Ultimate Sacrificeby Sharon Levy
New Scientist, September 6, 1997
SALMON were essential to the native people of the American Northwest. The fish were so important to their lives and culture that many of the region's Indians thought of themselves as salmon. People of the Tlingit tribe on the Alaskan coast adopted chinook, coho and chum salmon for their family crests. So it is fitting that a study of ancient Tlingit bones has changed our view of salmon ecology.
When graduate student Thomas Kline arrived at Iliamna Lake in southwest Alaska in the early 1980s, he was awestruck by the sight of thousands of sockeye salmon in mass migration. He was looking for a new way to assess the importance of these fish as a source of food and energy in their freshwater habitat. The key, it turned out, was a study by other researchers of the diets of prehistoric humans in this region. They had found that human bones from Tlingit archaeological sites contained chemical traces of the salmon the people had relied on, in the form of a mix of isotopes that could only have originated in the sea.
The elements carbon and nitrogen always exist as a mixture of isotopes. In the sea the heavy, stable isotopes nitrogen-15 and carbon-13 make up a larger proportion of the mix than they do in freshwater or on land. This is reflected in the isotopic mix of these elements in the flesh of migratory fish such as salmon. Salmon spend most of their adult lives at sea, and return to their native rivers to spawn. The fish don't eat during their spawning migration, so their tissues retain high levels of marine-derived nutrients-and the characteristic isotope ratios that go with them. Kline realised that by measuring the concentrations of these isotopes in other living things, he would be able to track nutrients from dead salmon as they moved into the food web.
"Isotopically speaking, you are what you eat," says Kline, who is now a researcher at the Prince William Sound Science Center in Cordova, Alaska. Any carnivore that eats a lot of salmon will raise the proportion of heavy nitrogen and carbon in its body. So will algae, aquatic insects and fish that take up nutrients released from decaying salmon carcasses. In 1985 Kline began to track nutrients as they moved from dead sockeye into the plants and animals of Iliamna Lake. He found that salmon play an important part in fuelling the lake's food web. They provide up to 90 per cent of the nitrogen in algae that live on the lake bottom, and up to 70 per cent of the nitrogen in plankton and juvenile sockeye.
Pacific salmon hatch out of eggs laid in the gravel of stream beds or lake shores. They migrate out to sea, where they spend most of their adult lives before returning to their freshwater birthplaces to spawn and die. The sockeye salmon that Kline studied migrate in dense groups and spawn in lakes. By contrast, coho spawn in steep headwater streams, where they disperse and defend mating territories.
Around the time that Kline began his studies in Alaska, Jeff Cederholm, a salmon biologist with the Washington Department of Natural Resources, became interested in the possibility that coho make an important contribution to the nutrients of their spawning streams. "High-latitude freshwater environments often lack levels of dissolved minerals, including nitrogen, that are necessary for the plant growth that forms the base of the food chain," Kline explains. Nevertheless, Cederholm's idea was not well received. "The prevailing belief was that coho carcasses would all wash out to sea, and the nutrient value wouldn't be retained," he says. Still, he knew from his own experience that the healthiest spawning streams were loaded with coho carcasses. And he was beginning to wonder if abundant carcasses might actually improve fish habitat. "Salmon are the only animals that return nutrients to the land from the sea," says Cederholm.
In 1984 and 1985 Cederholm took hundreds of coho carcasses from hatcheries, dropped them into streams and then tracked their fates. He found that in relatively undisturbed streams, most carcasses were retained-often caught on the same fallen logs that help create pools where young salmon feed and shelter. And from long hours of observation and carefully reading tracks, he found that a surprising array of creatures feed on the carcasses. Large animals like bears, raccoons and skunks often pull the fish onto the bank, where the leftovers are scavenged by shrews, mice and small birds. Coho spawn in the autumn and their bodies are there for wildlife to feed on in winter-the hungriest time of year in Pacific Northwest forests.
Cederholm's work provided evidence that coho play a crucial role in their freshwater habitats. Robert Bilby, an ecologist for the Weyerhaeuser Company, a timber and paper firm based in Tacoma, Washington, decided to apply Kline's analytical technique to assess the impact of coho on the food web in Washington streams. "With Kline's technique, we had a quantitative way to measure this," he says. Bilby traced heavy isotopes of nitrogen and carbon from coho as they were taken up by every part of the stream ecosystem-from the algae and bacteria that coat rocks on the stream bed to plants growing on the banks. He found that up to 30 per cent of the nitrogen and carbon that builds the bodies of algae and aquatic invertebrates comes from the sea via returning coho, and up to 18 per cent of the nitrogen in vegetation beside the streams is derived from marine sources.
Bilby's most dramatic results came from studies of juvenile coho. During critical phases of their growth, the young fish depend on the nutritive legacy of their dead parents. Between 25 and 40 per cent of the nitrogen and carbon in juvenile fish is derived from spawned-out adults. Young coho hatch in March or April, but do not return to the sea until the following spring. "Mortality rates of young smolts are very high," says Bilby, "but the larger young fish grow, the better their chances of surviving both the inland winter and life in the ocean." The nutrients provided by adult carcasses boost juvenile growth and survival in winter, when other food is scarce.
Bilby fears that coho may be caught in a vicious circle of decline (see "An upstream struggle"). As the number of returning adults decreases, so does a stream's ability to support growing juvenile fish. "There are no other sources to make up for missing nutrients from adult salmon, because we have unproductive, nutrient-poor streams," he says.
So last year fisheries managers in Oregon and Washington started putting spawned-out hatchery salmon into streams instead of dumping them in landfills. But this can only be a temporary fix, Bilby points out. "We need to allow as many adults as possible to return to spawn," he says. "Over several generations, this will build nutrient capital." Bilby and Cederholm are now working together, attempting to assess the numbers of returning adults that will be needed to keep the coho population of an entire watershed healthy. Their results may influence regulations governing coho harvests in Washington state.
Meanwhile, James Helfield, a graduate student at the University of Washington, is starting work on a project aimed at discovering how far the nutritive legacy of the salmon extends. He plans to explore the interactions of salmon, eagles, bears and riverbank vegetation, comparing growth rates and isotope ratios along streams with and without spawning salmon. To discover whether eagles and bears carry significant amounts of salmon-derived nutrients into the heart of northwestern forests, Helfield will count bear and eagle faeces and analyse their heavy-nitrogen content. "It's a dirty job, but someone's got to do it," he remarks. "If there is a link between salmon spawning and riparian growth, it could have some important implications for management and conservation of forests. Moreover, if other animals-like bears-play a role in this interaction, then spawning runs and river ecosystems may be affected by the loss of bear habitat."
Grizzly bears and bald eagles congregate at sites where migrating salmon are easily captured or where large numbers of salmon carcasses are available. There is evidence that changes in salmon distribution influence the reproductive success and migratory routes of eagles. Now isotope tracing is giving new insights into the complex relationship between salmon and their predators.
A team led by Charles Robbins and Grant Hilderbrand of Washington State University has studied captive black bears to discover how heavy isotopes accumulate in the animals' bodies. The researchers found raised levels of heavy isotopes in blood plasma taken from bears up to a week after they had eaten salmon. A longer-term record of diet is provided by the animals' fur, which grows from midsummer to late autumn, and so contains a ratio of heavy isotopes that reflects the amount of salmon consumed during that period. The team found that Alaskan grizzly bears trapped in the wild had low blood plasma levels of heavy isotopes in early summer-before the return of spawning salmon. But hair samples from the same bears contained high levels of the isotopes, indicating that they rely heavily on salmon for at least part of the year.
Robbins and Hilderbrand also studied museum specimens of grizzly bears from the Columbia River area, on the Oregon-Washington border-a population that has been extinct since 1931. Analysis of bones and hair from the long-dead animals revealed that up to 90 per cent of the carbon and nitrogen in their diets had come from salmon. "The surprising thing," says Robbins, "is that every single Columbia River bear we tested had consumed salmon. Even bears which are 700 to 800 miles from the ocean had isotope signatures similar to coastal bears." Grizzlies can live without salmon-all the surviving populations in the inland Rocky Mountains do. But taken together with the abundance of grizzlies described by early settlers along the Columbia River, Robbins and Hilderbrand's findings suggest that bears flourish where there is a healthy salmon population.
Hilderbrand is continuing his work with Alaskan grizzlies, trapping bears as they emerge from hibernation in spring, in midsummer before the salmon runs, and in autumn after they have fed heavily on salmon. He measures isotope signatures in blood and hair samples and collects information on changes in body weight and proportion of body fat in the bears. "Summer and fall food sources are extremely important to bears," says Chuck Schwartz, a wildlife biologist with the Alaska Department of Fish and Game who is working with Hilderbrand. "They need nutrient-rich food in large quantities to reproduce successfully and to prepare for hibernation." The salmon return just when the grizzlies need them most.
Though Alaska's salmon runs are still relatively healthy, Schwartz believes the state has already started down the road that has been travelled by California, Oregon and Washington. "We are having large-scale logging and increased pressures for agriculture and development," says Schwartz. "All of this affects the watersheds. Alaska is not heavily dammed, but the other problems salmon face in the contiguous US are starting to happen: logging, erosion and destruction of streamside vegetation." In addition, the salmon face significant pressure from fishing. On some of Alaska's more popular rivers, the anglers stand arm to arm along the banks during salmon runs. Schwartz hopes his work with Hilderbrand will produce hard data to show that bears need salmon. It is evidence that may help protect Alaskan salmon if their numbers start to dwindle in the future.
In Northwest Indian mythology, the salmon are a noble people who live in a huge house under the sea, where they take on human form. At the right time, they change into fish and run up the rivers, sacrificing themselves for the sake of humankind. We are just beginning to grasp the significance of the salmon's role in the ecosystems of the Pacific Northwest. It remains to be seen what sacrifices modern humans are prepared to make to keep the salmon running.
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An upstream struggle
IN THE 1940s, fewer than 8000 fishermen in Alaska were landing about 230 000 tonnes of salmon annually. By the mid-1960s, the number of fishermen had more than doubled, the number and length of their nets had tripled, but their total annual catch had dropped by more than half. Then the state began to manage salmon based on the number of returning adults rather than demand for fish. Today most runs of Alaskan salmon are healthy.
Runs in Washington, Oregon and California have not fared so well. An analysis of statistics from 1864 to 1979 suggests that salmon numbers have declined by more than 50 per cent since the arrival of Europeans in America. In addition to heavy fishing, loss of freshwater habitat has taken its toll: dam building, water diversion and pollution from agricultural runoff, logging and sewers all make life difficult for the fish. Many runs have disappeared completely, and others are rapidly dwindling.
The US National Marine Fisheries Service recently made a controversial decision to list coho salmon as a threatened species in California and southern Oregon. The listing was opposed by the hydropower industry, loggers and ranchers while environmentalists and fishermen say measures to protect salmon may be too little and too late.
It is clear that coho are in trouble. They are extinct in 55 per cent of their former range in California, Oregon and Washington. And though the overall catch of wild coho has been controlled in recent years, stocks have continued to decline, most likely due to loss and alteration of their freshwater habitat.
: The effect of salmon carcasses on Alaskan freshwaters by T. C. Kline, J. J. Goering and R. J. Piorkowski, in In freshwaters of Alaska: ecological syntheses, edited by A. M. Milner and M. W. Oswood, Springer-Verlag, New York, p. 179-204 (1997).
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