Occurrence patterns of fishes collected by net cages placed at surface and bottom layers along a seawall at the Furuhama Park in innermost Tokyo Bay

Abstract

Occurrence patterns of fishes were compared between net cages（0.5 × 0.4 × 0.4 m） with three internal materials（nylon net, bamboo shoot and oyster shell）placed at surface and bottom layers along a seawall at the Furuhama Park in innermost Tokyo Bay from April 2016 to March 2019. A total of 1,574 individuals of 17 species representing 9 families were collected, fishes collected at the surface numbering 1,189 individuals of 13 species（13.2 ind./cage）and those at the bottom 385 individuals of 11 species（4.2 ind./cage）.The dissolved oxygen was 6.2 ± 2.26（mean ± SD）mg/L and 4.2 ± 2.58 mg/L at the surface and bottom layers, respectively, and the respective times when the hypoxic oxygen water of 3.0 mg/L and lower appeared were 4 out of 88 cages and 33 of 89 cages at the surface and bottom layers. Three gobiids, Tridentiger obscurus, Chaenogobius gulosus and Tridentiger trigonocephalus, were dominant species occupying ca. 90% of the total individuals collected in this study; however, the first and third were collected by both the surface and bottom cages, but the second species occurred at the surface layer only. This study suggested a possibility that a certain structure created at the surface layer of a seawall would become a habitat for some fishes during the time when hypoxic waters arises.