Shimanto Estuary Research Articles

Estuarine dependency of Lates japonicus in Shimanto Estuary, Japan, inferred from otolith Sr:Ca

Understanding estuarine dependency is essential to clarify the life history of euryhaline species and propose adequate management measures. Lates japonicus is a rare fish, endemic of the estuaries and coastal waters of southeast Japan, and categorized as endangered in the Red Data Book of Japan. To reconstruct the life history of L. japonicus in Shimanto Estuary and understand estuarine residency at different ontogenetic stages, the otoliths of juveniles, young and adults captured in the estuary were analyzed using microstructure examination and elemental analysis. Otolith Sr:Ca ratio indicated that all juveniles, captured in estuarine seagrass beds, hatched within the estuary, and recruited into the seagrass beds without dispersion to coastal areas. Similarly, the otolith Sr:Ca ratio of six young and adults captured in the estuary suggested that they mainly remained in estuarine waters during all their life after recruiting in the estuary. No evidence of long-term residency in coastal waters after the juvenile stage was obtained. These results suggested that the population of L. japonicus in Shimanto Estuary relies on estuarine habitats at all life stages.

A New Genus and Species of Paracalliopiidae (Crustacea: Amphipoda) from the Shimanto Estuary, Western Japan

A New Genus and Species of Paracalliopiidae (Crustacea: Amphipoda) from the Shimanto Estuary, Western Japan

A Morphological and Meristic Comparison of Larval and Juvenile Temperate Bass, Lateolabrax japonicus, from Various Sites in Western and Central Japan

Comparison of larval and juvenile specimens of the temperate bass, Lateolabrax japonicus, collected from 11 sites in western and central Japan, revealed that specimens from the Chikugo estuary located at the innermost Ariake Bay, differed markedly in morphology from other specimens. In the Chikugo specimens, melanophores were distributed more sparsely, the head was proportionately smaller and the numbers of dorsal pterygiophores and vertebrae were significantly lower. Larval and juvenile stages of the species were redescribed using specimens from the Shimanto estuary.

Annual changes in the hatching period of the dominant cohort of larval and juvenile ayu Plecoglossus altivelis altivelis in the Shimanto Estuary and adjacent coastal waters during 1986-2001

: Larval and juvenile ayu from seven year-groups (1986, 1987, 1992, 1995, 1996, 1999 and 2000) were collected in the Shimanto Estuary and adjacent coastal waters. The present study focuses on the variation among year-groups in the hatching period estimated by age determination using the otolith. The hatching period of the dominant cohorts in the 1986, 1987 and 1992 year-groups occurred from late October to mid-November. A delay of the hatching period of the dominant cohort was observed from the 1995 year-group, which occurred in late November to early December, and the dominant periods in the 1996, 1999 and 2000 year-groups were observed from early to late December. It appears that the delay of the hatching period of dominant cohorts was not due to a delay of spawning, but a high mortality of early hatched larvae. The water temperature in autumn in the coastal waters adjacent to the Shimanto River has tended to rise over the past 20 years and this trend was especially notable in the late 1990s. The delay in the hatching period of the dominant cohort observed from the 1995 year-group was likely to be related to the rise in seawater temperatures in autumn.

Habitat shift of ayu Plecoglossus altivelis altivelis in early stages from waters adjacent to the bank to the center of flow in the Shimanto Estuary

: To clarify the habitats of the ayu in the early stages of development in the Shimanto Estuary, the size and hatching date of the larval and juvenile ayu were examined. Sizes of larvae and juveniles were larger in the center of the flow area than in the waters adjacent to the estuary’s banks, which suggests that larvae shift habitat to the center of flow from the waters along the estuary’s banks. This shift began at approximately 20 mm body length. However, the resident term in the waters adjacent to the estuary’s banks changed with the hatching dates; that is, the early and late-hatched larvae became short and long-term residents, respectively. A similar phenomenon was found also in the juveniles collected in the freshwater section that were migrating upstream. The growth rates of larval ayu in the waters adjacent to the banks tended to decrease with hatching dates. The fluctuations in growth rate with hatching date appear to be a factor leading to the variation of resident term.

Species composition and seasonal occurrence of fish larvae and juveniles in the Shimanto Estuary, Japan

To clarify the characteristics of the larval and juvenile fish community in a Japanese estuary, monthly collections were conducted in the shallow waters of the Shimanto Estuary, Japan from July 1985 to June 1987. A total of 49 101 fishes of more than 100 species from 44 families were sampled over the 2-year period. Numerically, Gerres equulus was most common, accounting for 20.02% of the total abundance. Rhyncopelates oxyrhynchus was ranked second, followed by Acanthopagrus latus, Acanthogobius flavimanus, Mugil cephalus cephalus, Parioglossus dotui, and Acanthopagrus schlegeli. Thus, fishes observed in the estuary were mostly marine species that spawn in the sea. The majority of these fish first appeared within the estuary at late postlarval and/or early juvenile stages. Based on their utilization of the shallow waters in the estuary, the majority of species could be categorized as residents. In the Shimanto Estuary, shallow waters are studded with shelters, especially eelgrass beds, which serve as important nursery grounds for coastal and euryhaline fishes.

Early life history of a catadromous sculpin in western Japan

To clarify the early life history of Cottus kazika (Cottidae), the ontogeny, osteology, distribution, migration and food habit were examined based on larvae and juveniles collected from coasts and estuaries in western Japan. The sequence of fin completion was pectoral (P1)-dorsal soft-ray part (D2)-anal (A)-dorsal spine part (D1)-pelvic (P2). However, that of calcification was P1-P2-D1-D2-A. Larvae and juveniles of C. kazika are distinguished from those of coastal sculpins of Japan by large preopercle spines, conspicuous parietal spines, the large and densely pigmented pectoral fin and the relatively deep body. C. kazika and Myoxocephalus polyacanthocephalus plus Enophrys bison share these four ontogenetic characters. From the relationship between the growth ring count on the sagittae and total length, the size of embryos at hatching is estimated at 5.2 mm. Larvae are estimated to reach 7 mm in 10 days, 9 mm in 20 days and 11 mm in 30 days after hatching. Hatching dates for larvae collected in the Shimanto estuary in 1987 peaked during the period around the new moon in January. Many free embryos ranging from 4.3 to 5.9 mm were collected using an aqualamp at a rocky shore outside the Yura River mouth, at high salinity (> 33 ppt), but none were collected in the estuary. From beach samples, seasonal changes in body size varied from year to year, and no consistent growth pattern was found, although juveniles were generally larger in late March and April than in January and February. Very few larvae were collected along shorelines where there were no adjacent rivers. Food habit of larvae and juveniles changed with growth. Polychaete larvae, cladocerans and copepods were found in preflexion larvae less than 6 mm. Gammarids were one of the important food items for larvae over 6 mm. Juveniles over 12 mm almost exclusively fed on gammarids. In the Shimanto estuary, larvae and juveniles stayed near the river mouth in January and February, within a stagnant layer where temperature and salinity remained about 12 °C and 20 ppt, respectively. Juveniles tended to move in March when the stagnant layer disappeared in the estuary.

Different Mortality in Larval Stage of Ayu <i>Plecoglossus altivelis</i> by Birth Dates in the Shimanto Estuary and Adjacent Coastal Waters

Different Mortality in Larval Stage of Ayu <i>Plecoglossus altivelis</i> by Birth Dates in the Shimanto Estuary and Adjacent Coastal Waters

Spatial Distribution of Larval Ayu <i>Plecoglossus altivelis</i> in the Shimanto Estuary, Japan

Spatial Distribution of Larval Ayu <i>Plecoglossus altivelis</i> in the Shimanto Estuary, Japan

四万十川河口内に出現するアユ仔魚

In the amphidromous form of ayu Plecoglossus altivelis Temminck et Schlegel, generally the prelarvae hatched in rivers in autumn are drifted down into the sea, where they live throughout the larval and early juvenile stages until spring. In the Shimanto estuary, however, an appreciable number of postlarval ayu occurred not only during March to May, but also during October to February. Furthermore, there was little difference in the size and age between the larvae occurring in the estuary and sea. In the estuary, they reside in the bank waters, during 10-75 day ages and 10-30mm SL. Thereafter, they seem to ascend the river without going out to the sea. The growth rate estimated from the otolith daily rings was higher in the estuary than in the sea beach.

Seasonal occurrence and food habits of larvae and juveniles of two temperate basses in the Shimanto estuary, Japan

Larvae and juveniles ofLateolabrax japonicus andL. latus occurred from January to May 1986 in the shallow waters of the Shimanto estuary.L. japonicus markedly outnumberedL. latus. Distinct ecological differences were recognized in habitats and food habits between the two species:L. japonicus mainly inhabited eelgrass beds composed ofZostera nana, whileL. latus appeared evenly in both eelgrass beds and non-eelgrass habitats; the former fed on copepods and cladocerans, while the latter fed on copepods and fish larvae. From these habitat and food habit analyses, estuaries were considered to be important as a main habitat forL. japonicus, but not forL. latus. The fact that ecological differences have occurred during the early life stages was inferred to be one of the possible keys to speculate on the speciation of the two species.

Occurrence of Larval and Juvenile Japanese Snook, Lates japonicus , in the Shimanto Estuary, Japan

A total of 54 postlarval, 12 juvenile and six young Lates japonicus were collected in the Shimanto estuary from August 1985 to January 1987. The larvae (4.3–7.6 mm SL) and juveniles (8.1–28.0 mm SL) occurred only in areas with eelgrass beds from late July to middle August. Temperatures and salinities at waters where any number of them were collected ranged from 27.8 to 31.9°C and from 2.8 to 18.2%, respectively. The larvae are very similar in general morphology to those of Mugilidae and some of Gobiidae, but are distinguished from others by the spines of preopercle, position of the anus and pigmentation patterns in the posterior part of body.

Occurrence of Larval and Juvenile Japanese Snook, Lates japonicus, in the Shimanto Estuary, Japan

Occurrence of Larval and Juvenile Japanese Snook, Lates japonicus, in the Shimanto Estuary, Japan

Seasonal Occurrence and Food Habits of Larvae and Juveniles of Two Temperate Basses in the Shimanto Estuary, Japan

Seasonal Occurrence and Food Habits of Larvae and Juveniles of Two Temperate Basses in the Shimanto Estuary, Japan