Testing Assumptions at a Large Riverine Sonar Project Using Acoustic Tags

The Yukon River currently supports one of the largest Chinook salmon Oncorhynchus tshawytscha and chum salmon O. keta subsistence fisheries in the state of Alaska. Located in a single-channel environment at river km 197, the Pilot Station sonar project has provided daily passage estimates for Chinook, chum, and coho salmon O. kisutch  for most years since 1986 and is an integral source of information used in making management decisions for the commercial and subsistence fisheries within the drainage.  Estimates of upstream migration of targeted fish species are produced from a combination of independently generated estimates of 1) fish passage past the site using shore-based sonar, and 2) species proportions based upon drift gillnetting at stations corresponding to sonar sampling strata. While the Yukon River at the sonar site is approximately 1000 m wide, sonar detection is limited to a practical effective range of approximately 300 m and testfishing is conducted with drift gillnets approximately 50 m in length. The accuracy of our estimates relies on meeting several untested assumptions about fish distribution, including that salmon remain bank oriented in this area, that the majority of salmon passage occurs within the detection range of the transducers, and that those salmon are both available and representatively sampled in the areas fished with gillnets. To test these assumptions, a two-year acoustic tagging project is being conducted. Approximately 150 adult Chinook salmon and 150 adult chum salmon will be captured downriver of Pilot Station, fitted with internal acoustic tags, and released. The trajectory of these tagged salmon will be recorded as they pass through an array of hydrophones deployed in the river at the Pilot Station sonar project. The three-dimensional track of each tagged fish will then be compared to 1) the area ensonified by the sonar system and 2) the established drift gillnetting stations.