快猫短视频

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鈥檚
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鈥檛 eat during their spawning migration, so
their tissues retain high levels of marine-derived nutrients鈥攁nd 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.

鈥淚sotopically speaking, you are what you eat,鈥� 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鈥檚 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. 鈥淗igh-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,鈥� Kline explains.
Nevertheless, Cederholm鈥檚 idea was not well received. 鈥淭he prevailing belief was
that coho carcasses would all wash out to sea, and the nutrient value wouldn鈥檛
be retained,鈥� 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.
鈥淪almon are the only animals that return nutrients to the land from the sea,鈥�
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鈥攐ften 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鈥攖he
hungriest time of year in Pacific Northwest forests.

Cederholm鈥檚 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鈥檚
analytical technique to assess the impact of coho on the food web in Washington
streams. 鈥淲ith Kline鈥檚 technique, we had a quantitative way to measure this,鈥� he
says. Bilby traced heavy isotopes of nitrogen and carbon from coho as they were
taken up by every part of the stream ecosystem鈥攆rom 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鈥檚 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. 鈥淢ortality rates
of young smolts are very high,鈥� says Bilby, 鈥渂ut the larger young fish grow, the
better their chances of surviving both the inland winter and life in the ocean.鈥�
The nutrients provided by adult carcasses boost juvenile growth and survival in
winter, when other food is scarce.

Vicious circle

Bilby fears that coho may be caught in a vicious circle of decline
(see 鈥淎n upstream struggle鈥�).
As the number of returning adults decreases, so does a
stream鈥檚 ability to support growing juvenile fish. 鈥淭here are no other sources
to make up for missing nutrients from adult salmon, because we have
unproductive, nutrient-poor streams,鈥� 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. 鈥淲e need to allow as
many adults as possible to return to spawn,鈥� he says. 鈥淥ver several generations,
this will build nutrient capital.鈥� 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. 鈥淚t鈥檚 a dirty job, but
someone鈥檚 got to do it,鈥� he remarks. 鈥淚f 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鈥攍ike
bears鈥攑lay a role in this interaction, then spawning runs and river
ecosystems may be affected by the loss of bear habitat.鈥�

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鈥� 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鈥� 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鈥攂efore 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鈥攁 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. 鈥淭he surprising thing,鈥� says Robbins, 鈥渋s 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.鈥� Grizzlies can live without salmon鈥攁ll 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鈥檚 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. 鈥淪ummer and fall food sources are extremely
important to bears,鈥� says Chuck Schwartz, a wildlife biologist with the Alaska
Department of Fish and Game who is working with Hilderbrand. 鈥淭hey need
nutrient-rich food in large quantities to reproduce successfully and to prepare
for hibernation.鈥� The salmon return just when the grizzlies need them most.

Arm-to-arm anglers

Though Alaska鈥檚 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. 鈥淲e are having large-scale logging and
increased pressures for agriculture and development,鈥� says Schwartz. 鈥淎ll 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.鈥� In addition, the salmon face
significant pressure from fishing. On some of Alaska鈥檚 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鈥檚 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.

* * *

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.