Feasibility of Estimating Salmon Abundance in the Koyukuk River Using Sonar, 2025

The Koyukuk River is a major tributary to the Yukon River and supports several fisheries for Chum Salmon (Oncorhynchus keta), Chinook Salmon (O. tshawytscha), and whitefish (Stenodus, Coregonus, and Prosopium spp.). In 2025, feasibility work began regarding a mainstem Koyukuk River sonar project to enumerate annual fish passage fish passage
Fish passage is the ability of fish or other aquatic species to move freely throughout their life to find food, reproduce, and complete their natural migration cycles. Millions of barriers to fish passage across the country are fragmenting habitat and leading to species declines. The U.S. Fish and Wildlife Service's National Fish Passage Program is working to reconnect watersheds to benefit both wildlife and people.

Learn more about fish passage . After initial review using remote sensing, boat-based bathymetric surveys were conducted between the junction of the two primary Koyukuk River mouths and the confluence of the Gisasa River. These surveys unveiled five candidate sites for a future large-scale sonar project, of which the most promising two were trialed with DIDSON imaging and HTI split beam sonars to determine fish passage orientation (i.e., distance from shore and depth). The trials took place at two sites near the Alaska Department of Fish and Game (ADFG) Check Station from June 30 to July 9 during high water. Water temperatures exceeded the 18°C threshold for fish heat stress in 84 out of 194 hours (43.3%). The site closest to the Check Station appeared reasonable as a site for future sonar work with more than 95% of the upriver-swimming fish observed in the area ensonified by the DIDSON passing within 17 m of shore on north bank and 29 m on south bank. In the split beam, more than 95% of the upriver-bound targets were within 15 m of north bank and 53 m of south bank, although the range on north bank was severely restricted. Future feasibility work must investigate (1) the estimated proportion and count of each fish species transiting the site, (2) the optimal split-beam sonar settings and configuration required by the site, (3) the optimal timing for sonar operation at the site, and (4) whether Nikolai Slough is allowing fish passage that would cause a sonar project at the site to under- or overestimate fish passage into the Koyukuk River.