Science advice on the use of timing windows as a mitigation measure

Timing windows are a mitigation measure that define periods in the year when a work, undertaking, or activity (WUA) can take place because the potential effects of that WUA on fish and fish habitat are reduced relative to other times of the year. Timing windows are an appropriate mitigation measure when the pressures caused by the WUA are transient, and when there is predictable variation in the vulnerability of fish and fish habitat to WUA pressures over time. Considerable variation was observed among established timing windows in Canada, reflecting the diversity of species and habitats across the country. However, there is limited science on the development, use, and effectiveness of timing windows. Thus, there is a need for a scientific process for their development and modification to facilitate their standardization and defensibility, and to validate their effectiveness. A conceptual model for the development and refinement of timing windows was presented that can be used to identify periods of the year when risks to fish and fish habitat from WUA pressures are lower. The conceptual model includes (1) the timing of life processes of species of interest, (2) the relative vulnerability of each life process to WUA pressures, (3) seasonal variation in environmental conditions, and (4) an assessment of how magnitude and persistence of the effects of WUAs vary due to modulation by environmental conditions. The four components of the conceptual model can be used to inform an assessment of the variation in the risk to fish and fish habitat from WUA pressures throughout the year. Although the model was presented for a single fish species or groups of species with similar life histories, conceptually, it can be adapted for communities or guilds of species. Based on this conceptual model, timing windows can be established for periods when the risk to fish and fish habitat is assessed to be reduced. Uncertainty (due to lack of knowledge, or to spatial and temporal variation in biological and ecological processes) can be managed by varying the duration of timing windows in accordance with risk tolerances and management goals. Timing windows can be modified by including site-specific information on species biology and environmental conditions, and could also be modified as required in response to observed variation in biological or environmental events. Timing windows are one of a suite of mitigation measures commonly prescribed by FFHPP to reduce the risk of harmful impacts on fish and fish habitat. If a period of reduced risk cannot be identified and the impact of a WUA pressure cannot be mitigated by using this measure, timing windows may not be effective, and greater emphasis should be placed on the use of other measures. A three-tiered approach for evaluating the effectiveness of timing windows was adapted from previous advice. The first tier consists of monitoring to determine the extent of exposure of fish and fish habitat to WUA pressures during the timing window. The second tier is designed to establish if timing windows reduce fish mortality and/or impairment of the habitat’s capacity to support life processes of fish. The third tier is an assessment of potential higher order consequences (e.g., above individual or site level) of the WUA pressure during the timing windows, and has a goal of increasing our understanding of their use. Challenges associated with the implementation of the proposed approach include estimating the risk to fish and fish habitat for complex and diverse fish communities in spatially and temporally variable environments, incorporating the effects of climate change, and difficulties evaluating the effectiveness of timing windows

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