Diel and tidal variations in larval fish exchange in the mouth region of the Gamtoos Estuary, South Africa
Diel and tidal variations in density of larval fishes were monitored over one neap and one spring tidal cycle in the mouth region of the warm temperate Gamtoos Estuary, South Africa. Data were collected over two 24h periods, using mixed method sampling with WP2 nets and a pushnet to sample both channel and margin waters. Larval fishes collected included 20 species from 12 families. Density varied with tide and diel state during the study period, with significantly higher catches recorded on the spring tide. Density of larval fishes peaked after the onset of darkness, predominantly associated with the nocturnal ebb tide. Large numbers of preflexion gobiid larvae were recorded on this tide state, this behaviour serving to facilitate movement out of the estuary for gobies that undergo a marine phase in their development. Spatial variations in larval fish distribution were also noted. Preflexion larvae were caught mainly in mid-channel waters, whilst postflexion larvae were more often encountered in the slower-flowing marginal waters. Data for a hydrodynamic model of tidal exchange of fish larvae in the estuary mouth region were collected concurrently with sampling and were used to model exchange of larval fishes over the neap and spring tidal cycle by extrapolating actual densities, using modelled tidal volume. Findings suggest that for some species, tidal exchange could escalate to millions of individuals over a single tidal cycle.