We investigated an aquatic animal community in a small floodplain water body of the Ohya River in the Tone River system of Japan in August 2020. We captured and observed native aquatic animals such as the diving beetle Copelatus weymarni, the water scavenger beetle Sternolophus rufipes, the Tokyo daruma pond frog Pelophylax porosus porosus, and the Japanese tree frog Dryophytes japonicus, inhabiting the lentic water areas in the study site. However, two invasive species, the pond loach Misgurnus dabryanus and the red swamp crayfish Procambarus clarkii were dominant. Most loaches were juveniles born during a reproductive season in 2020. Our results indicate that the inter-levee floodplain water bodies may serve not only as suitable habitats for native wetland aquatic animals but also as breeding sites for invasive species that have negative effects on wetland ecosystems.

Chara corallina Klein ex Willd. var. corallina, a macrophyte that grows in lakes and ponds, is a charophyte (Charales, Charophyceae) species. In Japan, C. corallina var. corallina is a threatened species as their populations have been declining in many regions. Several aquatic plant surveys of Lake Izunuma have reported that there are more than 30 aquatic plant species in the lake. However, information regarding charophytes in the lake is limited, as they have only been mentioned in two previous reports. We discovered a specimen of C. corallina var. corallina, a species which has not yet been reported in Lake Izunuma, from unidentified specimens collected in the lake. The specimen was obtained incidentally during an aquatic plant survey conducted by one of the authors in 1992 and was identified in this study based on morphological observations. We report the present discovery to improve the record of charophyte occurrence in historic flora records of Lake Izunuma. Of note, it is possible that C. corallina var. corallina and other charophytes in Lake Izunuma are now extinct.

Abstract Based on the morphological examination of the specimens, the recently established alien crayfish in Niigata Prefecture was identified as Pacifastacus leniusculus (Decapoda: Astacidae). Ectosymbiotic branchiobdellidans on the crayfish were composed of a single species, Sathodrilus attenuatus. (Annelida: Clitellata). Such a combination of crayfish and ectosymbionts suggests that the Niigata population has been derived from populations in the upper course watershed areas of Fukushima Prefecture or by illegal introduction.

One of the “100 of the World’s Worst Invasive Alien Species”, Eichhornia crassipes (Mart.) Solms, was recorded from lake Izunuma in Japan. In total, 1,140 shoots were immediately exterminated from the lake. This species is known for its high growth potential. When one E. crassipes shoot was introduced in early June, it was estimated that it increased to over 1,000 shoots by early October. However, E. crassipes cannot become established under the current climate conditions of lake Izunuma, because it is sensitive to low temperatures. If global warming progresses as predicted, by 2076–2095 the climate in the lake may become suitable for the establishment of E. crassipes. It is important to monitor the introduction and initial extermination to prevent further invasion of E. crassipes.

The breeding distribution and population of the Great Cormorant Phalacrocorax carbo, a large piscivorous waterbird, have increased rapidly throughout Japan, causing significant damage to inland fisheries. They are increasing rapidly in northern Japan, and damage controls are carried out in each prefecture without any basic information on their population and breeding. The breeding distribution and nest number of the cormorants were examined throughout Iwate Prefecture in the 2021 breeding period. A total of 412 nests were confirmed in six colonies, with more than 90% concentrated in the two colonies made in the riparian forests of the Kitakami river. These colonies were newly established within the last few years. We believe that these were established in new locations due to the recent population control efforts at several other colonies in Iwate Prefecture.

The ectosymbiotic branchiobdellidan, or crayfish worm, Sathodrilus attenuatus (Annelida: Clitellata) are known to be attached on the alien freshwater crayfish, Pacifastacus leniusculus (Crustacea: Decapoda), in many populations in eastern Hokkaido, Japan. The branchiobdellid species have been rarely found on the alien crayfish collected from brackish Lake Harutori in downtown Kushiro City likely due to their lower salinity tolerance. In this study, we deduced the pathway of introduction of the alien crayfish to Lake Harutori and the adjacent freshwaters by (1) survey of S. attenuates abundance on P. leniusculus collected from lakes and marshes around Lake Harutori, (2) salinity tolerance experiments of S. attenuates, and (3) review of official reports and interview surveys to collect information on the introduction of P. leniusculus in downtown Kushiro City. The branchiobdellidans were found on alien crayfish collected from Kayanuma, Kushiro River, in Kushiro Marsh, near Kushiro City (mean abundance was 34.4), whereas the branchiobdellidans were not found on the alien crayfish collected from the artificial pond (Hyotan-ike) in Tsurugadai Park near Lake Harutori despite Hyota-ike being freshwater. Results of laboratory experiments showed a low tolerance of the branchiobdellidans to salinity. All branchiobdellidan individuals died within 60 mins in 30‰ and 15‰ solutions of seawater, and died within 250 mins in the 10‰ solution. In the 5‰ and 0‰ solutions, all the individuals survived for 24 hours. Based on the field survey of branchiobdellidans and under laboratory experiments, the pathway of introduction of P. leniusculus in downtown of Kushiro City has been deduced and a review of official reports and an interview survey was also carried out to check the deduced pathways. We propose that P. leniusculus with S. attenuatus were introduced from Kushiro Marsh to the artificial freshwater pond (Tombo-ike) which is about 100m upstream from the brackish Lake Harutori. Then the alien crayfish and branchiobdellidan passed down the overflow channel to Lake Harutori where the host survived but the ectosymbiont died due to the salinity. P. leniusculus could be collected from Lake Harutori then introduced into the artificial freshwater pond (Hyotan-ike) where they remained free of S. attenuatus.

The medaka fish (Oryzias latipes species complex) were collected in Mino City, Gifu Prefecture, Japan, on November 2021. A total 74 individuals were collected and observed for morphological characteristics, of which 24 individuals had a metallic luster on their dorsal surface. These morphological characteristics are consistent with those of the “Taigaiko-medaka”, and therefore they are aquarium medaka varieties. Other aquarium medaka varieties such as blue-colored medaka and those with many silvery scales were also collected. These were presumed to have been abandoned in the field by aquarium owners because they were not needed after breeding selection.

In April 2022, one subadult male Pseudohelice subquadrata (Dana, 1851) was collected from the Sagamigawa River Estuary, Kanagawa Prefecture, located on the eastern side of the middle Honshu Island, Japan. The species, known from tropical and subtropical regions of the Indo-Pacific, has been rarely found on Honshu Island. This is the northernmost record for this species and the first record from Sagamigawa River.

In June 2019, the establishment of smallmouth bass was confirmed in Kotogawa Dam Reservoir in Yamanashi Prefecture, Japan. The elevation of the reservoir is 1,453.5 m, which is the highest elevation of a water body where smallmouth bass have been established in Japan. In 2020, we evaluated the status of the population of smallmouth bass and captured the bass using gillnet, spear gun, and angling. A total of 640 smallmouth bass were caught, and the highest catch per unit of effort of fish caught was by gill netting. The growth rate of the bass was slow, and the females spawn over 20 cm in total length. Based on the results of the capture, it is considered effective to use gill nets to target mature bass over 20 cm in total length from mid-May, when the surface water temperature exceeds 12°C, to mid-July, and to target immature bass after mid-July to reduce spawning by bass in the following year. Additionally, spawning beds of the bass were very difficult to find by divers in the reservoir, because most of the bottom of the shore of the dam was covered by gravel formed by water level fluctuation. Therefore, it is important to develop other effective methods of finding spawning beds and removing larvae.

In May 2021, a yellow toned Amur catfish was caught by a set net in Lake Izunuma-Uchinuma, Japan. The catfish was 681 mm in total length, and most parts of its body color was yellow. Dark colored body areas of the catfish increased when reared under dark conditions. In contrast, the area of yellow color on the body of the catfish increased under light conditions. The body color of the yellow toned catfish changed according to its rearing environment．After 100 days of rearing under light condition, the body color of catfish become completely yellow. In lake Izunuma-Uchinuma, populations of various fish species have begun to recover as a result of largemouth bass eradication activities. The yellow discolored catfish collected this time also appears to be as a result of the recovery of the catfish population in the lake.