Influence of Eastern Oyster (Crassostrea virginica) aquaculture overwintering on eelgrass (Zostera marina)

Description

Eelgrass (Zostera marina) provides fish habitat to many commercial fish species and is considered an Ecologically Significant Species (ESS) in Atlantic Canada. Various activities related to oyster aquaculture may cause disturbances and alterations to eelgrass beds. One such practice, in the southern Gulf of St. Lawrence, is the placement of oyster bags on the bottom during the winter, ice-covered period as opposed to when oysters are cultured in plastic mesh bags attached to long lines floated at the water surface during the open water season.. Since the bottom substrate of these lease areas is often characterized by eelgrass habitat, concern had been expressed by habitat regulators about physical damage of placing the oyster bags directly on the bottom that may be caused to eelgrass by these over-wintering activities.

This project was designed to assess the potential impact that the practice of overwintering oyster bags had on eelgrass beds in the southern Gulf of St. Lawrence and to study eelgrass winter ecology and its susceptibility to disturbance during this period. The project also examined the environmental performance of a newly developed bivalve aquaculture technology (Horizontal Rope Floating Rack system). This floating rack system rests on the substrate while ensuring the oysters themselves do not make contact with the benthos. We hypothesized that this system would cause less benthic disturbance than current culture structures while also lessening oyster mortality. The results of this study contribute to scientific advice for regulatory decisions and best management practices to minimize and/or mitigate potential negative impacts of oyster aquaculture on eelgrass habitat.

Findings

Overall, oyster overwintering practices had no apparent impacts on eelgrass beds. Other than canopy height, which would be expected to vary with the arrival of new spring shoots, the only eelgrass response variable that showed a significant difference from the control group was shoot density. Furthermore, this variable was only statistically significantly reduced by mesh bags and only for one of three replicates included in the study. There were no bare quadrats observed during the percent cover assessment of the overwintering site as would be expected if this area was severely impacted by shellfish aquaculture.

Program Name

Program for Aquaculture Regulatory Research (PARR)

Year(s)

2012 to 2014

Principal Investigator(s)

Simon Courtenay
Research Scientist, Fisheries and Oceans Canada, Canadian Rivers Institute, Department of Biology, University of New Brunswick
Email: simon.courtenay@dfo-mpo.gc.ca

Team Member(s)

Monica Boudreau, Biologist, Fisheries and Oceans Canada, Gulf Fisheries Centre, Gulf Region

Marc Skinner, Stantec Consulting Ltd./ Canadian Rivers Institute

Collaborative Partner(s)

André L. Mallet, President, L'Étang Ruisseau Bar Lte