Ocean and Estuarine PredatorsIdaho Fish & Game Report to the Director 5/1/98
Do predators control the recovery of Idaho's salmon and steelhead?
Conclusion: Although birds, fish, and mammals consume salmon in the ocean and estuary, there is no evidence that predators select for, caused the decline of, or are preventing recovery of Idaho salmon and steelhead. Nevertheless, dramatic increases in bird predation at the Columbia River mouth since 1990 appear to be a problem that should be addressed to increase survival of all stocks.
Caspian terns and cormorants nesting on man-made islands may take 20 to 40 percent of the salmon and steelhead smolts that arrive at the Columbia river estuary between mid-April and mid-July. Tern nesting in the estuary increased from none prior to 1984, to more than 8,000 pairs after dredge spoils were used to create islands. Cormorant nesting increased from 200 pairs prior to 1987, to 6,000 pairs after portions of the islands were protected from erosion with large rock. Most of the increase of 30,000 large fish-eating birds has occurred since 1990. Consumption estimates do not include unknown numbers of gulls, which also feed on smolts in the estuary. Bird feeding activity is intense where smolts are concentrated by wing piling and dredge islands to a 600-foot wide navigation channel. There is no evidence that this predation is selective for Idaho stocks, or caused their decline, although highly stressed fish are known to be more vulnerable to predators. Bird colonies in the estuary consume many more smolts than the 5.5 million "saved" by transportation around the dams in 1997. The NMFS, USFWS, and USACOE have discussed alteration of man-made islands to discourage bird nesting in the Columbia River estuary. IDFG strongly supports immediate implementation of measures to return bird predation to low levels.
In some years predators such as pacific mackerel may deplete juvenile salmon in nearshore areas before they move to the open ocean. These impacts increase when concentrations of predators move north during ocean warming cycles. Steelhead and sockeye salmon may be somewhat less vulnerable than chinook and coho because they move offshore as soon as they leave the estuary. Idaho spring-summer chinook and similar downriver stocks are vulnerable as they move north along the coast enroute to feeding areas off the Aleutian Islands. Fall chinook could be affected even more because they remain inshore longer, disperse north and south of the Columbia river, and are smaller than spring-summer chinook and steelhead when they enter the ocean. Snake River fall chinook returns have been relatively stable since 1975, however.
MARINE MAMMAL PREDATION
California sea lions on the West Coast have increased at more than five percent annually to about 170,000 animals following passage of the Marine Mammal Protection Act in 1972. The more common pacific harbor seal has shown similar increases. Relatively small numbers of seals and sea lions become a problem primarily in areas where salmonids concentrate or are caught in fishing gear. In other areas seals and sea lions generally feed on other fish; consequently this source of mortality has little significance for most stocks. Peak numbers of 3,000 seals and 300-500 sea lions consume salmon, steelhead, and other fish in the lower Columbia river from the estuary to Bonneville Dam, primarily during the winter months. Much of this time salmon and steelhead do not comprise the majority of their diet however. Salmonid predation that does occur is not likely to select Idaho fish over other stocks present in the area.
Distinct populations of killer whales that inhabit the inside passages of British Columbia feed almost exclusively on salmon. They prey primarily on salmon stocks resident in, or returning to those waters. Idaho steelhead pass through these areas on their return to the Columbia River, however killer whale predation is not know to be a factor limiting salmon and steelhead populations in general.
Appendix 3.9 IDAHO's ANADROMOUS FISH STOCKS:
Their Status and Recovery Options
Report to the Director Idaho Fish & Game 5/1/98
Issue Paper: Ocean and Estuarine Predators
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