When the air fills with dust, we react by coughing. Oddly enough, young salmon do the same thing when the water fills with sediment.

Scientists studying the effects of sediment on young coho salmon in the Fraser River used blood sugar levels and the frequency of coughing to measure the amount of stress the sediment caused in the fish.

Extra stress is only one of the effects sediment can have on fish, says Ian Birtwell of the Department of Fisheries and Oceans in Vancouver. Too much sediment in streams can harm or even kill fish and other aquatic creatures.

Varying levels of sediment occur naturally in streams, usually related to fluctuations in water flows and levels, he says.

"Aquatic organisms are subjected to these natural variations, and have obviously adapted their life cycle to accommodate them, " says Birtwell.

However, fish and other aquatic organisms can suffer if sediment levels increase beyond what they have adapted to.

Some of the effects of sediment are direct. In searching scientific literature, Birtwell has found two reports of fish kills associated with the release of water from dams.

In 1970, the opening of a sluice gate in a dam on the Shuswap River in British Columbia resulted in the huge increase in sediment downstream. Dead whitefish were found with their gills completely clogged with mucus and sediment.

About 20 years later, a reservoir was drained on another river, releasing water with a high sediment concentration. The incident caused fish kills for 3.5 kilometres downstream.

Just as seriously, it reduced the population of benthic invertebrates for over 200 kilometres downstream. Benthic invertebrates, insects and other tiny creatures that live in and on the stream bottom, are a major food source for other animals. In this case, it took two months for the invertebrate population to recover.

Increased sediment in a stream can affect fish in other ways. The more sediment in the water, the less light penetrates it. Many fish hunt their prey by sight, so reduced visibility makes it harder for them to find food.

Of course, some fish are both predators and prey. In sediment-laden water, where they can't be seen easily, juvenile salmon are more likely to escape larger fish that prey on them.

However, as the amount of sediment in the water increases, the advantage of shelter is not enough to offset the difficulty the young fish experience in finding food. In fact, some studies have shown that at higher concentrations of sediment young coho salmon reduce their feeding or stop altogether.

Salmon eggs are especially susceptible to sediment. If fine sediment enters a salmon spawning stream and settles on and around the eggs, many of them will never hatch. The sediment can also make the stream bottom unsuitable for spawning in future years.

Some of the more recent research on the effects of sediment on salmon eggs comes from the Yukon, Birtwell says. The research was part of the review of regulations for placer mining in the territory.

In one Yukon study, salmon eggs exposed to 10 percent fine sediments had a survival rate ranging from 86 percent to 91 percent. When the sediment level was increased to 40 percent, the egg survival rate plummeted to 35 percent.

For more information about the effects of sediment and regulations controlling it, contact Environmental Protection, Environment Canada, Whitehorse.