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箱山 洋

Competition for food among six clone amagos, Oncorhynchus masou macrostomus, was investigated in a laboratory experiment with different rates of food input but with the total food supply controlled. Whether and how the clumping of a food resource increases the equality of food sharing among competitors were examined. They shared foods more equally at a high input rate (1 food item per 1 sec) than at a low input rate (1food item per 10 sec). In the VTR analysis, foraging behaviour was decomposed into four steps: detection, approach, feeding and return to a deep position. Indevidual differences in any of these steps can produce inequality in food gain. The most important factor that enhanced the inequality for the slow input rate was individual differences in relative gain ability after approach, presumably due to stronger interference. Pecking behaviour was observed at the low input rate but not at high input rate. Two mechanisms reducing the inequality at high input rate were established: a) Fish approaches less likely after a successful capture of a food item presumably due to handling time. b) Interference interaction between fish at approach is reduced so that non-dominant individuals were less likely to avoid dominant fish.

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