Declining salmonid runs on the west coast of North America are a result of many factors, including habitat destruction, hydropower development, overharvest, and ill-conceived hatchery practices. One factor that has received little attention is inter- and intraspecific predation and competition in estuaries. Populations of salmonid predators and possible competitors in estuaries have increased dramatically since the 1970s. In particular, estuarine populations of marine mammals and piscivorous birds have increased significantly. The purpose of this paper is to review the status of salmonid predators and competitors in West Coast estuaries, highlighting current data gaps about these problems.

Since enactment of the Marine Mammal Protection Act of 1972, Northwest populations of California sea lions (Zalophus californianus) and harbor seals (Phoca vitulina) have increased steadily and are now healthy, productive, and growing. Many salmonid populations, on the other hand, are unproductive and decreasing, and some are listed as threatened or endangered under the federal Endangered Species Act (Snake River spring/summer and fall chinook salmon (Oncorhynchus tshawytscha), Snake River sockeye salmon (O. nerka), and Sacramento River winter chinook salmon).

Estuaries are excellent habitat for marine mammals, providing protected haul-out sites and a high abundance of a variety of prey species, including salmonids. However, the degree to which marine mammals prey upon salmonids is poorly understood. Harmon et al. (1994) documented a relatively high prevalence of marine mammal tooth and claw abrasions on adult salmonids passing Lower Granite Dam on the Snake River from 1990 to 1993. These were mostly attributed to sea lions. Their data plus more recent data (Table 1) indicate that, although the prevalence of marine mammal attacks are relatively high, there are no obvious trends. As marine mammal populations have continued to grow, prevalence of tooth and claw abrasions has not. Moreover, these data provide no information on how many salmonids were actually caught and consumed, nor how many died migrating to Lower Granite Dam (720 km from the mouth of the Columbia River). What these data indicate is that marine mammals do attack and eat salmonids and that there are factors in addition to salmon and marine mammal population abundance that influence marine mammal attacks.

The West Coast California sea lion population is growing rapidly with no indications that it is reaching an optimal sustainable population (OSP) level (i.e., an asymptotic population exponential growth curve). From 1975 to 1994, pup counts of sea lions in California increased at an annual rate of 5.2% and increased regionally 8% per year (Barlow et al. 1995, U.S. Dep. Commer. in prep.). In 1992, sea lion populations were estimated to be approximately 11,000, 17,000, and 30,000 along the coasts of Oregon, Washington, and California, respectively. Total U.S. population of California sea lions was estimated to be between 161,000 and 181,000 in 1994.

Although sea lion food habit studies have provided information on the occurrence of salmonid parts in sea lion diets, most studies have not estimated how many salmonids were consumed. The best evidence for severe marine mammal predation on adult salmonids comes from the Ballard Locks in Seattle, Washington, where the California sea lions have been estimated to consume about 40% of the steelhead (O. mykiss) run since 1985/1986 (U.S. Dep. Commer. in prep.).

In the Columbia River, sea lions feed primary on fishes other than salmonids, such as eulachon (Thaleichthys pacificus), lamprey (Lampetra spp.), Pacific herring (Clupea pallasi), rockfish (Sebastes spp.), and northern anchovy (Engraulis mordax). Nevertheless, salmonids may contribute substantially to the sea lion diet at specific times and locations. There also appear to be more salmonids in the diet now (28% in the 1990s vs. 13% in the 1980s), but this may be a result of improved laboratory procedures (U.S. Dep. Commer. in prep.).

Other feeding studies have been conducted off Cascade Head, Oregon; Rogue River, Oregon; and the Russian River, California. At Cascade Head, mackerel, Pacific herring, skates (Raja spp.), cephalopods, spiny dogfish (Squalus acanthias), Pacific sand lance (Ammodytes hexapterus), rockfish, and northern anchovy were the primary prey, with 33% of the samples containing salmonid remains. In the Rogue River (Roffe and Mate 1984), lamprey were the dominant prey, with steelhead comprising 19% and chinook salmon 3% of the diet by weight. Hanson (1993) found California sea lions were efficient salmonid predators, with a catch rate (number of successful captures/number of attack attempts) of 74% at the mouth of the Russian River. Roffe and Mate (1984) calculated that sea lions consumed less than 1% of the spring chinook run and less than 5% of the summer steelhead run in the Rogue River. More research is needed to identify the relationship between salmonid run sizes, alternative prey populations, and the number of salmonids eaten by sea lions. Sea lion populations have increased dramatically since 1977/1978, when Roffe and Mate performed their research. It would be valuable to repeat their study.

In 1977, an estimated 100 harbor seals resided in the Columbia River estuary during May/June, although there are now over 900 (Fig. 1). These are probably minimum estimates because many adults migrate to adjacent estuaries during May/June to pup. Highest numbers in the Columbia River are usually in the fall, winter, and early spring, especially during the annual eulachon run.

Northwest harbor seal populations are growing about 7% per year, with no indications when they will reach OSP level. Only an estimated 2,000 harbor seals resided in Oregon waters in the 1920s, but about 10,100 were found in 1992. Washington's population was estimated to be 34,134 in 1993 and California's over 30,000 in 1994.

Generally, harbor seals do not feed on salmonids as frequently as California sea lions. In Puget Sound, harbor seals feed primarily on Pacific hake (Merluccius productus), Pacific tomcod (Microgadus proximus), walleye pollock (Theragra chalcogramma), flatfish, Pacific herring, shiner perch (Cymatogaster aggregata), plainfin midshipman (Porichthys notatus), and sculpins, with some predation on adult salmonids (steelhead, and pink (O. gorbuscha) and chinook salmon) in the fall.

In Washington outer coastal estuaries and in Oregon, harbor seals have been found to feed primarily on eulachon, northern anchovy, flatfish, crustaceans, smelt, and sculpin. The number of scats containing salmonid remains ranges from 1% to 50%, depending on area, season, and method of analysis (U.S. Dep. Commer. in prep.). In the Russian River, primary prey were flatfish, octopus (Octopus spp.), Pacific whiting, hagfish (Eptatretus spp.), plainfish midshipman, cusk-eels, smelt, Pacific herring, and perch. Harbor seals had a catch rate (number captured/number of attacks) of only 19%, much less than sea lions (Hanson 1993).

There are many known avian predators of juvenile salmonids, including cormorants (Phalacrocorax spp.), terns (Sterna spp.), brown pelicans (Pelecanus occidentalis), sooty shearwaters (Puffinus grisens), common murre (Uria aalge), mergansers (Mergus spp.), gulls (Larus spp.), belted kingfisher (Megaceryle alcyon), grebes, loons (Gavia spp.), and herons. Avian predators of adult salmonids include bald eagles (Haliaeetus leucocephalus) and osprey (Pandion haliaetus).

Two species, sooty shearwater and common murre, reside primarily outside estuaries but eat a considerable number of baitfish and some juvenile salmonids (fish that are often similar in size). In the 1970s, the sooty shearwater population residing during the fall off Oregon was estimated to be over 2 million and consumed about 27.2 million kg of prey (mostly squid and anchovy) (Wiens and Scott 1975). The common murre population along the Oregon coast is estimated to be about 700,000 birds (R. Lowe, U.S. Fish and Wildlife Service, Newport, OR, pers. commun.). The Oregon Institute of Marine Biology (1982) estimated that common murres consumed 60,000 smolts per day off Coos Bay. These birds concentrated their feeding on recently released hatchery salmonids at the mouth of the bay.

Cormorants are known to consume large numbers of migrating Atlantic salmon (Salmo salar) smolts (Kennedy and Greer 1988), significantly affecting local fish abundance (Birt et al. 1987). The double-crested cormorant (Phalacrocorax auritus) population off the Pacific coast has increased dramatically over the last decade (Table 2) and is estimated at 48,980 nesting pairs. Large population increases have been observed in British Columbia (1,560%) and Oregon (632%) since 1950 and 1979, respectively (Table 2). However, no data are presently available indicating that this species is concentrating its feeding on salmonid smolts and affecting any specific salmon runs.

The Pacific Coast population of Caspian terns (Sterna caspia) increased 70% from 1960 to 1981 (Gill and Mewaldt 1983) and is probably still increasing. They are known to eat shiner perch, sculpins, coho (O. kisutch) and chum salmon (O. keta), northern anchovy, smelt, rainbow trout (O. mykiss), and jacksmelt (Atherinopsis californiensis). Caspian terns inhabit northwestern estuaries from April to August, when adults typically arrive at northwest estuaries in April from southern U.S. Pacific Coast or Central America.

Presently, one of the largest nesting colonies of Caspian terns (about 3,000 nesting pairs) on the West Coast is on Rice Island in the upper Columbia River estuary, an island made from dredged material. This colony became established after 1980, apparently as a result of successful breeding and immigration from other areas. Although there are no Caspian tern food habit data available from this colony, research by Hinton et al. (1992a, 1992b) showed that juvenile salmonids are the dominant fish in this area. Therefore, it is likely that smolts are consumed by birds from this colony. Assuming a breeding population of about 6,000 birds, an average fish weight of 10 g, and some standard biological energy conversions (see Furness and Cooper 1982), I estimate that up to 11 million fish (an unknown number of which would be salmonids) are potentially consumed each season by this colony.

Although several freshwater studies have reported substantial salmonid predation in fresh water by fishes (see Poe et al. 1991), research by McCabe et al. (1983) and others indicates that large piscivorous fishes that feed on salmonids are not common in most West Coast estuaries. The primary fish predators in estuaries are probably adult salmonids or juvenile salmonids which emigrate at older and larger sizes than others. This would include cutthroat trout (O. clarki) or steelhead smolts preying on chum or pink salmon smolts.

Outside estuaries, many large piscivorous fish populations reside just offshore and may consume large numbers of smolts. These fishes include Pacific hake, Pacific mackeral (Scomber japonicus), lingcod (Ophiodon elongatus), spiny dogfish, various rockfish, and lamprey (Beamish et al. 1992, Pearcy 1992, Beamish and Neville 1995).

Inter- and intraspecific competition in estuaries is not well documented. Research on the feeding habits of juvenile salmonids in Northwest estuaries indicates a high diet overlap between different species (different salmonids and nonsalmonids) and between hatchery and wild salmonids. However, there are no conclusive data indicating that competition is occurring (Myers 1980, McCabe et al. 1983, Fisher and Pearcy in prep.). Information on salmonid growth rates and prey availability during different years, with and without hatchery influences, would be valuable.

Increased competition may result from the introduction of exotic species which can alter estuarine food webs. Numerous vertebrate and invertebrate species have been introduced into West Coast estuaries. San Francisco Bay in particular has an invertebrate fauna that is now dominated by exotic species that have altered the bay's food web. For example, the Asian clam, Potamocorbula amurensis, has reduced abundances of both phytoplankton and copepod populations (Kimmerer et al. 1994). The recent introduction of the freshwater Asian shrimp, Exopalaemon modestus, into the Columbia River (personal observation) could impact salmonid resources. If E. modestus becomes abundant and preys on and reduces the abundance of the Corophium spp. amphipods (the primary prey for juvenile salmonids), smolt growth and survival could be reduced. In general, exotic species can impact salmonid runs if they displace native populations but do not replace them as important salmonid prey.

American shad (Alosa sapidissima) were introduced along the West Coast in 1871, 1885, and 1886 (Craig and Hacker 1940) and have been identified as a potential estuarine competitor to salmonids. The largest shad run on the West Coast is in the Columbia River, which reached a peak of 4 million in 1990, declining to 2 million in 1994 (Fig. 2) (ODFW and WDFW 1995). Although the potential for competition between shad and salmonids exists, the migrations in estuaries do not coincide. Adult shad migrate into estuaries primarily in June-July and are not actively feeding. Juvenile shad migrate through the estuary during September-October when most juvenile salmonids have left estuaries.

Estuarine populations of many marine mammals and birds are increasing. These predators may eat a substantial number of salmonids, depending on location, timing, and salmonid and alternative prey abundance. Data on the feeding habits of these predators are critically needed.

Documentation of competition for food in estuaries is very limited. Given the status of many wild salmonid populations, research is needed to identify possible hatchery vs. wild smolt interactions in estuaries.

Introductions of exotic species threaten estuarine food webs. Since most exotic species are introduced by large ship ballast water, legislation forcing ships to off-load, treat, or exchange ballast water at sea, would reduce the risk of exotic species introductions.

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Hinton, S. A., R. L. Emmett, and G. T. McCabe, Jr. 1992b. Benthic invertebrate and sediment characteristics in subtidal habitat at Rice Island, Columbia River estuary, December 1991 and March 1992. Report to U.S. Army Corps of Engineers, Contract E96920018, 22 p. (Available from Northwest Fisheries Science Center, 2725 Montlake Blvd. E., Seattle, WA 98112-2097.)

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Table 1. Prevalence of marine mammal tooth and claw abrasions on chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) at Lower Granite Dam on the Snake River.*

*Data from Jerry Harmon, National Marine Fisheies Service, Northwest Fisheries Science Center, 2725 Montlake Blvd. E., Seattle, WA 98112-2097.

Table 2. Number and percent increase of breeding double-crested cormorants (Phalacrocorax auritus) in the Northwest (from Carter et al. 1995).

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