Estuarine use by spotted grunter Pomadasys commersonnii in a South African estuary, as determined by acoustic telemetry

Abstract

Factors influencing estuarine use and marine excursions by spotted grunter Pomadasys commersonnii in the Great Fish Estuary, South Africa, were studied using manual and automated telemetry methods. In all, 20 individuals, ranging from 362 mm to 698 mm total length (TL), were caught and tagged with acousticcoded transmitters in the estuary. Tagged individuals spent most time in the estuary (67%), interspersed with a number of marine excursions — 15 in all. The number (6–53) and mean duration (0.8–9.5 days) of sea excursions varied among individuals, and were correlated significantly with tide, time of day, fish size and various environmental conditions. On average, fish departed the estuary on the outgoing tide at night and returned to the estuary on the incoming tide during the day. Spotted grunter <400 mm TL spent more time in the estuary than larger fish (403–698 mm TL). The number of fish recorded in the estuary was significantly correlated with barometric pressure, wind direction and sea temperature. The spotted grunter under study exhibited a high degree of estuarine use, highlighting the importance of this habitat to this overexploited, estuarinedependent fishery species.

Keywords: environmental variables; estuarine-dependence; estuarine conservation; fish movement; Great Fish River

African Journal of Marine Science 2008, 30(1): 123–132