マアナゴの生態学的研究-1-
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概要
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In the Seto Inland Sea the conger eel, Astroconger myriaster (Brevoort) is a commercially important species to be caught by small sized trawlers. In the present work, growth of the fish has been studied on the basis of morphometric surveys conducted during 1952-56 for 1, 960 individuals from fishing grounds off Marugame and 1, 892 off Takuma (Table 1 and Fig. 1). 1) Frequency distribution of the anal length composition and seasonal changes in the modes (Figs. 2 and 3) have suggested that the fish under study consist of two major age groups: the young exploited during August to December and the older staying in the areas over the year. The field data indicated the average growth from summer to summer to be about 50mm. 2) Relation between total length (L) and anal length (l) of 400 specimens collected in 1956 may be expressed as L=14.38+2.45l. From the anal length composition obtained at Marugame during 1952-56, the mean anal length of the young and the older groups was separately computed for the coordinate months of the years (Table 3). The results revealed that the annual growth averages 50mm. in a close agreement with the one determined from the field data. However, the young group from Marugame was found a little different in the growth trend from that of Takuma (Fig. 4). The difference may be partly attributable to the particular topography of Takuma area where the tidal currents from the outer sea join each other and receed to different directions. According to the least square method, the growth of the conger eel in these areas may be formulated as follows: For the young group occurring during August to December l=73.143+2.150a (Marugame), logl=1.8327+0.1288logl (Takuma); For the older group occurring in early months of the following year l=110.464+1.484a (Marugame), l=105.133+1.851a (Takuma), where l is anal length and a is age by ten day period. 3) In the conger eel under report, relation between body weight (w) and anal length (l) is indicated as in Fig. 5 and by a formula log w=-5.8326+3.614 log l. Assuming that the fish complete their metamorphosis in a year after hatching, it is inferable that the conger eel in the areas are composed of the two-year-old and the three-year-old groups.
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