Ages and growth rates of agulhas bank horse mackerel trachurus trachurus capensis — Comparison of otolith ageing and length frequency analyses

Abstract

A total of 138 433 horse mackerel was measured on the Agulhas Bank during four surveys in the years 1990–1993. Length frequency analyses served to separate the first four component distributions in the annual weighted length frequency distributions and to quantify longitudinal and depth-related shifts of the modal lengths in the length distributions in 1992 and 1993. Otoliths of 1 364 specimens were aged. The precision of the readings was assessed by the Average Percentage Error. Lengths-at-age obtained from otolith ageing and length frequency analyses with Bhattacharya’s method and MULTIFAN agreed well. Horse mackerel on the Agulhas Bank grow slower than previously thought. Except for a longitudinal 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 increased with increasing depth and were inversely related to bottom temperature. Bimodal length frequency distributions in several of the age-groups point at two spawning units and/or two reproductive seasons. The two-stock hypothesis is discussed and recommendations for future surveys are given. © 1995 Taylor & Francis Group, LLC.