Visual Perception of Density and Density-Dependent Growth in Medaka (Oryzias latipes): A Suitable Model for Studying Density Effects in Fish

High stocking densities have negative effects on fish. However, the mechanism mediating density perception and growth inhibition is still unknown. This study was conducted to confirm the occurrence of growth inhibition and evaluate changes in growth-related factors in fish reared under high-stocking-density conditions and to determine the role of vision in density perception of medaka. In the graduated-stocking experiment, growth inhibition was clearly observed in fish reared at higher densities, although environmental factors, such as water quality, dissolved oxygen, and feeding conditions, were the same in each experimental group. Differences in growth were observed between the 6-fish and 8-fish groups, indicating that medaka have a superior sense that allows them to accurately perceive the number of individuals in their surroundings. In the pseudo-high stocking experiment, the inner 2-L tank in both groups contained six fish; however, the outer 3-L tank in the pseudo group contained several fish, while that of the control group contained only water. Growth inhibition was observed among the fish in the inner tank of the pseudo group despite having similar spatial density with the control group. These findings suggest that vision is important for density perception. The gene expression of growth-related and metabolic-regulatory hormones decreased in the high-density group. Furthermore, neuropeptide Y expression increased, while pro-opiomelanocortin expression decreased in the high-density group. This study is the first to report that fish can visually perceive density and the resulting growth inhibition, and concluded that medaka is a suitable model for studying density effects and perception in fish.