Abstract.Directly counted bacteria increased in abundance in water passing through a trout farm in north‐east England and were not removed by a settling lagoon. Total bacteria in the effluent were such that they might potentially bring about immediate increase in the bacterioplankton populations of recipient rivers. Directly counted bacteria increased cumulatively through rows of well‐stocked fish ponds arranged in series, thus all ponds contributed to the increase in suspended bacteria. In contrast, bacterial numbers varied irregularly through rows of low‐density stocked and fallow ponds. The increase in bacteria in well‐stocked ponds was greatest shortly after the daily feedi