Science Advisory Report  2013/017

Framework for Assessing the Ecological Flow Requirements to Support Fisheries in Canada


  • Riverine ecosystems support many fisheries for which the Department has a regulatory mandate, including recreational, commercial or Aboriginal (CRA) fisheries.  Examples of fisheries dependant on the natural flow regime include (but are not limited to) many high profile fisheries, including Atlantic Salmon, six species of Pacific Salmon, Atlantic and Lake Sturgeon, Arctic Char, American Eel, Shad and Gaspareau or Alewife, Dolly Varden, Sea Trout, etc.
  • This scientific review specifically examined the “ecological flow requirements to support fisheries”.  This Science Advisory Report (SAR) provides advice on the management of “the flow regimes and water levels required to maintain the ecological functions that sustain fisheries associated with that water body and its habitat”.
  • The scope of this framework is for application of the Fisheries Act and intended to guide the assessment of ecological flows required to sustain a fishery (recreational, commercial, or Aboriginal), including potential future fisheries.
  • The scientific literature supports natural flow regimes as essential to sustaining the health of riverine ecosystems and the fisheries dependant on them.  Riverine ecosystems and the fisheries they sustain are placed at increasing risk with increasing alteration of natural flow regimes.
  • The probability of degradation to ecosystems sustaining fisheries increases with increasing alteration to the natural flow conditions.  Thus, the assessment of alterations to the flow regime should be considered in a cumulative sense, and not only on a project-by-project basis.
    • Cumulative flow alterations <10% in amplitude of the actual (instantaneous) flow in the river relative to a “natural flow regime” have a low probability of detectable impacts to ecosystems that support commercial, recreational or Aboriginal fisheries.  Such projects can be assessed with “desktop” methodologies.
    • Cumulative flow alterations that result in instantaneous flows < 30% of the mean annual discharge (MAD) have a heightened risk of impacts to fisheries.
  • For cumulative water use >10% of instantaneous discharge or that results in flows < 30% of the mean annual discharge (MAD), a more rigorous level of assessment is recommended to evaluate potential impacts on ecosystem functions which support fisheries.  For the purposes of this science advice, and as a basis to assess the impacts of flow alteration on fisheries, a minimum of 20 years of river flow data is recommended to establish the “natural flow regime”.  These data can be obtained from analysis of the:
    • Unaltered condition
    • Current condition (use existing technical guidance from various jurisdictions where this exists)
  • A “natural flow regime” can be defined as a flow regime that is only affected by the variability in hydrological inputs and outputs (precipitation, evaporation) and natural water storage (such as groundwater) and for which the response in terms of amplitude, timing, duration and frequency of events is unaltered by human impacts.
  • If the “natural flow regime” must be calculated with hydrologic modeling, it is recommended that data with the finest available time-scale be used. For most situations, this will be daily discharge data but in special circumstances (e.g. hydroelectric projects) hourly data may be preferred.
  • A floor value or ‘cut-off limit’ should be part of the overall prescription to conserve and protect fisheries, and should not simply be considered during low flow events.  Some jurisdictions in Canada currently have established methodologies to specify this ‘cut-off limit’.  In general, the development of such policy guidance is encouraged (refer to Linnansaari et al. 2013 for further information on various Canadian jurisdictions).
  • Given the inherent uncertainty in many of the ecological flow methodologies described, the use of adaptive management based on long-term and follow-up monitoring (a process based on the Before/After/Control/Impact experimental design) with multiple control locations is recommended.  
  • The science of assessment of ecological flows for fisheries is still evolving.  It is recommended that the scientific guidance in this document be periodically revisited in order to ensure that it reflects current scientific knowledge (i.e. 5 years).
  • Given the uncertainty around key relationships between flow and aquatic resources, further scientific investigation of the ecosystem-scale changes that affect fisheries subject to flow alteration is recommended. The objective of this research should be to define ecological flow assessment criteria to better inform both fisheries management decisions and policy and guidance development.

This Science Advisory Report is from the March 6-8, 2012 National Peer Review on “Standardized Framework for the Assessment of Instream Flow Needs in Canada”. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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