According to the Canadian Fisheries Act, it is against the law to introduce "deleterious substances" into fish-bearing waters. That seems simple enough: you're not allowed to harm fish.

But what, exactly, does "deleterious" mean in legal terms? How can you tell whether the run-off from a landfill or a mine site is deleterious? And how can you prove it in a court of law?

These questions are important to scientists in Environment Canada and the Department of Fisheries and Oceans, the two federal agencies responsible for enforcing the Fisheries Act.

The established way to determine whether effluent from an industrial site is deleterious to fish is to test it on living fish or other aquatic creatures, says Benoit Godin, head of Environment Canada's Environmental Contaminants Section in Whitehorse. The procedure is called a "bioassay".

The standard bioassay for assessments is called the 96-hour acute lethal static test. Rainbow trout are placed in water containing the material to be tested, or into the material itself, for 96 hours. The survival rate of these fish is compared to other fish in uncontaminated water, establishes whether the material is deleterious. If more than half of the fish in the material being tested die within 96 hours, it is considered to have failed the bioassay test.

Although rainbow trout are commonly used for freshwater tests, testing laboratories also use other organisms such as tiny creatures called Daphnia magna, or water fleas, and luminescent bacteria.

Because Daphnia individuals have a life cycle of only a few weeks, they can provide information about the material's effects on growth and reproduction as well as on the immediate health of the organism. "With the Daphnia, you look for immobility, because they move all the time when they are healthy," says Godin.

The luminescent bacteria offer another way of measuring how harmful a material is. The microscopic organisms give off measurable amounts of light. As individuals die, the amount of light given off by the group decreases. Laboratory instruments can measure how quickly the light fades and infer how harmful the material is.

Although the smaller organisms provide useful information, the preferred method is to test suspect material on fish, Godin says.

Because these test results might have to stand up to scrutiny in court, bioassays have to meet very strict standards, Godin says.

Any testing laboratory used for legal bioassays must be able to prove that its procedures are impeccable, and that its stock of fish and other creatures is healthy and suitable for testing purposes.

"There's a whole technology built up around providing suitable organisms for bioassays," says Godin. "And these stocks have to be kept healthy and uncontaminated. The laboratory will periodically calibrate its fish, for example."

Because the legal standards for bioassays or toxicity testing are so strict, Environment Canada's Pacific and Yukon Region has set up its own testing laboratory in North Vancouver.

"We would use other labs for research or exploratory work," says Godin, "but if it's got legal implications we would only use our lab. It has been instrumental in many Fisheries Act prosecutions."

Not every bioassay of material from an industrial site goes through Environment Canada's laboratory. Companies or agencies operating under a water licence are generally required to test their own discharged material on a regular basis. Those tests are carried out by a private laboratory at the company's expense, and the results are submitted to the Yukon Territory Water Board, which then passes them on to all interested agencies, Godin says.

Environment Canada or the Department of Fisheries and Oceans usually undertakes its own tests only if the regular tests indicate a problem, if there is a spill or some other kind of emergency, or as part of a regular audit of industry practices.

For more information, contact the Environmental Protection Branch of Environment Canada, Whitehorse.