志々伎湾におけるヒラメ稚仔魚の着底過程

概要

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Successive diel sampling was conducted from May 11 to 12, 1983, to elucidate the settling process of a flounder Paralichthys olivaceus in a nursery ground at Shijiki Bay, Hirado Island.A beam trawl with 2-m width was used for collecting flounders just before and after metamorphosis. Six hundred eighty-six flounders, consisting mostly of settling larvae and just settled juveniles ranging from 10 to 13mm in standard length, were collected after thirty-one hauls. Efficiency of the gear, was found to be 26.8%, as evaluated from actual flounder densities assessed by scuba-equipped quadrat method and repetition of experimental tows following the assessment. Population of the flounder inhabiting the nursery ground was estimated from flounder densities, gear efficiency and area of the nursery ground. The population size of mid-and late-phase metamorphosing larvae markedly changed with sampling time, exhibiting increases during ebb and low tides and decreases during flood and high tides. These changes seemed to be attributed to vertical movements of flounder synchronized with the tidal phase. The flood-swimming up and ebb-settling down movements enable metamorphosing larvae to migrate towards near-shore nursery grounds.Heavy mortalities possibly occurring with settlement were preliminarily discussed in relation to starvation and predation.
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