Age, growth, reproductive biology and diet of Spondyliosoma emarginatum (Cuvier 1830) in the West Coast National Park, South Africa

Master Thesis


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University of Cape Town

Comprehensive morphometric measurements and counts, external colouration, reproductive behaviour and diet of steentjies (Spondyliosoma emarginatum) in Langebaan Lagoon were recorded between September 2004 and February 2005. Sagittal otoliths were extracted and age estimates recorded from both whole and sectioned otoliths in order to estmate growth and mortality parameters. A total of 302 steentjies (139-312 mm fork length) was sampled of which 243 were females, 55 were males and four were individuals with hermaphroditic gonads. Macroscopic and microscopic examination of gonads and monthly gonadosomtic indices suggest a spring spawning season and that steentjies are protgynous hermaphrodites with males significantly than females (t = 64.75, df = 301, p < 0.001). During the reproductive season, males develop a blue colouration. All males with this colouration had testes classified as either active or ripe and had significantly larger gonad masses than those without mating colours (U = 3.5, Z = 6.48, p $lt; 0.001). The length-at-50%-maturity for females was 171 mm (FL) and for males was 204 mm (FL). The size at 50% sex change was estimted as 250 mm (FL). Analysis of foregut contents showed steentjies to be opportunitistic ominivores, changing from plant to animal food as they age. The average percent error and coefficient of variation values for whole otoliths were 4.59% and 6.00% respectively, and 10.89% and 14.53% for sectioned otoliths. Although the result of the whole otolith study provided more precise age estimates, there was a strong indication of bias so the sectioned otolith results were used for calculating the growth model. Growth (by sex) was modelled using a model developed for sex changing sparids that allows for accelerated growth after sex change. Steentjies appear to have shorter life spans than most sparids; the oldest individuals sampled attaining less than 7 years of age. Growth rates appear to be relatively high. The smallest, youngest fish were all female, with males only present in the larger size classes. The estimated mortality for steentjies was 0.7y[-1]. Such a high mortality rate implies that further increase in fishing pressure on this species may not be sustainable in Langebaan Lagoon.

Includes bibliographical references (leaves 47-50).