To evaluate the habitat function for fish of an artificial sandy beach constructed in the inner Tokyo Bay, occurrence patterns of Plecoglossus altivelis altivelis were examined based on samples collected monthly at both low and high tide from March 2018 to February 2019, using a small seine net in the sandy beach of Furuhama Park, Ota City, Tokyo. The number and mean body length（± SD）of specimens collected were 1,817 and 20.6 mm（5.4 mm）at high tide and 218 and 17.4（3.5 mm）at low tide, respectively. Because these sizes correspond to the phase of growth where fish gain swimming ability, they are considered to migrate positively to and from the sandy beach of Furuhama Park and the canal on the flow and ebb tides, respectively. The results suggest that the sandy beach of Furuhama Park has a habitat function as a nursery ground for P. altivelis altivelis at least.
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.
In order to clarify fish assemblage structures in small fragmented seagrass beds inside
harbors facing the open sea, daytime seine net sampling was conducted over a seagrass bed
and adjacent bare sandy area in Hiraiso Harbor, Ibaraki Prefecture, eastern Japan, from May to August 2014. A total of 1126 fish individuals from 29 species and 562 individuals from 20 species were collected in the seagrass bed and sandy area, respectively. Despite the lack of significant differences in environmental properties（water temperature, salinity, turbidity and dissolved oxygen）, except for mud content, between the seagrass bed and sandy area, the mean numbers of fish species and individuals per haul were significantly higher in the former throughout the study period, with species composition also differing remarkably between the two habitats.Larger-sized individuals also tended to be more abundant in the seagrass bed compared with the sandy area. These results indicated that fragmentary and small seagrass bed may provide an important habitat for some species.
Seasonal patterns of larval and juvenile fish assemblage in a limnetic zone of Lake Kitaura, a shallow large inland-sea lake（7 m maximum depth, 36 km ²）in Ibaraki Prefecture, eastern Japan, were examined by monthly larval net sampling at surface and middle layers from April 2010 to March 2012. A total of 5,272 individuals representing 8 families and 11 species were collected during the study period. Mean numbers of fish species and individuals were more abundant from spring to summer, with non-occurrence in most months during late autumn and winter. The most abundant species were Japanese icefish Salangichthys microdon, pond smelt Hypomesus nipponensis and three gobiids Tridentiger brevispinis, Rhinogobius sp. and Acanthogobius lactipes. Of these, icefish and pond smelt occurred abundantly at both layers in early
spring（March and April）, whereas three gobiids at middle layer in most months during late spring and summer（May to September）. Species composition also differed between early spring and late spring to summer. Occurrence patterns and developmental stages of the five abundant species suggested that they would disperse from spawning grounds in littoral habitats and/or inflowing rivers and have more or less different pelagic larval duration and swimming layer in the limnetic zone.
ハゼ−エビ共生における条件的関係について太平洋での知見は限られているため，ツマグロスジハゼの条件的共生に関する生態学的研究は重要である。本研究では，南日本の干潟において，満潮時と干潮時にテッポウエビの巣穴口周辺を定量的に観察することで，ツマグロスジハゼの巣穴外活動を調査した。満潮時，ツマグロスジハゼは10 分間の観察時間のうち約30％のみを巣穴口前で過ごしていた。また，ツマグロスジハゼは，ときどき巣穴口から10 cm 以上離れることがあったが，ほとんどの個体は観察時間中に巣穴口前に戻っていた。干潮時の調査では，潮溜りでの巣穴外活動が確認されたが，満潮時に比べてその時間は短いことが分かった。さらに，雑食性や肉食性魚類，カニ類の接近に伴いツマグロスジハゼが巣穴内に戻ることが明らかになった。今後，近縁種であるスジハゼ類によるテッポウエビの巣穴利用に関する研究を進めていくことで，本属の条件的共生の進化過程が明らかになるかもしれない。