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Research Document - 2006/034

Indicators and Thresholds for Use in Assessing Shellfish Aquaculture Impacts on Fish Habitat

By Cranford, P.J., R. Anderson, P. Archambault, T. Balch, S.S. Bates, G. Bugden, M.D. Callier, C. Carver, L.A. Comeau, B. Hargrave, W.G. Harrison, E. Horne, P.E. Kepkay, W.K.W. Li, A. Mallet, M. Ouellette, and P. Strain


The purpose of this research document is to provide science advice needed to allow DFO Habitat Management to make and justify management decisions related to the potential harmful alteration, disruption and destruction (HADD) of fish habitat by shellfish aquaculture. The overall goal of this exercise and of our recommendations is to promote the avoidance and mitigation of a HADD. Our specific objectives were the following:

  1. identify, evaluate and make recommendations regarding a range of quantitative indicators (measures of habitat and ecosystem status) that could be used to monitor for potential shellfish aquaculture effects; and
  2. provide science-based decision support for the development of an environmental monitoring framework, based on identification of predetermined impact limits (operational thresholds) intended to trigger management actions.

A wide range of ecosystem and habitat status indicators and methodological approaches were identified to support industry management and each was initially screened based on habitat impact predictions and observations. Selected indicators were classified based on associated strengths and weaknesses using predefined criteria, including: availability of operational thresholds; regulatory needs; cultured species; scales of impact addressed; cost/benefit; and the needs of responsive management. A habitat assessment framework is recommended for shellfish aquaculture that addresses the need for a consistent and transparent decision-making approach that is science-based, and reflects both fish habitat and ecosystem concerns.

The highly diverse Canadian shellfish aquaculture industry (e.g. species cultured, husbandry method, and stocking density) and regional differences in environmental impact risks (related primarily to geographic and hydrodynamic factors) were identified as important considerations for our evaluation of shellfish aquaculture impact assessment options. Recommendations are made towards establishment of an environmental monitoring framework that incorporates sufficient flexibility to be of use in a wide range of environmental settings and that is both effective and practical for current aquaculture operations that range from less than 0.5 to 500 hectares.

A primary recommendation of this report is that habitat assessments could be based on a tiered approach that recognizes that an increased risk to fish habitat requires an increase in monitoring effort. Various levels of monitoring could be triggered based on assessments of environmental sensitivity and risk (e.g. dispersive vs. depositional environment and presence of sensitive habitat), the nature of the operation (e.g. size, species and husbandry), and previous measurement and verification of environmental impacts. Inherent within the recommended framework is that ongoing monitoring programs could be continually adaptive to changes in our state-of-knowledge on potential environmental impacts, indicators and related methodologies. It is important to maintain an ability to add or remove indicators to monitoring programs based on sound science.

The recommended multi-tiered impact assessment approach addresses the potential for benthic marine habitat impacts in the immediate vicinity of each shellfish aquaculture lease and it therefore parallels science recommendations for finfish aquaculture monitoring in Canada. Scientifically defensible thresholds are available for benthic biogeochemical indicators (sulfides and redox potential) and these could be used to define the hypotheses that need to be addressed in an operational monitoring program. Effective measures are also available for mitigating benthic organic enrichment impacts, and these can be linked to the operational thresholds incorporated in a responsive management framework.

Ecosystem-level interactions with dense shellfish aquaculture populations are more complex than for finfish culture and many potential and observed effects on fish habitat cannot be assessed using only site-specific benthic habitat indicators. Measurements with selected far-field impact indicators are needed under certain conditions to compliment benthic operational monitoring. The inability to fully define quantitative operational thresholds for many valid and highly relevant indicators of habitat and ecosystem status (particularly those describing the structure and dynamics of pelagic habitat), owing to present gaps in our knowledge of ecosystem dynamics, should not preclude their potential use. Surveillance sampling programs based on water column parameters are needed under conditions where environmental impact assessments and ongoing monitoring data indicate a relatively high risk that bay-scale impacts will occur. Of particular importance is the need to assess the impacts of longline mussel culture operations on suspended particle concentrations and distribution and the pelagic food web (micro-flora and fauna) in extensively leased coastal embayments. Industry shellfish stocking information for all farms within a management area is considered fundamental to assessments of shellfish aquaculture impacts on fish habitat. The use of sound science practices is required for the design of monitoring programs (statistically valid sampling approaches) and for the analysis of habitat status indicators and data (e.g. quality assurance/quality control).

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