Assessment of factors leading to oyster mortality in Tracadie Bay (New Brunswick) and development of physical and biological tools for management

Description

In August 2016, significant mortality of up to 75% of suspended oysters in some farms occurred in the northern branch of Tracadie Bay. Field observations were consistent with an anoxic event (i.e., water conditions characterised by an absence of dissolved oxygen and elevated hydrogen sulfide levels in the water column). The cause of the anoxic event is thought to be eutrophication which is an increase in nutrient levels in the local waters that causes an increased growth of microscopic plant life and a lack of oxygen in the water for animal life. As eutrophication is a major threat globally, it is important to increase knowledge and understanding of how productivity of cultured stocks might be impacted by the ecosystem in which they exist, and of how aquaculture operations impact the ecosystem.
The objectives of this project were to:

1. Develop a hydrodynamic model capable of reproducing current flow patterns within Tracadie Bay
2. Determine areas within the bay that are at risk of hypoxia (when the body is deprived of oxygen)
3. Determine thresholds of effect of anoxia, hydrogen sulfide and temperature on cultivated oysters using a combination of field and laboratory work.

Together, field and laboratory work provide powerful tools for managing oyster farming in Tracadie Bay and in other shellfish aquaculture areas for the foreseeable future. The hydrodynamic model, in conjunction with the spatial assessment of organic content and benthic flux, will help identify areas at risk of nutrient-induced impacts.

While the capacity to predict areas at risk of impact is useful for management, it is also directly applicable for shellfish growers and their operations. Ultimately identifying thresholds of oyster survivorship will provide baseline information for early warning signs and, potentially, indicators for growers to take action in the event of a future anoxic event. Findings from this work also are of relevance for other fixed gear aquaculture around the world, resulting in an increase in scientific capacity of the aquaculture industry.

Findings

A water circulation model was developed and validated for Tracadie Bay based on lidar data acquired in 2011 (via PARR-2011-G-19), five years prior to the oyster mortality incident. The bay had changed substantially from 2011 to 2017 with circulation being dramatically reduced due to the narrowing of an inlet in the northern part of the bay. A number of scenarios were tested to determine the most effective means to improve circulation; ultimately re-opening the northern inlet proved to be the best option.

Secondly, a series of laboratory experiments confirmed the plausibility of anoxia as a mechanism for oyster mortality. When exposed to anoxic conditions, oyster mortality occurred within one to two weeks in both static and flow-through chambers. However, survivorship essentially doubled when the seawater was initially treated with the broad-spectrum antibiotic (chloramphenicol) suggesting that the growth of anaerobic bacteria accelerates the mortality rate.

These findings support the initial hypothesis that an anoxic event was the ultimate cause of oyster mortality in the field.

Publications

• Coffin, MRS, Clements, JC, Comeau, LA, Guyondet, T, Maillet, M, Steeves, L, Winterburn, K, Babarro, JMF, Mallet, MA, Hache, R, Poirier, LA, Deb, S, Filgueira, R.The killer within: Endogenous bacteria accelerate oyster mortality during sustained anoxia. (Limnology and Oceanography – under review post revision).
• Deb, S, Guyondet, T, Coffin, MRS, Barrell, JP, Comeau, LA, Clements, JC. Effect of inlet morphodynamics on estuarine circulation and implications for water quality. (pending submission)
• Winterburn, K, Steeves, L, Coffin, MRS, Filgueira, R. Crassostrea virginica mortality under anoxic conditions: influence of temperature and exogenous bacterial sources. (pending submission)

Program name

Aquaculture Collaborative Research and Development Program (ACRDP)

Years

2017 – 2020

Principal investigator

Michael Coffin, Biologist, Fisheries and Oceans Canada, Gulf Fisheries Centre, Gulf Region
Email: Michael.Coffin@dfo-mpo.gc.ca

Team member(s)

• Thomas Guyondet, Research Scientist, Fisheries and Oceans Canada, Gulf Fisheries Centre, Gulf Region
• Luc Comeau, Research Scientist, Fisheries and Oceans Canada, Gulf Fisheries Centre, Gulf Region
• Rémy Haché, New Brunswick Department of Agriculture, Aquaculture and Fisheries (DAAF)
• Sylvio Doiron, New Brunswick Department of Agriculture, Aquaculture and Fisheries (DAAF)
• Joannie Thériault, Commission of the Environment for the municipality of Tracadie, NB
• Jose Babarro, Instituto de Investigaciónes Mariñas CSIC
• Ramon Filgueira, Dalhousie University
• Michael van den Heuvel, University of Prince Edward Island

Collaborator(s)

• André Mallet, L’Étang Ruisseau Bar Ltd. (ERB)