Marine fish larvae on the move – behavioural variability and flexibility and their implications for dispersal

Jeffrey M Leis (1, 2)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania 7005,

2 Ichthyology, Australian Museum Research Institute, Sydney, New South Wales 2010

Most marine, demersal, bony fishes have a complex life history including potentially dispersive, pelagic larval stages. Dispersal of marine fish larvae, once thought to be essentially a physical process with passive larvae, is now recognized as a biophysical process in which behaviour of both adults and larvae plays an influential role. The where and when of spawning (adult behaviour) and whether eggs are planktonic, strongly influence dispersal trajectories and outcomes. Currents vary in velocity with depth, so vertical distribution indirectly influences horizontal movement even in weakly swimming, smaller larvae. Although larval fishes are small, when swimming is orientated, swimming abilities of larvae allow both escape from viscous hydrodynamic forces, and direct influence on dispersal outcomes.  All behaviours change ontogenetically.  It is important to distinguish variance among individuals around mean behaviours from behaviours with temporal or spatial variation in central tendency. Behavioural variance among individual larvae is high, which ensures that dispersal outcomes will be varied. Behaviours that are temporally or spatially variable introduce additional temporal and spatial variation in dispersal outcomes.  Laboratory and in situ observations and experiments providing examples of both types of behavioural variation are discussed along with their implications for dispersal. Locating settlement habitat depends on sensory ontogeny, and on sensory cues, both those that emanate from settlement habitats, and those that do not. The settlement transition from pelagic to demersal habitat is dangerous. Post-settlement processes determine if dispersal and successful settlement result in population connectivity and/or range changes, or not.