The role of environment in shaping resilience of Pacific oysters to summer mortality syndrome: A comparison of intertidal and deep-water culture sites

In 2021, Fisheries and Oceans Canada, in partnership with Vancouver Island University and Taylor Shellfish Canada, initiated a two-year research project to address climate change impact knowledge gaps for Pacific oysters. These oysters attract a premium price in the marketplace but opportunities for the local oyster industry to expand are limited by the industry’s ability to produce and grow the shellfish. A major issue continues to be the high mortality rates experienced by oyster farms due to summer mortality syndrome, with farms experiencing up to 90% mortality in recent years. The specific causes of BC oyster mortality events are unknown, but recent research has shown field mortality to be positively correlated with temperature (typically occurring when summer temperatures are >20°C), reproductive conditions, and the presence of certain pathogenic bacteria. Further research was needed to determine if using the intertidal zone as a refuge from summer mortality may be a viable option for shellfish growers. Following analysis of data collected during field and lab assessments in 2021 and 2022, advice can be provided to the BC shellfish industry on ways to develop oyster resilience and suitable mitigation strategies for summer mortality.

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Understanding and Assessing Impacts

The BC shellfish industry generates $47 million dollars annually and employs up to 800 people in direct full-time jobs. The industry primarily produces Manila clams and Pacific oysters, the latter typically for high-end markets, such as upscale restaurants, that offer farmed oysters on the half shell. These oysters attract a premium price in the marketplace. An increased demand exists for BC oysters, but opportunities for the local oyster industry to expand are restrained by the industry’s ability to produce and grow the shellfish. A major issue continues to be the high mortality rates experienced by oyster farms due to summer mortality syndrome, with farms experiencing up to 90% mortality in recent years. Summer mortality of Pacific oysters is very common in BC and the BC Shellfish Growers Association estimates that, on average, 50% of adult oysters cultured in deep-water sites die from summer mortality each year. Anecdotal evidence from oyster farmers indicates that summer mortality on farms in Baynes Sound has substantially increased over the past decade, with losses occurring every summer. Mortality rates typically range from at least 30% but can extend to near total loss. The specific causes of BC oyster mortality events are unknown, but recent research has shown field mortality to be positively correlated with temperature (typically occurring when summer temperatures are >20°C), reproductive condition, and the presence of certain pathogenic bacteria.

For additional climate information, look at the Resources section of this example (below). 

Identifying Actions

Anecdotal evidence from BC shellfish growers suggests that intertidally-grown oysters do not suffer from the massive mortality events observed in deep-water-cultured oysters and transplantation of stressed animals from deep-water to intertidal may facilitate recovery. Consequently, using the intertidal zone as a refuge from summer mortality may be a viable option for shellfish growers. However, implementation must be planned and carried out well in advance, as severe mortality events can happen rapidly. There remains a lack of research on how the varying physical conditions and oyster physiologies associated with deep-water and intertidal culture sites may promote or reduce the severity of summer mortality events. This project investigates summer mortality of intertidal and deep-water cultured oysters to determine how specific environments may shape oyster resilience/susceptibility to the various drivers of summer mortality and to test the use of the inter-tidal zone as a mechanism to promote physiological resilience and/or offer refuge from summer mortality.

Implementation

Field assessments took place in summer of 2021 and summer of 2022. In March 2021, Pacific oysters were deployed at three commercial farms in the Baynes Sound region, each comprising a deep-water site and an intertidal site. Following one year in the deep-water or intertidal site, randomly selected oyster bags from the deep-water/intertidal site were transferred to the corresponding deep/ intertidal site (e.g. reciprocal transplant between sites). This allowed for assessment of whether movement from deep-water to intertidal reduces mortality rates during the high-temperature summer months and whether movement from deep-water following time in the intertidal produces a more resilient oyster. At each sampling time point, percent survival was calculated, and reproductive status, size and condition were assessed. Physical environmental conditions including temperature, salinity and oxygen content were monitored at all deep-water and intertidal sites.

Oysters from intertidal and deep-water sites were also collected at regular intervals (in September 2021 and September 2022) for use in laboratory-challenge experiments. These experiments compared the survival of intertidally- and deep-water-cultured oysters under separate and/or coinciding temperature and pathogen (Vibrio) stress conditions.

Outcomes and Monitoring Progress

Key results to date:

1. There was a clear benefit (i.e. increased survival) for oysters that spent one year in the intertidal zone prior to deployment in deep water but the effect was farm-specific.
2. No obvious benefit was gained by transferring oysters from deep waters to the intertidal zone during the summer.
3. Challenge experiments demonstrated that intertidal animals have an increased tolerance to heatwave (↑survival time).
4. There was an extended period of low oxygen conditions following heatwave conditions at all farms in summer of 2022.
5. An increased incidence of Viral Gametocytic Hypertrophy was observed in oysters following a heatwave event in 2021.

Data from field and laboratory experiments are currently being analyzed. Following this, advice can be provided to the BC shellfish industry on whether using the intertidal zone as a summertime refuge to develop oyster resilience is a suitable mitigation strategy for summer mortality.

Next Steps

The results from these experiments will inform future proposals examining the impacts of oceans and warming on shellfish.

Resources

Additional Climate Information:

Using climate change projections enables better adaptation decisions. To learn how to choose, access, and understand climate data, visit ClimateData.ca’s Learning Zone.  

Visit ClimateData.ca and click “Explore by Variable” for future climate projections related to temperature and precipitation, which can be used to inform adaptation planning.