広島湾におけるクロダイ稚魚の出現状況と年変動

広島湾においてクロダイ稚魚の着底・成育に適した環境条件を把握する目的で，同湾沿岸の稚魚の生息密度を明らかにした。稚魚は2006–2009年6–7月に広島湾内18定点で採集した（n=2,488）。さらに，調査定点のうち最も高い稚魚密度（35.83 inds./100m2）を観測した能美島鹿田において，2013–2016年の7–9月にかけて稚魚を採集し（n=1,588），密度の年変動を観察した。その結果，広島湾の全平均密度は3.78 inds./100m2となり，密度の高い4定点はいずれも厳島，能美島西岸であった。密度が高い定点の特徴として，潮流が物理的に遮られた静穏域の砂浜帯で，淡水流入があることが挙げられた。密度には年変動があり，近年は減少傾向にあることが判明した。広島湾クロダイの資源加入量が低下している可能性が示唆された。

To obtain basic information on the optimal nursery grounds for black sea bream Acanthopagrus schlegelii recruits, juvenile density in Hiroshima Bay was calculated. Juveniles (n=2,488) were collected from 18 sampling stations around Hiroshima Bay using surf zone nets during the early summer from 2006 to 2009. to assess the annual fluctuations of juvenile density, samples were continuously caught from one station (St. 1) on the western side of Noumishima Island from 2013 to 2016. Relatively high densities were recorded from four stations (St. 1, 2, 6 and 10) located along the Ohno Seto Strait and the western side of Noumishima Island. These sampling stations shared three common characteristics that likely explain the observed higher juvenile density, namely (1) moderate currents with calm areas protected by physical barrier, (2) sandy muddy beaches (tidal flats) and (3) freshwater inflows or run-off. Juvenile density showed an annual decreasing trend, suggesting that fish stock levels of black sea bream are declining in Hiroshima Bay.