Report: Status and Restoration of Native Oysters in Galway Bay 2018-2023

The Marine Institute, in partnership with Cuan Beo and Bord Iascaigh Mhara (BIM), recently published a report on the status and restoration of native oysters in Galway Bay 2018-2023. The report represents an incredible amount of effort by collaborators in the Marine Institute, Cuan Beo, BIM, and the Clarinbridge Co-Op, and offers insight into our native oyster restoration progress in Inner Galway Bay to date and the historical context for restoration efforts. The transferability of our considerations, learnings, and progress to native oyster projects cannot be understated. The report is broadly structured across 8 themed considerations arising during our continued oyster reef restoration, namely: distribution of native and pacific rock oysters in Inner Galway Bay, settlement, growth, and survival rates of native oysters in Inner Galway Bay, and the effects of Bonamia infection, temperature, and salinity on native oyster populations.

 

Key points from the report include:

  • Survey time series (2011-2023) shows regular recruitment but very high mortality rates in larger size classes. This mortality is not due to fishing. Mortality rates increased during the time series. Bonamia ostrea is present in oysters in Galway Bay. Prevalence varies annually and seasonally. Intensity of infections is usually less than 30%. Current understanding suggests that Bonamia infection is responsible for high mortality of larger size classes of oysters.
  • Spat settlement can be enhanced by providing bivalve shell material (cultch) for settlement. Small scale trials show that settlement occurs in all areas when this material is provided and indicates that substrate availability is limiting recruitment.
  • The success of large scale sub-tidal deployments of cultch depends on location and the type of cultch used. Scallop shell cultch deployed in a specific area resulted in settlement of 13.9million spat in 2021 and 5.9 million spat in 2022 and provides substrate for successive annual settlements given that it remains available at the sediment surface over time. Survival to 1 year was 45% and significantly higher than spat released from spatting ponds. Growth is seasonal and stops in winter.
  • Oysters tolerate a broad range of temperatures and salinities but prolonged (days) exposures to reduced salinities, especially when temperatures are high, can be lethal or can reduce feeding rates. Modelled estimates of daily temperature and salinity in inner Galway Bay indicate that estuarine areas, that previously supported oyster stocks, are now high risk areas due to low salinity events. Low level mortality due to unsuitable combinations of temperature and salinity is predicted to occur periodically during the life span of a cohort of oysters.
  • Habitat suitability assessment is key to successful oyster restoration.

 

You can find the report by clicking the photo or link below:

Report: Status and Restoration of Oyster Galway Bay 2018-2023