BIOLOGICAL EFFECTS ON FISH.
Pollutants might effect a given population without being lethal to adult organisms in many ways. i) Migration: Mechanism used for orientation and navigation by migrating organisms is not well known, but in some cases chemo toxicants clearly plays an important role. Sub-lethal concentration of pollutants may interfere with the normal migration pattern of organisms thereby change the composition of population or species diversity. Salmon, trout and many other fishes have been excluded from their home streams by pollution, though it is not known whether the reason is that a chemical cue has been masked or because the general chemical environment of pollution is offensive to the fish .On the other hand, heavy siltation and flow of heated coolant water may block migratory channels and long distance migratory fishes during some phases of their life history may be adversely affected by highly localized pollution of river. ii) Incidence of diseases: A long-term exposure of sub lethal concentration of pollutants may make an organism more susceptible to a disease. .
It is possible that some organic pollutants will provide an environment suitable for the development of disease producing bacterial and viruses .In such case, even though the pollutant is not directly toxic to the adult organism it could stillhave a profound effect on the population of the species over a longer period .iii) Behaviour: Much of the day-to-day behaviour of a species may also be mediated by means of chemo toxic responses. The finding and capture of food and the search for a mate during the breeding season are included in this category of activity, and again any pollutant interfering with.
6the chemo receptors of the organism would interfere with the behavioural patterns essential to a survival of the population.iv) Physiological Processes: Pollutant may interfere with various physiological processes without necessarily causing death, which may interfere in the survival of a species.