ABSTRACTFish farms are continuing to grow in numbers and in size. They cause concern because of their location in areas of high‐quality water, frequently in the headwaters where there is little dilution for large volumes of effluent. This jeopardizes the water quality, and may affect the ecology of the river ‐ migratory fish in particular.The use of chemicals for the treatment of disease is causing concern, particularly if the river is used for potable abstraction. Little information is available on the low‐level effects and the detection of chemicals such as antibiotics and hormones. The chemicals are not controlled nationally, the only control being through consents.With the continued growth of the fish‐farm industry, problems are likely to increase in the future unless a responsible attitude to their development is