Rarely Gasterosteus aculeatus populations occur where many individuals have lost their protective dorsal spines, ventral spines, pelvic girdle and lateral bony plates. Reduction in predation pressure is currently viewed as the causative factor. On the lsle of North Uist (Outer Hebrides) 10 spine‐deficient G. aculeatus populations, sympatric with various fish and bird predators are described. All of the spine‐deficient morph sticklebacks are found in acidic oligotrophic peat moorland lakes and pools where Ca2+ is ≤ 2.5 mg/1. It is proposed that calcium deficiency may have been an important selective agency in the evolution of skeletal reduction in these unusual populations.

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Pike fry were kept in 0,054-m3 tanks at densities of 50, 100 and 150 (277, 555, 833 fry m-') for seven weeks during which the development of social and feeding behaviour was observed. Zooplankton, macro-invertebrates and perch fry were provided sequentially as food; the pike fry were allowed to feed ad libitum.Zooplanktivorous fry stopped growing at 22 mm whilst, in the presence of abundant suitable alternative prey, 14% turned cannibalistic at 5 weeks of age; cannibals subsequently grew rapidly (mean 1.88 mm day-'). Cannibalism ensued in all tanks when the ratio of predator size: prey size was r.2: 1.Fry tended to space-out evenly in the tanks with no overt aggression or territoriality. Behaviour was typified by remaining still for long periods, particularly subsequent to the onset of cannibalism. Cannibals were attracted by fry movements which often initiated attacks.Daily per capita mortality rates showed no crowding effects before cannibalism but significant density-dependent mortality due to cannibals. Consumption rates of cannibals varied between 0.63 and 6.0 fry per cannibal per day. Cannibals accounted for 5496% of daily mortality in the experimental tanks. These results are discussed in relation to proposed mechanisms of pike population density regulation.

After varying periods of food deprivation the foraging behaviour of parasitized and nonparasitized sticklebacks was measured in both undisturbed and recently frightened fish. Parasitized sticklcbacks forage significantly more actively and recover more quickly after a frightening stimulus than d o control fish. They also react more quickly to food deprivation than do uninfected control fish. Seventy-two hours without food is sufficient to suppress the fright response in parasitized fish. and causes them to forage at the same rate as when undisturbed. Non-infected controls failed to forage successfully after a frightening stimulus. even when they had been without food for 96 h. Frightening stimuli can affect profoundly the normal foraging behaviour of hungry fish.