The PSC Mission hydroacoustics (HA) program uses stratified counting method to estimate daily salmon passage across the river. Passages in nearshore areas (60m from the left-bank and 30m from the right-bank) are monitored by the 2 shore-based sonar systems which also simultaneously monitor fish’s swim speed and direction of travel. Passage beyond the nearshore monitoring areas is sampled by a mobile transecting vessel with a downward looking transducer. The mobile system can estimate density of the offshore fish, but not swim speed or direction. Offshore fish are assumed to have a similar swim behaviour to the nearshore fish for the derivation of offshore fish passage rate through a mobile flux model 1.
In recent years, Food, Social and Ceremonial (FSC) fisheries have operated a driftnet fishery in the vicinity of the HA site. During fisheries openings, driftnets often cross the transect line of the mobile transecting vessel and severely impact the migration behaviour of salmon. When trying to evade the net, the salmon in offshore area deviate from their normal upstream migration behaviour by holding or milling thereby violating the assumption of uniform behaviour between nearshore and offshore fish used in the current estimation method. During the recent hydroacoustic review process and in responding to a memorandum of understanding between DFO, PSC and Sumas First Nation, PSC Hydroscoustics staff identified such evasive milling behaviour during fisheries as a source of likely bias. Therefore, alternative estimation methods should be investigated and developed for fishing days or opening hours to minimize bias due to fishing impacts.
For this project, we intend to explore the impact of replacing the data collected during fishing hours with an alternative dataset to derive the salmon passage during fishing hours. The method constructs the alternative dataset from the data outside the fishery opening hours (the non-fishing hours) using a projected ratio of fishing hour to non-fishing hour passages from the previous non-fishing day(s).
