Mathematical modeling of preference intensity of Japanese Medaka for environmental factors in streamflow

The preference intensities of Japanese Medaka Fish (Japanese killifish, Oryzias latipes) to the three environmental factors of water depth, current velocity and cover were discussed in this paper. Mathematical models for preference intensities of Japanese Medaka to the three environmental factors were constructed in laboratory open-channel experiments. A simple genetic algorithm was newly introduced to search for the optimal functional representation of preference intensity. The models were then verified by laboratory water-tank experiments and on-the-spot examinations. The results indicated that the maximum preference level of Japanese Medaka was determined at a water depth of 9.4 cm and at a current velocity of 2.8 cm/s, and that the All-cover condition had a markedly high level compared with other cover conditions. The relative weights of the three environmental factors were evaluated as 0.32 for depth, 1.0 for velocity and 0.66 for cover, when the maximum weight was normalized to be unity. The weight values suggested that the environmental preferences of Japanese Medaka were not greatly affected by water depth. Judging from the good agreement between the predicted and observed distributions in considering the two factors of current velocity and cover, the two factors obviously affected the environmental preferences of Japanese Medaka.