Abstract
Movies of schooling of Gnathopogon elongatus elongatus and small juvenile Mugil cephalus in a tandk were analyzed to examine the distance-dependent behavior which is a principle of the organiza-tion of fish schools. Dynamic properties were determined in the fluctuation of the nearest neighbor distance by using power spectrum information.
Power spectra for the two species had a pattern analogous to each other; the spectral density was greater and flat at the lower frequencies, and decreased approximately in proportion to minus second power of frequency at the higher frequencies. When schools were stable and organized normally, the observed spectra corresponded relatively well to the theoretical results from the assumption of the first order system where a random force was input.
Such a property of the fluctuation of the nearest neighbor distance was consistent in spite of the differences in average distances and directions to the nearest neighbor among species and school sizes. In the two-fish school of Gnathopogon, however, the value of the time constant was greater than in the eight-fish schools.
