Research Document - 1999/207

Optimal Production of Chinook Salmon from the Taku River.

By S.A. McPherson, D.R. Bernard, and J.H. Clark

Abstract

Optimal production of adult chinook salmon Oncorhynchus tshawytscha from the Taku River was investigated with information from a stock assessment program (1973-1997) and catch sampling programs of the Canadian inriver gillnet fishery, U.S. gillnet fishery in Taku Inlet, the U.S. commercial troll and the U.S. recreational fishery near Juneau. Stock assessment was based on aerial surveys and mark-recapture experiments to estimate abundance of large (mostly age-1.3 and older fish) salmon over the spawning grounds. Relative age composition was estimated from 1973 through 1997 at a carcass weir on the Nakina River and during mark-recapture experiments on other tributaries. Additional mark-recapture experiments using coded-wire tags provided estimates of harvest in fisheries and abundance of emigrating smolts. Spawning abundance that would produce maximum-sustained yield (N_MSY) was estimated at 19,597 and 28,738 large salmon from fits of two traditional stock-recruit models to the data. No autocorrelation among residuals was detected in these fits, however, measurement error in estimates of spawning abundance and production, mostly from aerial surveys, reduced confidence in the accuracy of N_MSY estimates. Inspection of more precise, individual estimates of spawner and smolt abundance, adult production, and smolt size provided evidence for early density-dependent survival of young salmon and later density-independent survival of young salmon while still in freshwater and while at sea. Because observed smolt production was near maximum over a range of about 30,000 to 55,000 large spawners, this level was recommended as the best estimate for N_MSY at this time. We recommend that the Alaska Department of Fish and Game and the Department of Fisheries and Oceans (Canada) adopt a biological escapement goal range of 30,000 to 55,000 large spawners for management purposes for this chinook salmon stock. This corresponds to an aerial survey goal of approximately 5,800 to 10,600 large spawners, counted in five tributaries. We also recommend continuation or implementation of several stock assessment components to improve estimation of population statistics and management of this stock.

Key words: chinook salmon, Oncorhynchus tshawytscha, Taku River, smolt abundance, spawning abundance, mark-recapture, age, sex and length composition, escapement goal, stock-recruit analysis.

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