Influences of illumination intensity on the nearest neighbour distance in coho salmon Oncorhynchus kisutch

Effects of illumination intensity on the nearest neighbour distance (NND) of coho salmon in an experimental aquarium were studied under illumination intensities ranging from 0 to 4000 Ix. The NNDs in sighted (intact) fish were divided into three groups according to the illumination intensities; the largest [1.15 times fork length (f.l.)] at 0 1x, the medium (0.78 to 0.84 f.l.) at 0.01 to 0.4 1x, and the least (0.60 to 0.66 f.l.) at 4–4000 1x. The fact that NNDs under 4 1x or higher illumination conditions were significantly smaller than those under 0.01 to 0.4 1x conditions suggests a change from photopic to scotopic vision. Blinded fish exhibited constant NNDs irrespective of illumination intensity and the value was similar to that of sighted fish under the 0 1x condition. The NNDs of blinded fish and sighted fish exposed to 0 1x were larger than the simulated NND calculated by hypothesizing that fish distribute randomly in the experimental aquarium. These results indicate that NND in a coho school is maintained primarily by vision, however, the NND is not determined by vision alone but by integrated senses.