Salmon dung and food pellets drift down, covering the seafloor faster than they can decompose. About 80 percent of the salmon feed stays in the water, either as uneaten pellets or sewage, said Chilean scientist Doris Soto, who studied the impact of salmon farms on Chilean lakes for nine years. Though fish farmers try to feed more efficiently, there will always be some waste. The waste is natural and the water can handle it, countered Canadian fish farm consultant Dave Conley. That's where they live,'' he said.
Environmentalists say the difference is numbers.
Salmon farms often have 180,000 to 250,000 fish at one site. Wild salmon don't stay in one place in such high densities, said Arthur Whitely, a founding member of the Marine Environmental Consortium in Seattle. The consortium convinced the Environmental Protection Agency to require waste permits for fish farms. At first, waste from a fish farm increases the amount of marine life below, as mollusks, crustaceans and other fish feed on the nutritious sediment. Studies found that as the sediment builds up, the number of animals below the nets increases, but the diversity diminishes until there are just hordes of worms, bacteria and fungi.