Age and growth of the Queen Mackerel (Scomberomorus plurilineatus) and Seventy-four (Polysteganus undulosus) of KwaZulu Natal, South Africa.

Abstract

This study provides information on the age and growth of two important Iinefish species, Scomberomorus pluriline~tus and Polysteganus undulosus. Age determination for both species was carried out using otoliths and growth was modelled using age- and length-based methods. For the age-based method various growth models were evaluated to determine which growth function best described the age-length data, whilst Shepherd's Length Composition Analysis was used to estimate growth parameters from length-frequency data. Preliminary stock assessments, based on limited catch data, were also attempted for both species. Age estimates for the pelagic migrant S. plurilineatus, derived from reading whole otoliths, ranged from 0 + to 6 + years. As validation by marginal increment analysis was inconclusive because of the seasonal occurrence of this species in KwaZulu-Natal waters, it was assumed that a single opaque band was laid down in the otolith annually. Reproducibility of age estimates evaluated using the average percentage error (APE) technique was good (9.4%). Von Bertalanffy growth parameters were poorly estimated from length-frequency data because multiple maxima were encountered on the fitting surface. However, from the age-length data, growth was adequately modelled by the von Bertalanffy growth equation: L t =9 3 5mmFL (l-e -0. 583yr-1 (t+o. 991yr) ) S. p!urilineatus are fully recruited to the fishery at the age of 1 + year and the ageiv at-50% maturity is 2 + years. Preliminary per-recruit analyses indicated that the spawner biomass of S. plurilineatus is at 50% of its unfished level. Polysteganus undulosus is an endemic, reef-dwelling sparid and large catches weremade earlier in the century. Age determination was carried out using sectioned otoliths collected in 1962 and 1963 before the collapse of the fishery. Age estimates ranged from 3 + to 20 + years. Marginal increment analysis indicated that active deposition of opaque bands occurred during winter but, because of the seasonal occurrence of P. undulosus in KwaZulu-Natal, validation was inconclusive. Reproducibility of the age estimates was low (APE = 18.2%) because of difficulties with band interpretation as a result of stacking on otolith margins in old fish. Von Bertalanffy growth parameters could not be adequately estimated from length frequency data because of the slow growth and longevity of this species. However, from the age-length data, no difference in growth rate between the sexes was observed, and growth for the combined sexes is described by the following logistic equation: L = 942mmTL t 1+e-O.277yr-l(t-S.178yrs) The age at full recruitment was found to be 12 + years and the age-at-50% maturity was 8.8 years. A preliminary stock assessment revealed that the spawner biomass of P. undulosus was already at 25% of its unfished level in the early 1960s.