Biological invasions by non-indigenous populations are one of the most serious problems for the conservation of genetic resources in natural environments, due to the likelihood of such populations becoming established and negatively impacting the genetic integrity of indigenous populations through hybridization and introgression. In central Japan, freshwater fish populations have been isolated by the Ibuki-Suzuka mountains between the Ise Bay basin (Gifu, Mie and Aichi prefectures on the eastern side of the mountains) and Lake Biwa water system (Shiga, Kyoto, Osaka and other prefectures on the western side), subsequently becoming genetically divergent. On the other hand, nonindigenous populations of freshwater fishes have been introduced to the rivers of the Ise Bay basin as a consequence of transplantation of commercially important Ayu (Plecoglossus altivelis altivelis) being accompanied by other species from Lake Biwa. Because some serious genetic disturbance of some species has already been reported from the former system, it is necessary to clarify the magnitude of such invasions and introduce measures for the conservation of native populations. This study focused on Japanese dace, Tribolodon hakonensis, one of the most common local fishes, which exhibits genetic differentiation between the Ise Bay basin and Lake Biwa system populations. Due to the superficial similarity between Japanese dace and Ayu, a bycatch of the former has been transported with Ayu from Lake Biwa to the other areas. To understand the magnitude of invasions from Lake Biwa, the PCR-RFLP method was used to discriminate between indigenous and non-indigenous mitochondrial DNA (mtDNA) haplotypes of Japanese dace in Gifu Prefecture rivers of the Ise Bay basin. Non-indigenous mtDNA haplotypes (Lake Biwa types) were found to be highly abundant in dam reservoirs, while less so in rivers, suggesting that Lake Biwa dace more easily adapt to the lacustrine environment of dam reservoirs than to river environments.
Mugilogobius flavomaculatus Huang, Chen, Yung and Shao, 2016 (Gobiiformes: Gobiidae) previously known only from Taiwan was newly recorded from Japan, based on a single specimen (30.9 mm standard length) from Iriomote-jima Island (24˚22'N, 123˚53'E), Yaeyama Islands, Okinawa Prefecture. The species is characterized by the following combination of characters: body with eight distinct black and seven yellow bands; cheek and operculum with dark net-like marking, surrounding five rounded pale blotches; caudal-fin base with a vertical black bar; first dorsal fin with a somewhat horizontal broad black band, rounded contour and non-filamentous spines; second dorsal- and anal-fin rays I, 8; and predorsal scales 19. Although M. flavomaculatus is similar to two presently unidentified Japanese congeners, Mugilogobius sp. 2 (Japanese name: Tanukihaze) and Mugilogobius sp. 3 (Mujina-haze), all three being assigned to the M. mertoni complex, the former is readily distinguished from the latter two species by the black and yellow bands on the body (body with X-shaped dusky markings in the latter), and (usually) I, 8 second dorsal- and anal-fin rays (usually I, 7). The new standard Japanese name Torahaze is proposed for the species.
Fertilized eggs of two manefishes (Caristius macropus and Paracaristius nudarcus) were collected off Shionomisaki, Wakayama Prefecture, Japan and hatched in an aquarium. Descriptions are provided for early-life stages of both species [C. macropus (6 specimens: 8.9–125.9 mm in standard length: SL; maximum rearing period 255 days) and P. nudarcus (26.8 mm SL; 64 days)], based on the reared larval and juvenile specimens. Characters known to be diagnostic for the genus Caristius, including small sized lateralline scales and serrated lower caudal-fin rays, appeared from 80.9 mm SL in juvenile C. macropus. Juveniles of the two species differed in dorsal fin shape and meristic characters. The occurrence of fertilized eggs in waters off Wakayama Prefecture, Japan indicated that C.macropus and P. nudarcus both spawn in Kuroshio waters.
A non-native brown trout Salmo trutta (407 mm in fork length, 717 g in body weight) was collected from the Otsukushinai River, southern Hokkaido, Japan, in 2019. This specimen was an immature male (0.4 g in gonad weight). The otolith Sr:Ca ratio profile and annual rings suggested that the specimen had migrated to the sea at the age of 4+ years and ascended the Otsukushinai River in the year of the seaward migration. This study is the first record of invasion of anadromous brown trout through the sea in Japan.
Changes in dietary habits and ecological niche of the Sakhalin taimen Parahucho perryi were estimated from the analyses of carbon and nitrogen stable isotope ratios in the Shumarinai Lake system, Hokkaido, Japan. Stable isotope ratios of both elements in 0+ fry in a feeder stream were within the range of stable isotope ratios in other fish species collected from the same stream, although nitrogen stable isotope ratios in co-existing 1+ juveniles were 0.84‰ to 2.10‰ higher than in the latter. Nitrogen stable isotope ratios in large individuals (54.5–69.5 cm fork length) in the lake were 1.40‰ to 4.28‰ higher than in other fishes. These results suggest that Sakhalin taimen begin predating other fishes at the 1+ juvenile stage, with larger individuals becoming a top predator in the Shumarinai Lake system.
A single mature migrating male (371 mm SL) Biwa salmon (Oncorhynchus masou subsp.), collected on 11 December 2019 off the mouth of the Yanagawa River, feeding the South Basin of Lake Biwa, Japan, represents the southernmost record of that subspecies from Lake Biwa and first specimen-based record from the South Basin. Together with anecdotal evidence, the record indicates that O. masou subsp. likely utilized South Basin feeder streams as spawning grounds before the latter were artificially revetmented.