We investigated the distribution and predation activities of northern Squawfish Ptychocheilus oregonensis at three locations where hatchery-reared juvenile salmonids Oncorhynchus spp. were released in Bonneville Pool, a Columbia River reservoir. Significant increases in northern Squawfish catch rate occurred at all three locations after releases. The timing and duration of elevated catch rates appear to be closely related to the release and subsequent residence time of the hatchery-released fish in the sampling area. Northern Squawfish caught after salmonid releases had a significantly higher frequency of occurrence and mean number of juvenile salmonids in their diet compared with fish caught before releases. Consumption indices, relative measures of consumption rates, were also higher at each location after release, Our results suggest that northern Squawfish aggregate to feed on hatchery-released juvenile salmonids in the spring. Management activities to reduce predation on juvenile salmonids should consider (1) removing predators from areas where they concentrate to feed on juvenile salmonids, specifically near hatchery release points where the residence time of juveniles may be prolonged, (2) releasing hatchery fish that are larger, and (3) adopting hatchery release plans that reduce the residence time of hatchery fish at the release location.
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Collis et al. (1995) studied this question.