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The escape behavior of wild and hatchery-reared juvenile Japanese flounder Paralichthys olivaceus from a dummy predator, gray goblinfish Minous monodactylus, was investigated under artificial light in an experimental tank. When the dummy predator approached, both the juveniles quickly emerged from the sand, escaping primarily by swimming close to the bottom (benthic swimming) rather than off the bottom (off-bottom swimming). It is suggested that juvenile flounders might escape in this manner from a diurnal visual feeder in the daytime, owing to the flounder being virtually undetectable visually when close to the bottom. The average escape swimming speed of an individual of 9.3 total length (TL)/s in wild fish and 9.8 TL/s in hatchery-reared fish was similar to the maximum speed of many fish species. Juvenile flounder escaped more quickly and had a higher swimming speed (v cm/s), when their distance (d cm) to the dummy predator decreased (v = −2.7d + 82.1, r = −0.33; anova, P

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.

To elucidate the recruitment and movement process of larval ayuin the surf zone of a sandy beach facing Tosa Bay, 31 collectionswere made with a small seine net at intervals of approximately 5days from November 1990 to April 1991. On the basis of seasonalvariations in larval abundance, size, age, duration of residence,and the occurrence patterns of ayu in the surf zone, larval ayuwere classified into two groups: short-term resident (

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.

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.

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.

In Shijiki Bay of Hirado Island, diel discrete-depth horizontal tows with a larva net in May 1983 resulted in a total of 2, 350 larval and juvenile red sea bream (2.1-10.7mm SL), and 347 larval black sea bream (2.4-6.2mm SL) being collected. During the daytime, black sea bream larvae were concentrated near the surface, but red sea bream larvae near the bottom, with these tendencies becoming more evident with their growth. During the nighttime, the larvae of both species were dispersed throughout the water column. In the surf zone of the sandy-beaches located in the innermost part of the bay, collections with a small seine net in June and July 1983 gave a total of 241 late-larval and early-juvenile black sea bream (6.2-20.1mm SL), but no red sea bream. The separation of juvenile habitats between these related species seemed to be a consequence of the species-specific vertical distribution pattern differentiated during the pelagic larval period. A possible mechanism for inshore migration in the black sea bream was discussed in relation to their surface occurrence.