|
...Continued
from top
And the chinook noses out the scent of the native stream it left years ago!
But over the last century, humans have built massive dams that have transformed the Columbia River Basin. Dams help churn out the bulk of the region's electricity, prevent floods, create artificial lakes for boating and fishing, and irrigate land for Idaho potatoes and Washington apples, among other crops. But dams invariably disrupt the natural migration patterns of Pacific salmon, and since the completion of more dams in the 1960s and '70s, salmon populations have steeply declined--some species are already extinct. Now a heated debate rages: Should dams be busted, or partly torn down, to meet the needs of salmon? Or do human needs outweigh any concern for saving salmon?
Q: WHAT'S SO IMPORTANT ABOUT SALMON?
Salmon are a valuable and healthy food source for millions of people, providing protein, essential fatty acids, and numerous vitamins and minerals. And they also play a vital role in the Pacific Northwest ecosystem. When bears eat salmon, for example, they pass nutrients from salmon into the forest via scat (feces) and urine. Riparian (streamside) forest ecologist Jim Helfield at the University of Washington tested bear scat samples and tree cores to identify ocean and soil nutrients near Alaskan streams. Turns out, nutrients from salmon (including nitrogen from ocean sources) passed via bear poop fertilize trees near streams--in fact, trees near salmon streams grow up to three times faster than trees along streams without salmon! "Salmon and bears help ensure the health of streamside forests," says Helfield.
Q: AREN'T MOST DAMS SALMON-FRIENDLY?
When a salmon confronts a towering dam, the fish seeks a flow path around it. If no such path exists? The salmon may spend the rest of its days swimming haplessly at the dam's base. Most Columbia and Snake River dams feature fish ladders, a series of steps and pools that let adult salmon climb gradually up and over the dams (see diagram, "HOW SALMON SURVIVE DAMS"). But some salmon have trouble finding a fish-ladder entrance. Others reach the top only to reel back from the gush of dam water, then exhaust themselves climbing again. Mostly, fish ladders have proved successful in allowing adult salmon to migrate past dams, but don't provide a downstream path for migrating juveniles.
Behind dams, lake fishes often thrive while river fishes like salmon suffer. Some lake fish prey on smolts that swim slowly in reservoirs--they're not drawn along quickly enough by a river current. Smolts also have to battle dam turbines, rotors or blades that generate electricity when turned by rushing hydroelectric dam water, which can shred young salmon. Other smolts spill over a dam's rim and get gas-bubble disease (similar to "the bends" that scuba divers get when they surface too quickly). When falling water becomes supersaturated with air, an excess of dissolved nitrogen gas is absorbed into a smolt's bloodstream, where it forms bubbles that block circulation and kill the fish.
Q: SHOULD DAMS BE BUSTED?
After four lower Snake River dams were completed in the '70s, the number of returning adult chinook salmon plummeted from around 58,000 adults to around 23,000 between 1974 and 2000. "We need to take down the four lower Snake River dams," says media director Melissa Pease of Save Our Wild Salmon in Seattle. One proposed strategy to restore salmon populations: Breaching, or removing the dam's earthen portion--allowing a river to flow around the concrete structure. Supporters claim these dams provide less than 5 percent of the region's electric power, as well as no flood control or irrigation water. Breaching is also much cheaper than removing an entire dam.
But opponents argue that breaching dams won't help save salmon, because their population decline is caused by other factors: unfavorable ocean conditions, spawning-habitat destruction, and overfishing. "Salmon runs in areas without dams have seen the same salmon declines," says Tom Flint, chairman of the Save Our Dams coalition. "Dams do have an impact on salmon, but it's very, very small." Opponents also maintain that dam-busting could severely impact the region's economy by eliminating a plentiful source of cheap electricity and of transporting crops downstream.
Q: WHAT'S BEING DONE TO HELP SALMON?
In the last three decades, the National Marine Fisheries Service has implemented an unusual tactic to save salmon: driving smolts around dams! Workers load them on trucks or barges and motor them past Bonneville Dam, the last dam on the Columbia. While the free ride has improved smolt survival, migrating adult salmon numbers remained mysteriously low until the '90s. The reason? The stress of diversion, collection, and transportation may increase the smolts' susceptibility to disease and predation in the sea.
Another possibility: Pacific Decadal Oscillation, a 20-year-cycle in which the Pacific Ocean warms and cools. When the ocean is colder, salmon thrive; when it's warm they languish. The Pacific entered a cool phase in the late '90s, and in 2001 the number of returning chinook to the Snake River skyrocketed to more than 190,000 from a 1995 low of about 3,000. "Returning salmon runs are the largest they've been since 1938--when the first dam was built on the Columbia River," says Tom Flint. However, most returning salmon are produced in hatcheries or fish farms, and wild salmon still hover on extinction. For wild salmon to survive, they need human help fast.
WHAT DO YOU THINK?
Should dams be busted? Why or why not? Check out these Web sites to help form your opinion:
./ps/ /flyfisherman/mfarnell/
THE DALLES DAM features fish paths, but altered the Columbia River's natural downstream surge and salmon's migratory patterns.
BEAR CHOW: Nutrients from salmon pass through bear poop to fertilize soil--part of the balance of the Pacific Northwest ecosystem.
SILVERY SOCKEYE salmon, one of six Pacific species, turn red before they spawn, or lay eggs. Only a few eggs of thousands escape predators to become adults.
HOW SALMON SURVIVE DAMS
An alternative to the current system would be the removal of earthen portions of dams, restoring the river's natural flow.
Re-routing Nature's Flow
Dams on the lower Snake and Columbia rivers interrupt the natural migrations of Pacific salmon and are considered a major factor in their declining numbers. Officials are debating alternatives to the present system for routing the fish around the dams.
A. Downstream
THE YOUNG GO TO SEA
1 SPILLWAYS
Some juvenile fish bypass dams through spillways built into the dams. One concern: The large volume of water spilling through these gates can produce excessive amounts of gases harmful to fish.
2 TURBINES
Others go through turbines, where high pressure and whirring blades kill up to 10 percent of smolts. Some of those that make it through end up disoriented and vulnerable to predators.
3 BYPASS SYSTEM ...
Large screens prevent fish from being drawn into the turbine intakes and direct them upward into a bypass channel. Some fish re-enter the river on the downstream side.
... AND TRANSPORT
At some dams, fish are transferred to barges or trucks that transport them past the Bonneville Dam, the last dam interrupting their migration to sea waters.
B Upstream
THE ADULTS' RETURN TRIP
Adult fish swimming upstream to spawn pass the dams through an artificial set of rapids and pools constructed of concrete. About 95 percent complete the journey, but how many live to reproduce is in dispute.
A FISHY LIFE-CYCLE
Natural Propagation ...
Spawning females deposit eggs in gravel nests in streams, where the eggs are fertilized by adult males. Males and females then die. Some salmon species do not die after spawning ...
Eggs hatch in streams and young fish live there until ready to migrate ...
Hatchery Propagation ...
Eggs from adult females are fertilized with sperm from multiple males to ensure genetic diversity. Eggs and young are cared for at the hatchery; smolts are released into streams ...
As smolts travel from streams to main rivers and on to the ocean, some must pass dams and all must dodge predators ...
Smolts enter the ocean ...
Survivors grow to maturity at sea ...
Ocean and river harvests reduce the numbers of returning salmon ...
Adult fish enter the rivers and move upstream toward spawning areas ...
On the way, they climb ladders at dams ...
Changes in body color and form occur ...
Did You Know?
* Chinook are the largest and least abundant Pacific salmon species.
* At maturity, chinook are generally 50 centimeters (20 inches) to 1 meter (39 in.) long and weigh around 7 to 14 kilograms (15 to 30 pounds). But some tip the scales at up to 27 kg (60 lbs).
* The National Marine Fisheries Service has calculated the risk at 50 percent that all Snake River runs of spring and summer chinook will be extinct in 100 years.
Cross-Curricular Connection
Health: Research and compare the nutritional value of different types of fish, including salmon.
[ILLUSTRATION OMITTED]
National Science Education Standards
Grades 5-8: structure and function in living systems * reproduction and heredity * regulation and behavior * populations and ecosystems * risks and benefit
Grades 9-12: interdependence of organisms * behavior of organisms * natural resources * natural and human-induced hazards
Resources
For more information and activities, visit the University of Washington School of Fisheries' Salmon in the Classroom Web site: ./sic/ Visit the following sites to learn about two opposing views. For saving salmon: .org For saving dams: .com
CHECK FOR UNDERSTANDING
Directions: Match the word in the left column with the correct phrase
in the right column.
1. spawn a. scent cells
2. turbines b. natural warming and cooling ocean
cycle
3. olfactory pits c. dam's hydroelectricity-producing
blades
4. olfactory receptors d. fish farm
5. hatcheries e. leave eggs behind to hatch
6. Pacific Decadal Oscillation f. juvenile salmon
7. smolt g. nostrils not connected to throat
8. alevin h. newly hatched salmon
ANSWER
1. e 2. c 3. g 4. a 5. d 6. b 7. f 8. h
|