A conservation officer told me that some fish, such as gaspereau (Alewives) migrate in huge numbers. Because of various stresses, it's not abnormal to see dead ones. Because there are so many of them, they document but do not investigate many of these deaths, unless citizens alert them or the media get involved. It is considered a 'road kill' type of kill off and they commonly label the deaths as tramua. However, if more prized fish, like salmon, of fish that exist in small numbers die, they most always send them to a labratory. In most cases the cause of the death is never found (samples are frequently of low quality).

He indicated there are a number of lab analysis that can be carried out. Some, especially tests for toxins, can be quite expensive. So, the expensive tests are done infrequently...more often is a polluter is known and suspected as the cause, and they can get a good court case.