EVALUATION OF THE BIOLOGICAL BASIS FOR A PREDATOR CONTROL PROGRAM ON NORTHERN SQUAWFISH (PTYCHOCHEILUS-OREGONENSIS) IN THE COLUMBIA AND SNAKE RIVERS

Over 900 000 northern squawfish (Ptychocheilus oregonensis) were remov ed from the Columbia and Snake rivers from 1990 through 1995 by sport- reward and directed fisheries in an attempt to increase production of salmon and steelhead (Oncorhynchus spp.). Our review suggests that low exploitation of northern squawfish can have disproportionately large benefits for salmon survival if exploitation does not stimulate a comp ensatory response among predator populations. The number of adult salm onids can be expected to increase with increased freshwater survival o f juveniles at the prevailing depleted numbers. Northern squawfish con sumed significant numbers of uninjured juveniles that would otherwise have survived migration. Northern squawfish consumed an estimated 16 m illion migrants per year or about 8% of the population. Population sim ulations suggest that the number of large piscivorous northern squawfi sh can be significantly reduced by exploitation rates of only 10-20%, which removal efforts from 1990 to 1995 have demonstrated are feasible . Compensation by northern squawfish or other predators is possible bu t is not guaranteed because of the relatively small fraction of the no rthern squawfish population that is affected by exploitation. We propo se that evaluations of northern squawfish removal be based on indirect measures because direct demonstration of salmon survival benefits is not feasible.