low salinity and other environmental stressors to support the selection of resilient stocks for aquaculture and restoration programs.
Material and methods: Juvenile oysters from two distinct origins, Mediterranean Sea (Sardinia, Italy) and North Sea (Koster Islands, Sweden), were deployed in February 2025 at a commercial oyster farm in Lima estuary (NW Portugal), characterized by strong salinity (~3-30 PSU) and temperature variations (~10-19ºC). Monthly assessments included survival, growth, pathogen prevalence (e.g., Bonamia ostreae, Marteilia refringens, Ostreid herpesvirus-1, Perkinsus spp), while microplastic (MPs) contamination was evaluated before deployment (T0) and at 1, 3, 6, and 12 months after deployment. Environmental conditions were continuously monitored to detect environmental stress events like flooding and heatwaves.
Results: Survival differed significantly between origins (p<0.0001), with Italian oysters showing higher survival than Swedish oysters throughout most of the study (> 75% vs ~42% after 10 months). Complete mortality of both stocks occurred after 12 consecutive days of near-zero salinity caused by severe storms (Figure1). Growth also differed significantly (p<0.001), with higher absolute growth rates in Italian oysters (0.03 mm day-1) than in Swedish oysters (0.003 mm day-1). MPs contamination varied between origins and sampling periods, with higher loads in Italian oysters (7.44 MPs ind-1 and 59.50 MPs g-1) than Swedish oysters (5.45 MPs ind-1 and 5.14 MPs g-1). Italian oysters showed a contamination peak in March (T1) relative to pre-deployment levels (T0), followed by a decline, whereas Swedish oysters maintained consistently low MPs levels throughout the study. Fibers were the dominant MPs type in both stocks, mainly colourless, blue, black, and grey. Although MPs concentrations in water varied over time, no clear correlation was observed between MPs concentrations in water and oyster tissues. Regarding pathological survey, no evidence of Marteilia refringens, Bonamia ostreae, OsHV-1, or Perkinsus spp. was detected. 
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Figure 1. (A) Kaplan-Meier survival curves comparing oysters of Italian and Swedish origins over 11 months; Average daily salinity (B) and temperature (C) in the estuary over 12-month period, expressed as average ± SD.
Discussion and conclusion: Differences in survival and growth between origins suggest population-level variation in the response of O. edulis to estuarine conditions, with contrasting acclimation dynamics between Italian and Swedish oysters. Growth patterns indicate origin-specific strategies, with higher absolute growth in Italian oysters. Putative MPs contamination was detected in all samples, with higher levels in Italian oysters. As the confirmation of polymer composition by FTIR spectroscopy is still in progress, these particles are classified as putative MPs. The lack of direct correspondence between MPs concentration in water and oysters suggests that MPs accumulation is influenced by factors other than ambient water concentrations. The non-detection of major oyster pathogens suggests low pathogen pressure during the study period and supports the potential suitability of the Lima estuary for O. edulis cultivation and restoration initiatives. Overall, this study provides preliminary evidence of population-specific differences in oyster performance and susceptibility to environmental stressors. These findings will help identify the most suitable stock for aquaculture and ecological restoration programmes in Portuguese estuarine systems.
Acknowledgments: This work is developed within the project ShellFishBoost (SBEP/0009/2023), financed by Sustainable Blue Partnership, European Union and FCT. The authors also thank AQUAGOMA for providing space and logistical support for the field trial.
References:
1Beck, M. W. et al. (2011). Oyster reefs at risk and recommendations for conservation, restoration, and management. BioScience, 61(2), 107–116. https://doi.org/10.1525/bio.2011.61.2.5
2Zu Ermgassen, P. S. E., et al. (2024). European native oyster reef ecosystems are universally collapsed. Conservation Letters, 17, e13068. https://doi.org/10.1111/conl.13068