Disappearance of a spring cohort in a population of the dragonet,Repomucenus valenciennei, with spring and autumn spawning peaks in Tokyo Bay, Japan |
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Authors: | Kou Ikejima Makoto Shimizu |
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Institution: | (1) Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, 113-8657 Tokyo, Japan;(2) Present address: Department of Global Agricultural Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, 113-8657 Tokyo, Japan;(3) Present address: Department of Marine Science and Resources, College of Bioresource Sciences, Nihon University, Kameino, Fujisawa, 252-8510 Kanagawa, Japan |
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Abstract: | The occurrence of eggs, pelagic larvae and juveniles, and settled juveniles of the dragonetRepomucenus valenciennei in Tokyo Bay, Japan, were investigated by plankton net and bottom trawl surveys between September 1990 and September 1991.
Eggs, and pelagic larvae and juveniles appeared from April to November (spring to autumn), peaking in both spring and autumn.
From the temporal pattern of egg and pelagic fish occurrence, and pelagic duration reported elsewhere (ca. one month), settlement
could be predicted as occurring from late spring to autumn. However, settled juveniles appeared from August to December, with
an abrupt peak in November. Aging from daily increments in the otoliths of settled recruits in 1990 indicated that the latter
comprised individuals which had hatched between mid-September and early November (i.e. autumn cohort), implying that individuals
which had hatched in spring to summer (April to August) were not recruited. Benthic hypoxia occurs widely in Tokyo Bay, from
June to October each year, particularly in the northern part, which is the main nursery area ofR. valenciennei. The timing of dissolved oxygen recovery, and appearance of settled fish coincided closely (i.e. November), indicating that
summer hypoxic conditions prevented the settlement of the spring cohort and hence recruitment to the population. |
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Keywords: | Disappearance of a spring cohort Repomucenus valenciennei spawning peaks daily increments hypoxia |
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