Riding the tide: estuarine movements of a sciaenid fish, Argyrosomus japonicus

The movement behaviour of juvenile dusky kob Argyrosomus japonicus (n = 25) was studied in the 12 km long Great Fish Estuary, South Africa, by collecting telemetry data in relation to tidal conditions over 6.5 mo. Two behavioural patterns were observed. Tagged fish either moved in circatidal rhythms over long stretches up and down the estuary, corresponding to the speed and direction of the rise and fall of the tide, or remained stationary, often in deeper or structured habitats. All individuals alternated between moving (mean 61% of the time) and stationary behaviour. Moving periods occurred during both light and dark periods. However, up-estuary movements were initiated most frequently at night, while down-estuary movements were initiated most frequently at dawn and dusk. The mean distance of the unidirectional movements was approximately 3.2 km. Ground speed for up-estuary movements (0.29 m s(-1)) was faster than for down-estuary movements (0.22 m s(-1)). These findings suggest that juvenile A. japonicus adopt 2 distinct feeding strategies within their turbid estuarine nursery habitats. 'Riding the tide', with minimal energy expenditure, may be a successful strategy when feeding on prey that follow the tidal current. Alternatively, in periods when they adopt a stationary behaviour, they may fulfil their foraging requirements by feeding on stationary and/or passing prey, while at the same time reducing predation risk in sheltered, structured habitats.