Experimental Study on the Fish Preference for Hydraulic Environments

Abstract

The preference intensities of river fish to environmental factors were discussed in this paper. Thethree environmental factors of water depth, current velocity and cover were selected as the principal factorsaffecting the availability and quality of the fish habitat in natural rivers. Japanese Medaka Fish (Japanesekillifish, Oryzias latipes) was selected as a river fish subject to quantify preference intensities becauseJapanese Medaka is now regarded as the symbolic fish for the restoration of countryside ecosystems in Japan.<br><br>Mathematical models for preference intensities of Japanese Medaka to the three environmental factors wereconstructed in laboratory open-channel experiments. A simple genetic algorithm was newly introduced tosearch for the optimal functional representation of preference intensity and to determine parameter values ofthe preference intensity models. The results indicated that the maximum preference level of Japanese Medakawas determined at a water depth of 9.4 cm and at a current velocity of 2.8 cm/s, and that the All-covercondition had a markedly high level compared with other cover conditions. The relative weights necessary forcalculating the combined preference level 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 valuessuggested that the current velocity has the strongest preference intensity among the three environmentalfactors and that the environmental preferences of Japanese Medaka were not greatly affected by water depth.