Ryukyu-ayu Plecoglossus Altivelis Research Articles

Does construction in the spawning ground improve the riverbed conditions making it suitable for spawning of the Ryukyu-ayu Plecoglossus altivelis ryukyuensis?

Maintaining a certain number of spawning grounds for endangered fish is particularly important in rivers and streams where the environment changes drastically through floods and exploitations. Ryukyu-ayu Plecoglossus altivelis ryukyuensis is an endangered annual fish inhabiting the few streams and rivers flowing through the Amami-Oshima Island, southern Japan, and spawns in the lower reaches at sites with the specific riverbed conditions. In the island, resident volunteers along with researchers modify riverbed manually to provide conditions suitable for the natural spawning of the fish every year. This study investigated the effects of these activities on the riverbed conditions and consequently on the spawning by Ryukyu-ayu in two rivers. Our results showed that riverbed hardness and the amount of silt significantly decreased in the modified areas, but not in the adjacent unmodified areas. In one of the two rivers, the modification suppressed the settlement of net-spinning caddisfly larvae Stenopsyche schmidi that promote consolidation of the riverbed. The riverbed conditions in the modified areas were similar to those in the sites where P. a. ryukyuensis indeed spawned. These results suggest that manual modification is efficient in improving riverbed conditions for the spawning of Ryukyu-ayu. However, the eggs of the fish were not found at the modified areas in both rivers. It will be necessary to develop effective and practical ways of selecting sites for modification without obstructing the natural spawning in this fish.

Using environmental DNA analyses to assess the occurrence and abundance of the endangered amphidromous fish Plecoglossus altivelis ryukyuensis.

The Ryukyu ayu Plecoglossus altivelis ryukyuensis is an endangered amphidromous fish that inhabits rivers in the Ryukyu Archipelago (Japan). Populations of the species have declined dramatically. Consequently, the Ryukyu ayu has been registered as a natural monument in Japan and monitoring surveys with direct catching are restricted legally. This restriction, unfortunately, makes monitoring of population abundances difficult and creates a barrier to both advancing understanding of the species’ status and the development of appropriate conservation plans.We developed a non-invasive monitoring methodology using eDNA analyses. We designed a specific quantitative PCR assay for the Ryukyu ayu using the mitochondrial ND4 region. Using this primer/probe set, we conducted eDNA analyses in three rivers on Amami-Ohshima Island. The DNA fragments were amplified from the eDNA extracted from natural water in each river. The numbers of DNA fragments detected were positively correlated with individual counts of fish obtained by visual snorkelling surveys. Our method does not contravene restrictions and facilitates abundance monitoring of this endangered fish species.

Facultative amphidromy involving estuaries in an annual amphidromous fish from a subtropical marginal range.

Variation in the life cycle of diadromous fishes can be explained by differential food availability between marine and freshwater habitats, since migration is often interpreted as a mechanism for exploiting food resources. Theoretically, a migration pattern of mainly remaining in freshwater occurs in tropical and subtropical habitats where fluvial productivity possibly exceeds marine productivity. However, in Yakugachi River, Amami-Oshima Island, southern Japan, low nutrient concentrations in the river suggest that food availability is limited for the subtropical Ryukyu-ayu Plecoglossus altivelis ryukyuensis. Since Ryukyu-ayu is an amphidromous fish that mainly grows in rivers after spending 2 months in the sea, limited food availability in rivers would force this species to migrate to other habitats with better food availability. Otolith increment and Sr:Ca analyses of 48 adult Ryukyu-ayu collected from the Yakugachi River revealed that all individuals visited estuaries more than three times after moving upstream. Although the specific growth rates of this species in the river had no correlation with the salinity profile in the fluvial period, this movement may be an adaptive choice because the salinity profile significantly affected the body size at maturity. Our results highlighted individual-based variations in amphidromous migration for utilizing estuaries, which could be explained by relatively higher productivity in estuarine than in freshwater and marine habitats.

Monthly occurrence and feeding habits of larval and juvenile Ryukyu-ayu Plecoglossus altivelis ryukyuensis in an estuarine lake and coastal area of the Kawauchi River, Amami-oshima Island, southern Japan

We examined monthly occurrence patterns and feeding habits of larvae and juveniles of the critically endangered Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the lower reach of the Kawauchi River, Amami-oshima Island, southern Japan, for two successive years. The study area was classified into a brackish lake (Lake Uchiumi) and a coastal zone (Sumiyo Bay) based on monthly variations in water temperature and salinity. Larvae and juveniles occurred from December to April in the first year and from January to February in the second year, respectively. Smaller-sized and earlier developmental-staged P. altivelis ryukyuensis individuals tended to emerge in larger numbers in Lake Uchiumi than in Sumiyo Bay. The present data suggest that Lake Uchiumi, where the physical environment is characterized by lower temperature and salinity than in Sumiyo Bay, would be suitable as a nursery ground for this fish, especially for earlier developmental-staged larvae. No individuals ≥25.0 mm body length, which is the size at which they begin their upstream migration, occurred in Lake Uchiumi or Sumiyo Bay in April or May, when upstream migration is assumed to reach a peak based on previous studies, in either year. The peak of upstream migration, therefore, may have occurred several months earlier in the years covered in this study due to higher water temperature than in typical years. To assess the feeding habits of this fish, the gut contents were observed. In both areas, copepods were abundant in P. altivelis ryukyuensis diet with calanoids and Oithona spp. being the most abundant. The diet composition differed between Lake Uchiumi and Sumiyo Bay and clearer ontogenetic variation in the composition of the diet was observed in Lake Uchiumi. Small individuals fed on tintinnid ciliates, but the proportion of these organisms in the diet gradually decreased with growth. Large individuals fed exclusively on harpacticoid copepods and insects including demersal species. Overall, larval and juvenile P. altivelis ryukyuensis exhibited a generalist feeding habit that tended to increase with growth. It is indispensable to adequately manage the environmental conditions in the lower reach of the Kawauchi River, especially Lake Uchiumi, to conserve P. altivelis ryukyuensis population.

Survival rates of larval Ryukyu-ayu Plecoglossus altivelis ryukyuensis under differing experimental conditions of temperature and salinity

Survival rates of larval Ryukyu-ayu Plecoglossus altivelis ryukyuensis under differing experimental conditions of temperature and salinity

Microsporidian Infection in Rryukyu Ayu Plecoglossus altivelis ryukyuensis on Amami Oshima Island

Cultured Ryukyu ayu Plecoglossus altivelis ryukyuensis was found to be infected with a microsporidian parasite on Amami Oshima Island in January 2009. The parasite formed numerous cysts on the visceral organs and skin of the diseased fish. High temperature seemed to suppress the development of the parasite; in 2009 and 2010, the disease occurred in the 0-year fish from April to August, and afterwards the prevalence decreased dramatically in September and October. These characteristics well resembled glugeosis in ayu P. altivelis altivelis in other areas of Japan. This parasite seems to be indigenous to Amami Oshima Island, because it was detected in the museum specimens of wild Ryukyu ayu caught in 1991. Average dimensions of spores from Ryukyu ayu were slightly shorter and wider than those of Glugea plecoglossi, but the ranges of the measurements were overlapped. From the above characteristics and molecular analysis of SSU rRNA, it was concluded that the present microsporidian was included genus Glugea, but the specific identity remained to be clarified.

Seasonal appearance and size of newly-hatched larvae of Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the Yakugachi and Kawauchi Rivers, Amami-oshima Island, southern Japan

Seasonal appearance and size of newly-hatched larvae of Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the Yakugachi and Kawauchi Rivers, Amami-oshima Island, southern Japan

The behavior of Ryukyu-ayu Plecoglossus altivelis ryukyuensis larvae during downstream migration from the Yakugachi River flowing into Sumiyo Bay, Amamioshima Island, southern Japan.

The behavior of Ryukyu-ayu Plecoglossus altivelis ryukyuensis larvae during downstream migration from the Yakugachi River flowing into Sumiyo Bay, Amamioshima Island, southern Japan.

Occurrence and body size changes of drifting land-locked Ryukyu-ayu Plecoglossus altivelis ryukyuensis larvae in the San-numata River, Okinawa-jima Island, Japan

The occurrence and body size of drifting land-locked Ryukyu-ayu, Plecoglossus altivelis ryukyuensis, larvae were investigated in the San-numata River, Okinawa-jima Island from November 2000 to March 2001. The water temperature in the river fluctuated from 13.8 to 17.9°C during the sampling period. The drifting larvae occurred from mid-December to mid-March with an estimated spawning peak during late November and early December. The notochord length of the drifting larvae ranged from 4.9 to 6.7 mm (5.7 ± 0.3 mm, mean ± SD), and became longer as the spawning months progressed. These results imply that the land-locked population possesses the same maturation attributes as the original amphidromous population.

A Checklist of the Parasites of Ayu (Plecoglossus altivelis altivelis) (Salmoniformes: Plecoglossidae) in Japan (1912-2007)

The ayu or sweetfish Plecoglossus altivelis altivelis is distributed in many rivers and some lakes in Japan and also occurs in rivers of the Korean Peninsula and along the east coast of China and northern Vietnam. In Japan, this species is one of the most important freshwater fishes for commercial fisheries, aquaculture, and recreational fishing. In the present paper, information on the protistan and metazoan parasites of ayu in Japan is compiled based on the literature published for 96 years between 1912 and 2007, and the parasites, including 29 named species and those not identified to species level, are listed by higher taxon as follows: Ciliophora (no named species), Microspora (1), Myxozoa (1), Trematoda (13), Monogenea (3), Cestoda (1), Nematoda (1), Acantho- cephala (3), Copepoda (4), and Branchiura (2). For each taxon of parasite, the following information is given: its currently recognized scientific name, any original combination, synonym(s), or other previous identification used for the parasite occurring in ayu; habitat (freshwater, brackish, or marine); site(s) of infection within or on the host; known geographical distribution in Japanese waters; and the published source of each locality record. There has been no record of parasites from a subspecies of ayu, the ryukyu-ayu Plecoglossus altivelis ryukyuensis. The myxozoan Mitraspora plecoglossi Fujita is transferred to the genus Hoferellus (as H. plecoglossi n. comb.), Acanthocephalus opsariichthydis Yamaguti, 1935 is determined to be the nomenclaturally correct spelling and date of that acanthocephalan species, and synonymy of the acanthocephalan Echinorhynchus oblitus Golvan with E. cotti Yamaguti is formally accepted.

Diet shift of larval and juvenile landlocked Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the Fukuji Reservoir, Okinawa Island, Japan

The diet shift of larval and juvenile Ryukyu-ayu Plecoglossus altivelis ryukyuensis was investigated for the landlocked population stocked in the Fukuji Reservoir, Okinawa Island. Gut contents of 274 specimens (5.1–31.4 mm in body length) including 31 yolk-sac larvae (5.7–8.0 mm) were examined. The diet of larval and juvenile landlocked P. a. ryukyuensis was mainly composed of zooplankton such as dinoflagellates, rotifers, copepods, and cladocerans. Feeding incidence was 48% in total and it gradually increased with growth. Diet analysis clearly demonstrated a diet shift during the larval and juvenile periods. For yolk-sac larvae and 5.0–19.9 mm fish, the diet was dominated by dinoflagellates Gymnodinium sp., and Peridinium sp., both numerically and by frequency, followed by rotifers that mainly consisted of Polyarthra spp. Diet breadth (Levin’s standardized index BA) diversified with growth, and feeding on copepods and cladocerans gradually increased in 10.0–14.9 mm fish. Diet overlap (Morisita’s index: C) showed that diet drastically changed in 20.0–24.9 mm fish with a specialization on cladocerans Bosminopsis deitersi and Diaphanosoma brachyurum. This diet shift probably corresponded with an increase of feeding ability resulting from an enlargement of the mouth, increased swimming ability, and the development of sense organs.

Morphological development of eggs, larvae and juveniles of laboratory-reared Ryukyu-ayu Plecoglossus altivelis ryukyuensis

ABSTRACT: The development of embryos, larvae and juveniles of the Ryukyu-ayu, Plecoglossus altivelisryukyuensis are described based on laboratory-reared specimens.The eggs were spherical, 0.98–1.18 mm (mean 1.06 mm)in diameter with an adherent membrane. The incubation period after fertilizationwas approximately 155 h at a water temperature of 19.7–22.0°C(mean 20.7°C). Newly hatched larvae were 5.0–5.9 mm(mean 5.5 mm) in body length (BL) with 59–62 myotomes.Within 5 days after hatching, the larvae had attained 6.4–8.2 mm(mean 7.8 mm) BL and had completely consumed their yolk.Notochord flexion began at 13.7 mm BL and was completedby 16.3 mm BL. The rudimental dorsal, anal, pelvic andadipose fins appeared at 14.7, 16.3, 21.8 and 21.8 mm BL, respectively.All fin rays reached the same fixed number of adult fish at about28 mm BL. The comb-like teeth began to form at approximately30 mm BL and were fully developed at about 40–50 mm BL.The proportions of P. a. ryukyuensis specimens,which were consistent with adult fish, occurred at approximately40 mm BL.

Evaluation of the Genetics of Growth and Resistance to Vibriosis in Ryukyu Ayu Plecoglossus altivelis ryukyuensis by Hybridization

Evaluation of the Genetics of Growth and Resistance to Vibriosis in Ryukyu Ayu Plecoglossus altivelis ryukyuensis by Hybridization