Unravelling the mystery of a unique giant trevally Caranx ignobilis aggregation in the Mtentu Estuary, Pondoland Marine Protected Area, South Africa

Abstract

The giant trevally Caranx ignobilis (family Carangidae) is an iconic apex predatory fish valued by recreational and commercial fisheries globally. A seasonal but episodic aggregation of up to ∼1 000 individuals takes place in the Mtentu Estuary in the Eastern Cape Province, South Africa, every year; however, the reason behind this unique aggregation is unknown. With the aim of understanding various aspects of their movement patterns, 10 individuals (sized 430–1 030 mm FL) were tagged with acoustic transmitters in the estuary and subsequently monitored by passive acoustic receivers in the estuary and adjacent coastal environment for up to 3.6 years. The results showed clear movement patterns, including repeated daily migrations between the upper estuary (∼4 km upstream) during the day and the mouth region or into the sea at night. Statistical modelling revealed that sea temperature was the main environmental variable associated with their pattern of daily estuarine presence/absence. During summer, the daytime temperature of the surface waters in the deep and thermally stratified Mtentu Estuary may be 3–6 °C warmer than the subsurface waters, providing a more suitable thermal environment for this tropical to subtropical species. Notwithstanding the possibility of other environmental drivers, we propose that adult C. ignobilis utilise the Mtentu Estuary during periods of cold coastal upwelling, primarily as a thermal refuge, specifically for daily re-warming after nocturnal feeding in colder water. The maintenance of this site as a no-take (no fishing) zone, as well as encouragement of tourism-linked incentives to increase local custodianship, are strongly recommended to protect this vulnerable aggregation.