MEPS 242:205-214 (2002)  -  doi:10.3354/meps242205

Behaviour of settlement-stage larvae of fishes with an estuarine juvenile phase: in situ observations in a warm-temperate estuary

Thomas Trnski*

Ichthyology, Division of Vertebrate Zoology, Australian Museum, 6 College Street, Sydney, New South Wales 2010, and Department of Environmental Sciences, University of Technology, PO Box 123, Broadway, Sydney, New South Wales 2007, Australia
*Correspondence address. E-mail:

ABSTRACT: Observations of behaviour were made on a total of 103 settlement-stage larvae (9 to 15 mm standard length [SL]) of 4 species of estuary-dependent fishes in Lake Macquarie, a barrier estuary on the central coast of New South Wales, Australia. Larvae were released at mid-depth over deep water or within sight of settlement habitat, and their behaviour was observed by SCUBA divers. Average near-surface swimming speed was 6.4 cm s-1 (7.1 body lengths [BL] s-1) for Acanthopagrus australis, 6.4 cm s-1 (6.5 BL s-1) for Rhabdosargus sarba, 10.9 cm s-1 (10.8 BL s-1) for Pagrus auratus (all family Sparidae) and 12.5 cm s-1 (11.2 BL s-1) for Girella tricuspidata (Girellidae). These speeds were faster than previously recorded for temperate fish larvae. It is hypothesised that body morphology determines swimming ability. At these speeds, the larvae are faster than ambient currents. The majority of larvae of all species swam in a directional (non-random) manner, but there was no evidence that the larvae were orienting to the shoreline or the sun. There were species-specific differences in depth profile: the 3 species that settle in seagrass beds preferentially swam near the surface; P. auratus, which settles over soft bottoms, preferentially swam to the bottom. Swimming near the surface in Lake Macquarie brings larvae into strong wind-driven surface currents that carry them to a shoreline and settlement habitat. Larvae of only G. tricuspidata (1 of 3 species that settle in seagrass) were rarely observed to settle when they were released within sight of a seagrass margin.


KEY WORDS: Larva · Fish · Behaviour · Swimming speed · Settlement · Orientation · Acanthopagrus · Girella · Pagrus · Rhabdosargus


Full text in pdf format