The shocking collapse of one of the West Coast's biggest wild salmon runs has prompted even cash-trapped fishermen to call for an unprecedented   shutdown of salmon fishing off the coasts of California and Oregon. The Sacramento River salmon run is usually one of the most productive on the Pacific Coast,providing the bulk of the salmon caught by sport and commercial trollers off California and Oregon. But only about 90,000 adult salmon returned to the Central Valley last fall--the second lowest number on record and well below the number needed to maintain a healthy fishery.That number is projected to fall to a record low of58.000 this year. The council,which regulates Pacific Coast fisheries,will choose between three management options:a total ban on salmon fishing off the coast of California and Oregon;extremely limited fishing in select areas;or catch-and-release fishing for scientific research. For consumers,it will be hard to find any Chinook,also known as king salmon.which are prized by trollers,seafood critics and upscale restaurants.There should still be abundant supplies of farm-raised salmon.but prices could be higher. Biologists and others are trying to figure out what caused the salmon collapse so they can make sure California's Chinook populations rebound. There are many potential factors,because wild salmon are born in streams and rivers, migrate to the ocean when they're juveniles and spend two to four years there before returning to spawn in the areas where they were born.In between they have to navigate the often dangerous waters of the Sacramento Delta and San Francisco Bay. The council has asked state and federal scientists to research possible causes, including water diversions,habitat destruction,dam operations,agricultural pollution,and ocean conditions.Many scientists point out that unusual weather patterns disrupted the marine food chain along the Pacific Coast in 2005,when thousands of seabirds washed up dead or starving because they couldn't find enough to eat. Researchers believe those poor ocean conditions also devastated the juvenile salmon that would have returned to the Central Valley last year.Young Chinook couldn't find the tiny shrimp and fish they depend onto survive. The passage is mainly about----.
the agricultural pollution soils the coast    , the declining number of salmon could bring fishing ban the disrupted food chain in the Pacific Coast the fishermen could make little profit of salmon