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

ABSTRACT:   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.     

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

ABSTRACT: 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.