Sharks caught in the protective gill nets off KwaZulu-Natal, South Africa. 10. The dusky shark Carcharhinus obscurus (Lesueur 1818)

Abstract

Between 1978 and 1999, a total of 5 626 dusky sharks Carcharhinus obscurus, constituting 20% of the total shark catch, was caught in the protective nets off KwaZulu-Natal, South Africa. The mean annual catch was 256 sharks (SD = 107.5, range 129–571). There was no significant linear trend in catch rate with time. Of the total C. obscurus catch, 677 (12%) were found alive and 217 of these (3.8%) were tagged and released. Only three tagged animals were recaptured. The size frequency distribution of the catch was trimodal, the modes of each sex consisting of small (mostly neonate), medium (adolescent) and large (mostly mature) sharks respectively. Geographical and seasonal distributions were characteristic for each of these size categories. Females significantly outnumbered males in all size categories, the greatest disparity (2.72:1) being in large animals. This probably reflects the movement inshore of near-term pregnant females to drop their pups. On 128 occasions, groups of five or more sharks were found together in a net installation, the largest group consisting of 113 animals. Group catches tended to coincide with the annual 'sardine run', a seasonal influx of Sardinops sagax. The sardine run affects the distribution of medium and large sharks. Few animals were sampled in mating condition and there were few newly pregnant or midterm females, but there are indications that the gestation period may be as much as two years. Teleosts dominated the diet in terms of frequency of occurrence (63%) and elasmobranchs in terms of mass (51.4%).

Keywords:

• CPUE
• DISTRIBUTION
• EMBRYOS
• GILLNETS
• LENGTH FREQUENCY
• LENGTH-WEIGHT RELATIONSHIPS
• MATURITY
• NURSERY GROUNDS
• REPRODUCTION
• SEASONALITY
• STOMACH CONTENTS
• TAGGING