TURBIDITY REDUCES PREDATION ON MIGRATING JUVENILE PACIFIC SALMON

We field tested the hypothesis that predation by piscivorous fish is r educed in turbid compared with clear water. The Harrison River (less t han or equal to 1 nephelometric turbidity units, NTU) is a clear tribu tary of the naturally turbid Eraser River (27-108 NTU), in British Col umbia, Canada. Age 0 juveniles of Harrison River stocks of Pacific sal mon Oncorhynchus spp. migrating seaward in spring obligately pass thro ugh turbid and clear reaches of these rivers. To test the hypothesis, we compared predation on salmonids by potential predators caught by be ach seine and by the rate of predator attack on tethered juvenile chin ook salmon O. tshawytscha in these two rivers. Of 491 predators examin ed, 30% of Harrison River piscivores had recently consumed fish compar ed with only 10% of Fraser River piscivores. Of those that ate fish, f ish prey per predator was significantly lower in the Fraser River (mea n = 1.1, N = 21) than in the Harrison River (mean = 1.7, N = 66). In a clear-water side channel of the Fraser River-Nicomen Slough (1-6 NTU) -both incidence of predation (37%) and number of fish prey per predato r (mean = 2.4, N = 19) were similar to values for the Harrison River. Loss of prey from tethers was significantly higher in the Harrison Riv er (23-61%) than in the Fraser River (10-24%). The loss of prey from t ethers was highest at dusk and near the bottom in the Harrison River; no spatial or temporal difference occurred in the turbid Fraser River. Therefore, our data support the hypothesis. During their seaward migr ation in the Fraser River system, age-0 Pacific salmon were less likel y to encounter and be consumed by fish piscivores in turbid water than in clear water.