Small species, such as fathead minnows and waterfleas, can be used to uncover the quantity of toxic chemicals being dumped into rivers and streams  Under new rules issued in a certain country, these indicator species will be used to test the effluent pouring out of sewage treatment plants and factories among the country' s waterways  If many of these animals die as a result of the test, the country is going to require that pollution levels be reduced
The new rules will apply to about 450 of the 2, 000 factories and sewage treatment plants that legally discharge into the country' s rivers and streams
If, after the test, the country does not require that pollution levels be reduced, then few or none of the individuals of the indicator species died as a result of the test.