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Age determination of the South African round herring Etrumeus micropus and length and age composition of the commercial catches, 1965-1973
Geldenhuys, N.D. (1978). Age determination of the South African round herring Etrumeus micropus and length and age composition of the commercial catches, 1965-1973. Investigational Report = Onderzoekverslag, 115. Dept. Of Industries, RSA: Cape Town. 16 pp.
Deel van: Investigational Report of South Africa = Onderzoekverslag. Republic of South Africa. Department of Industries. Sea Fisheries Branch: Cape Town. ISSN 0379-1084
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| Abstract |
Otoliths were used for the determination of the age and the growth of the round herring Etrumeus micropus, a minor but regular constituent of the South African multispecies pelagic fishery. Opaque zones are formed mainly from December to March in fish longer than 10,0 cm in standard length and hyaline otolith margins from May to September. The relation between fish length and otolith radius was found to be curvilinear. Growth in standard length is ex- pressed by the following von Bertalanffy equation: Lct = 26,19 [1-e-0.3334(t + 0.1664) ]. The length frequency distribution of commercial catches shows a bimodal pattern attributed to the shoaling behaviour of round herring which results in the absence of some intermediate length groups. The first mode represents small fish in the approximate standard length range 4,0- 12,5 cm and the second, large fish of about 15,0- 23,0 cm. In terms of age, the relative dominance of the modes changed during the period 1965- 1973, three-year-old fish being dominant in 1965, 1972 and 1973 and O-year-olds in the intervening years. The 1969 and 1970 year-classes thus seem to have been particularly strong. The occurrence of gonads in the active to ripe-and-running stages showed that 50 per cent of fish are sexually mature at a standard length of 17 ,0 cm (or an age of three years) and that all are mature at 22,0 cm (or five years). The presence of small fish in the samples in every month indicates that spawning takes place throughout the year . |
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