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A NUMBER of workers have investigated, in experimental animals, the effects of tranquillizing drugs on learned behaviour motivated by fear. John et al.1 have suggested that reserpine inhibits conditioned avoidance behaviour by interference with learned associations, whereas Weiskrantz2 believes that reserpine reduces sensory perception. Miller3 suggested that chlorpromazine reduces fear motivation and that this is independent of its sedative action or of an effect on motor co-ordination.

SUMMARY Recovery processes operating within a stressed ecosystem are dependent in part on natural avoidance behaviour or acclimatization of organisms to otherwise unfavourable conditions. Adaptation to an altered environment can be achieved through either genetic or non-genetic means. Five case histories are presented which illustrate in situ behaviour of fishes to stress conditions.