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SUMMARY. The seasonal variation in population density of Gammarus pulex was studied in a Dorset chalk stream. The numbers increased markedly in June and July and reached a maximum of c. 10000m−2 in September whilst the most rapid decline in density occurred in October‐November and reached a minimum of 820 m−2 in February. The animals occurred in greater densities in habitats containing Ranunculus or Callitriche than in those devoid of vegetation. The population structure was determined monthly and was split into juveniles (length <4mm), immature males, immature females, mature males and ovigerous females. The percentage of juveniles (39–76) was always the highest of any of the categories. Ovigerous females were found at all times of the year. The sex ratio varied with the time of year both for immatures and matures, although there was approximately a 1:1 ratio for the mature individuals. Seasonal variation in biomass showed a maximum of 7.l g dry wt m−2 in September and a minimum of 1.4 g dry wt m−2 in March. Production was calculated by two methods giving values of 12.9 g dry wt m−2 year−1 and 12.8 g dry wt m−2 year−1.

Atlantic salmon, Salmo salar L., smolts are vulnerable to entrainment in a variety of man‐made structures as they migrate downstream. The ability of acoustic bubble screens to deflect smolts from potential hazards was assessed. Screens were deployed, in turn, across one of the two identical channels through which the millstream of the River Frome flowed and the efficiency of these screen systems was tested by counting smolts by video recording in each channel. It was concluded that these screens deflected a significant number of smolts. Efficiencies were greater at night than in daylight because of smolt behaviour.

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Stone loach and bullhead were given a choice of Chironomus and Asellus prey in experiments using solitary fish and fish in company . Solitary fish ate more than fish in company . The effect of light and substratum type on feeding was investigated . Both species ate more prey items on gravel than silt when a significant difference was observed . Bullhead ate more than loach in the light on both substrata. The only experiment in which loach ate more than bullhead was on a silt substratum in the dark . It is concluded that these two species utilise different components of the available food resources in chalk streams by adopting different habitats.