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Science Advisory Report 2009/071

Pathways of Effects for Finfish and Shellfish Aquaculture


  1. The science review was provided with proposed activity – stressor – effect linkages for each of seven categories of stressors: chemicals, escapes, light, noise, nutrients, pathogens and structure. The linkages identified in the provided draft Pathways of Effects (POEs) were determined to be comprehensive but relatively simplistic representations, as they did not communicate the full complexity of all relationships and feedbacks that may occur. There is reasonable evidence that cumulative and/or cascading effects occur across pathways, and vary geographically and temporally and by level of activity. An appreciation of these complexities is essential to understanding the linkages. It was also noted that the provided POEs did not directly identify large-scale synergistic ecological functions and yet they are recognised by science as being important. The supporting scientific papers provide charts to complement the POE diagrams.
  2. Chemicals enter the aquatic environment during normal aquaculture practices. They are released directly into the water column (pesticides, antifoulants and disinfectants) or in faeces and constituents of medicated food (drugs). Hazards have been determined for most of these compounds but field data on exposure and effects is limited. While data are collected on use patterns of therapeutants, access to these data is limited, which greatly hinders the characterization of pathways and of effects.
  3. Potential hazards to wild populations posed by aquaculture escapes have been identified, but the probabilities and magnitudes of effects for such are not well known. Targeted in-depth investigations are needed in well-defined ecological systems where escapes and interactions are known to be occurring.
  4. There is evidence that light used for aquaculture operations would have only a local effect.
  5. Apart from acoustic deterrents, the effects of noise associated with aquaculture are generally short-term and localized and insufficient to cause injury.
  6. Overall there is scientific evidence that bivalve and finfish aquaculture can affect nutrient flow in both pelagic and benthic environments. There is strong empirical and modeling evidence that increased deposition of organic matter from cultured and non-cultured/fouling organisms has the potential to alter local benthic habitat. Linkages between bivalve filtration and nutrient removal are well known. Some of the effects of release of nutrients from aquaculture to the water column are less well understood.
  7. The extent to which pathogens released from aquaculture sites are stressors requires knowledge of infection and disease in wild aquatic populations. In Canada and other jurisdictions, pathogen surveillance of wild animal populations is virtually non-existent and should be established. Without this knowledge the extent to which pathogens are stressors cannot be assessed. There is scientific evidence that pathogens present in wild populations are the source of initial infections in aquaculture animals and some evidence that aquaculture animals release pathogens in their environment. However, evidence of pathogen transfer from aquaculture animals and/or products to wild populations is very limited.
  8. Considerable physical structure is added or removed as part of all types of aquaculture activity. This includes both non-living (ropes, buoys, anchors, etc.) and living (fish, bivalves) components. Diverse biological assemblages may colonize this structure and affect the ecosystem both locally and at larger spatial scales.
  9. There is substantial evidence and understanding from a broad range of environments that the major factors influencing the Pathways of Effects include i) water column characteristics (e.g., current flow, stratification, temperature, salinity, dissolved oxygen concentration); ii) bathymetry (e.g., depth, bottom topography); iii) operational practices (e.g., cultured species, non-cultured/fouling organisms, feed characteristics, stocking density); and, iv) the biological, chemical and physical characteristics of the receiving environment.
  10. Significant knowledge gaps exist for some of the key stressor-effect linkages that make it difficult to complete Pathways of Effects diagrams. The research required to address these knowledge gaps is identified within each of the stressor categories.

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