Larval supply and settlement success of estuary-dependent fishes

Many coastal marine fish species that are targeted by commercial and recreational fishers have an estuary-dependent juvenile phase.

Research Scientist: Dr Tom Trnski

Many coastal marine fish species that are targeted by commercial and recreational fishers have an estuary-dependent juvenile phase. In these species, juveniles require the estuarine environment to develop. The juveniles use the estuary for shelter from predators and strong currents, as a source of food, and perhaps their development is faster in an estuary than would occur in the marine environment.

My interest is in the physical and behavioural influences that determine the successful recruitment of estuary-dependent fishes to their estuarine habitat. My current research project is based at Lake Macquarie on the New South Wales central coast. Lake Macquarie is a large estuary connected to the ocean by the narrow Swansea Channel. This channel restricts tidal flow into and out of the estuary, and all fish larvae entering (and leaving) the estuary must pass through the channel. I measure the abundance of settlement-stage larvae entering the estuary through Swansea Channel with a fixed, fine-meshed channel net. Variation in abundance of the estuary-dependent fish larvae is compared with physical factors such as wind speed and direction, rainfall, atmospheric pressure, lunar phase, and magnitude of the flood tide. My aim is to determine which of these physical factors are important in delivering settlement-stage larvae to the estuary.

The four species under investigation are Yellowfin Bream (Acanthopagrus australis), Tarwhine (Rhabdosargus sarba), Snapper (Pagrus auratus), all in the family Sparidae, and the Luderick (Girella tricuspidata), family Girellidae. All four species are targeted by commercial and recreational fishers. All except the Snapper are dependent on estuarine seagrass beds to complete their juvenile phase. These three species are largely responsible for the perception that local seagrass beds have a nursery role. Snapper seem to prefer soft mud habitats while they are juveniles, and as juveniles are not entirely dependent on estuaries.

The behaviour of the settlement-stage larvae of these four species also plays a role in their recruitment success. The speed and directionality of their swimming, and their vertical distribution in the water column all play a role in determining where they settle in the estuary. For example if the larvae swim near the surface, they are more prone to influence by wind-driven currents in the estuary. Behavioural observations on Bream, Tarwhine and Luderick indicate they generally swim within a few centimetres of the surface. This means wind strength and direction play a role in dispersing the larvae in the estuary.