Stock Assessment of Chinook, Sockeye, and Chum Salmon in the Nushagak River

Alaska Department of Fish and Game, Fishery Manuscript Series No. 12-05, Anchorage

In 2005, the Alaska Department of Fish and Game began using dual frequency identification sonar (DIDSON) to measure escapement on the Nushagak River. The DIDSON replaced an earlier acoustic system (Bendix) that had been in use for many years. Over the course of several years, the two systems were operated side-by-side in the four  spatial strata that are summed to produce the total salmon passage estimates. This comparison found the DIDSON detected a higher number of fish than the older system, particularly in the offshore strata. From the relationship between DIDSON and Bendix raw passage described in previous work, conversion factors for each species were calculated by first apportioning the daily sonar passage estimate to species by strata using DIDSON and Bendix daily estimates from 2002 to 2004 on the south bank and 2002 to 2005 on the north bank. Summing these strata estimates by species we determined strata conversion ratios for sockeye Oncorhynchus nerka, chum O. keta, and Chinook O. tshawytschasalmon. We expanded this analysis by applying these strata conversion factors to annual species passage estimates from 2002 to 2011 to come up with total conversion factors of 1.11, 1.27 and 2.08 for sockeye, chum and Chinook salmon, respectively. By applying these conversion factors to historical Bendix passage estimates, we produced revised total run and brood tables for Nushagak sockeye, chum, and Chinook salmon composed solely of DIDSON or equivalent estimates.