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Fish passage issues

Activities that cause physical or physiological impediments to fish movement or migration.


Pathways of Effects diagrams have been developed by Fisheries and Oceans Canada as a tool to communicate potential effects of development proposals on fish and fish habitat and were developed through extensive consultation. It is expected that these diagrams will be updated to describe new activities and stressors as required.

Fish passage issues


Change in total gas pressure: Dissolved gases may become supersaturated when air gets trapped in water and submerged to sufficient depth (e.g., at the base of spillways associated with hydroelectric facilities). Total gas pressure above certain levels may subject organisms to injury or mortality.

Change in salinity: Increased volume freshwater flows into estuaries at certain times can decrease salinity levels which can affect the diversity and abundance of sensitive sea grass and also affect the distribution of some fish species. Alternatively, fish eggs, larvae may not tolerate higher salinities of the marine environment that may result from decreased freshwater flows. The quantity and seasonal timing of freshwater flows are critical at sensitive stages (e.g., larval, hatching).

Change in thermal cues or temperature barriers: Temperature often serves as a behavioural cue for fish. Anadromous fish, such as salmon, and shellfish, need temperature to trigger reproductive behaviour. Thermal pollution resulting in higher temperatures can cause a shift in the timing of reproduction and changes in the community structure.

Interbasin transfer of species: Diversion channels can promote the interbasin transfer of water which can promote insurgence of invasive species or other non-native aquatic organisms.

Incidental entrainment impingement or mortality of resident species: Fish may become entrained through intakes, turbines, spillways, etc. or impinged at screens and can result in injury or mortality.

Change in access to habitat/ migration: An alteration in water depth, flow, and/or substrate size causing a disruption in access to fish habitats essential for various life processes within given fish populations such as spawning and rearing.

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