AGES AND GROWTH-RATES OF AGULHAS BANK HORSE MACKEREL TRACHURUS-TRACHURUS-CAPENSIS - COMPARISON OF OTOLITH AGING AND LENGTH FREQUENCY ANALYSES

A total of 138 433 horse mackerel was measured on the Agulhas Bank dur ing four surveys in the years 1990-1993. Length frequency analyses ser ved to separate the first four component distributions in the annual w eighted length frequency distributions and to quantify longitudinal an d depth-related shifts of the modal lengths in the length distribution s in 1992 and 1993. Otoliths of 1 364 specimens were aged. The precisi on of the readings was assessed by the Average Percentage Error. Lengt hs-at-age obtained from otolith ageing and length frequency analyses w ith Bhattacharya's method and MULTIFAN agreed well. Horse mackerel on the Agulhas Bank grow slower than previously thought. Except for a lon gitudinal trend, growth rates of West and South Coast specimens appear to be similar. Mean annual length increments amounted to 15, 7, 5 and 4 cm during the first four years of life and so resembled the growth rates of Namibian specimens. Lengths-at-age in most age-groups also in creased with increasing depth and were inversely related to bottom tem perature. Bimodal length frequency distributions in several of the age -groups point at two spawning units and/or two reproductive seasons. T he two-stock hypothesis is discussed and recommendations for future su rveys are given.