Life history of the dusky kob Argyrosomus japonicus (Sciaenidae) off the east coast of South Africa

Abstract

The life history of Argyrosomus japonicus off the east coast of South Africa was studied using biological, size composition and tagging data from fish caught in the estuaries, surf zone and nearshore marine environment of three coastal regions. Capture methods included hook and line (all habitats), seine-netting (estuaries and surf zone), and gill-netting and trawling (estuaries only). Median total length (TL) at maturity was 920 mm (5 years) for males and 1 070 mm (6 years) for females. All males > 1 100 mm TL (7 years) and all females > 1 200 mm TL (8 years) were mature. Adult fish are found predominantly in the nearshore marine environment, but they also frequent estuaries and the surf zone. Spawning takes place in the nearshore environment, from August to November in KwaZulu/Natal, and from October to January in the Southern and South-Eastern Cape regions. A large proportion of the adult population migrates to KwaZulu/Natal to spawn, although spawning may continue on their return to the Cape. Early juveniles of 20-30 mm (~4 weeks) are recruited into turbid estuaries along the entire East Coast. They remain in the upper regions of the estuaries until they grow to about 150 mm. Juveniles > 150 mm are found in estuaries and in the surf zone. Fish < 400 mm are found throughout estuaries, but larger specimens are less abundant in the upper reaches. Fish < 1 000 mm generally do not migrate long distances, remaining as separate allopatric populations around their natal estuaries until they reach maturity. It is postulated that the protection from predation afforded by estuarine and surf-zone nursery areas has allowed A. japonicus to evolve a life history with a large size at maturity, thereby 'preadapting' it to a migratory life-style. The life history of A. japonicus, including late maturity, the use of estuarine and surf-zone nursery areas, and a migratory adult population which forms concentrated spawning aggregations, is particularly vulnerable to the activities of man.