Variation in population structure and life-history parameters of steentjies Spondyliosoma emarginatum: effects of exploitation and biogeography

Abstract

The size and age structures, sex ratio, size- and age-at-sex-change, growth and mortality rate of the protogynous hermaphrodite, steentjie Spondyliosoma emarginatum, were compared to determine the effects on population structure and life-history parameters of exploitation (comparing an unexploited reserve and an exploited area in Langebaan Lagoon) and biogeography (comparing Langebaan Lagoon and Struisbaai on the south-west coast of South Africa). There were few large fish and a sex ratio skewed towards relatively more females in the exploited area compared with the reserve in Langebaan. The age structure of female steentjies was truncated in the exploited area but the male age structure was unaffected, possibly because of the poor relationship between size and age of males caused by accelerated growth-at-sex-change. There were no differences in the size- and age-at-sex-change between the reserve and the exploited area, suggesting that steentjies have not shown a physiological response to exploitation in Langebaan and are unable to compensate for a skewed sex ratio. The comparison between Struisbaai and Langebaan showed that the growth rate of steentjies in Struisbaai was slower and the size-at-sex-change smaller than those at Langebaan. Linefish stock assessments may have to account for regional differences in physiological parameters.