We field tested the hypothesis that predation by piscivorous fish is reduced in turbid compared with clear water. The Harrison River (Յ1 nephelometric turbidity units, NTU) is a clear tributary of the naturally turbid Fraser River (27-108 NTU), in British Columbia, Canada. Age-0 juveniles of Harrison River stocks of Pacific salmon Oncorhynchus spp. migrating seaward in spring obligately pass through turbid and clear reaches of these rivers. To test the hypothesis, we compared predation on salmonids by potential predators caught by beach seine and by the rate of predator attack on tethered juvenile chinook salmon O. tshawytscha in these two rivers. Of 491 predators examined, 30% of Harrison River piscivores had recently consumed fish compared with only 10% of Fraser River piscivores. Of those that ate fish, fish prey per predator was significantly lower in the Fraser River (mean ϭ 1.1, N ϭ 21) than in the Harrison River (mean ϭ 1.7, N ϭ 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 predator (mean ϭ 2.4, N ϭ 19) were similar to values for the Harrison River. Loss of prey from tethers was significantly higher in the Harrison River (23-61%) than in the Fraser River (10-24%). The loss of prey from tethers 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 migration in the Fraser River system, age-0 Pacific salmon were less likely to encounter and be consumed by fish piscivores in turbid water than in clear water.

Well-functioning food webs are fundamental for sustaining rivers as ecosystems and maintaining associated aquatic and terrestrial communities. The current emphasis on restoring habitat structure-without explicitly considering food webs-has been less successful than hoped in terms of enhancing the status of targeted species and often overlooks important constraints on ecologically effective restoration. We identify three priority food web-related issues that potentially impede successful river restoration: uncertainty about habitat carrying capacity, proliferation of chemicals and contaminants, and emergence of hybrid food webs containing a mixture of native and invasive species. Additionally, there is the need to place these food web considerations in a broad temporal and spatial framework by understanding the consequences of altered nutrient, organic matter (energy), water, and thermal sources and flows, reconnecting critical habitats and their food webs, and restoring for changing environments. As an illustration, we discuss how the Columbia River Basin, site of one of the largest aquatic/riparian restoration programs in the United States, would benefit from implementing a food web perspective. A food web perspective for the Columbia River would complement ongoing approaches and enhance the ability to meet the vision and legal obligations of the US Endangered Species Act, the Northwest Power Act (Fish and Wildlife Program), and federal treaties with Northwest Indian Tribes while meeting fundamental needs for improved river management.

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Off-channel ponds in the upper reaches of the Coldwater River, British Columbia, were major rearing areas for juvenile coho salmon (Oncorhynchus kisutch). Chinook salmon (Oncorhynchus tshawytscha), steelhead trout (Salmo gairdneri), and Dolly Varden char (Salvelinus malma) were generally scarce in the ponds, although they were numerous in the main river. Coho salmon were predominant at "natural" river sites while steelhead trout was the main species at sites with "rip-rap" bank stabilization. Catches of juvenile coho were much lower in the main river than in the ponds where they were the main species, and were more variable in the river. Population density and biomass estimates of juvenile coho in the ponds ranged from 0.100 fish∙m−2 and 1.00 g∙m−2 to 1.00 fish∙m−2 and 5.15 g∙m−2, compared with density estimates of 0.08–0.23 fish∙m−2 in the river. The coho population in the ponds consisted of 0+ and 1+ age-groups in similar proportions, while in the main river the 0+ age-group was much more abundant. The growth rate of coho in the ponds was faster than in the main river, with pond fish reaching mean lengths of 62–79 mm at the end of the first growing season, compared with 53 mm in the main river. Smolt outmigration from the main study pond occurred in late spring with peak outmigration in May and June coinciding with peak river discharge and increasing water temperatures in the main river and pond.

Increasing empirical evidence indicates the number of released individuals (i.e. propagule pressure) and number of released species (i.e. colonization pressure) are key determinants of the number of species that successfully invade new habitats. In view of these relationships, and the possibility that ships transport whole communities of organisms, we collected 333 ballast water and sediment samples to investigate the relationship between propagule and colonization pressure for a variety of diverse taxonomic groups (diatoms, dinoflagellates and invertebrates). We also reviewed the scientific literature to compare the number of species transported by ships to those reported in nature. Here, we show that even though ships transport nearly entire local communities, a strong relationship between propagule and colonization pressure exists only for dinoflagellates. Our study provides evidence that colonization pressure of invertebrates and diatoms may fluctuate widely irrespective of propagule pressure. We suggest that the lack of correspondence is explained by reduced uptake of invertebrates into the transport vector and the sensitivity of invertebrates and diatoms to selective pressures during transportation. Selection during transportation is initially evident through decreases in propagule pressure, followed by decreased colonization pressure in the most sensitive taxa.

The migration of young adult Lampetra tridentata out of the Nicola River during 1984–85 through 1987–88 was estimated to be approximately 176 000,19 000,90 000, and 102 000; at the same time, approximately 749 000, 909 000, 920 000, and 605 000 large ammocoetes, age 4 and 5 yr, left the river. Migration of young adults started in September, with the largest number migrating from mid-March to mid-May. Ammocoetes behaved similarly except that movement continued throughout the year. Most young adults metamorphosed at age 4 or 5 and were age 5 or 6 when they migrated to sea. It was not determined when lamprey from the Nicola River reached salt water; however, L. tridentata entered the Strait of Georgia from the Fraser River from March until July. Maturing adults first returned to the Nicola River from the ocean in August when water levels in the river were at their lowest. This is the first estimate of abundance of young adult L. tridentata in any river in the Fraser River drainage. The very large number of young adults and older ammocoetes leaving the Nicola River indicate that parasitic lamprey are abundant in the Fraser River and that they are potentially a common predator of commercially important fishes.