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SUMMARY. 1. The life cycle of salmonid fishes is described.

2. The performance and environmental requirements of the various life stages of the trout (Salmo trutta L.) are reviewed, (a) The literature gives predictive relationships between water temperature and rate of embryonic development, food requirements and growth rate, (b) Water temperature, intragravel oxygen supply rate, water pH, the occurrence of mechanical shock, disturbance of spawning gravels, sedimentation and water chemistry can all influence the survival of the intragravel stages, (c) The survival and/or well-being of the free-swimming stages and the success of spawning are influenced by such factors as dissolved oxygen concentration, pH, water depth, water velocity and water chemistry.

3. Human activities such as impoundment, river transfer, drainage works, land improvement, afforestation and deforestation can all influence trout populations via changes in flow regime (and related effects such as sedimentation), temperature regime and water chemistry.

4. Man can also influence trout populations directly by cropping for food and/or sport and by artificial stocking.

5. Examples of practical application of present knowledge are given and some future research needs are listed.