White Sturgeon in the Lower Columbia River: Is the Stock Overexploited/

Abstract
We used computer simulation to examine potential yields and sustainable exploitation rates for white sturgeon Acipenser transmontanus in the lower Columbia River. Simulated yields varied with assumptions used for mortality, growth, and stock-recruitment relationship, and with size restrictions imposed on harvest. The stock-recruitment function had the greatest influence on our results. Maximum yields (sustained or at 100 years) ranged from 0.2 to 2.9% of unexploited biomass. Maximum yields were produced with exploitation rates ranging from 0.02 to 0.20, and yields declined or collapsed at higher rates. Size restrictions (sport and commercial harvest of fish only between 92 and 183 cm or 122 and 183 cm, respectively) resulted in higher yields and higher supportable exploitation than no size restrictions. However, size windows did not prevent collapse of the fishery under higher exploitation rates when a stock-dependen t recruitment function was used. The fishery on the Columbia River has expanded dramatically in recent time. Most recent yields were about 30% of the peak yield realized when the stock was overfished before 1890, and were at least three times the sustained yields expected from our most optimistic simulations. Cultural development of the river could have reduced the productivity of the stock and made it vulnerable to overexploitat ion. The present fishery appears to be overexpanded. Current yields probably cannot be sustained, and current harvest risks eventual collapse of the fishery. Overexploitation of long-lived, relatively slow- growing fishes is a well-known problem (Ricker 1963; Adams 1980; Francis 1986). Expanding fisheries typically produce increasing yields in a "fishing-up" process (Ricker 1975) that cannot be sustained. Yields can decline dramatically, often to levels that represent a small fraction of the peak (Francis 1986). If the fishery expands beyond the effort producing maximum sustained yield (MSY), stocks can be quickly and seriously overfished. Recovery to a level supporting MSY can take many years (Adams 1980; Francis 1986). The fishery for white sturgeon Acipenser transmontanus in the Columbia River is susceptible to each of these problems. White sturgeon may live to 100 years or more (Scott and Crossman 1979); Columbia River white sturgeon commonly exceed 30 years in age (Hess 1984). Although white sturgeon can reach tremendous sizes (reports from the Colum- bia River exceed 450 kg), documented growth is relatively slow (Hess 1984). Increase in weight from age 5 to age 30 averages less than 20% per year. A commercial fishery for white sturgeon started on the Columbia River in the 1880s. The fishery expanded to a peak yield of nearly 2.5 million kg (80,000 fish) in 1892 and then declined to less than

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