Effect of natural light conditions on the use of cover in concrete block structures by Japanese dace Tribolodon hakonensis

Abstract

The effects of light intensity on the use of cover by Japanese dace, Tribolodon hakonensis, under natural and experimental conditions were examined. Concrete blocks were placed to provide cover in a study pool and the numbers of dace in open water and under cover were recorded. During mid-afternoon, the number of dace in open water decreased while the number under the block cover increased. An experimental tank was also prepared, one part of which was exposed to direct sunlight (5000–50 000 lx) and the other part was screened to allow moderate light intensity (shade rate: 27% or 93%). The dace were able to select either part. In higher daylight intensity, the dace were induced to stay longer in the shaded part. These results show that the dace use cover as a shelter with lower light intensity as compared to direct sunlight. This study demonstrates that the preparation of lower light intensity shelters by providing additional cover is an important preservation technique in the freshwater ecosystem.