Factors Influencing Fish Composition in an Australian Intermittently Open Estuary. Is Stability Salinity-Dependent?

The ichthyofauna of the intermittently open Shellharbour Lagoon in southeastern Australia was sampled over 14 months to investigate factors influencing species composition. Zostera capricorni and bare sand habitats were sampled every two months with a fine-mesh seine. Of the 10 277 fish caught representing 19 families and 27 species, the most numerically dominant species were Pseudogobius olorum and Pseudomugil signifer. The most abundant economically important species were Acanthopagrus australis, Mugil cephalus and Myxus elongatus. Mean number of fish and species was significantly higher over Zostera than bare sand during months immediately after the estuary opened when juvenile marine fishes recruited, demonstrating the estuaryʹs nursery function. In contrast to other intermittently open estuaries in Australia and South Africa, Shellharbour Lagoonʹs fish assemblage appears resilient to perturbation and does not undergo distinct seasonal variations. This may be a result of the estuaryʹs short open phases providing less opportunity for marine fishes to enter. Perhaps more importantly, the less dramatic fluctuations in salinity between open and closed phases maintains salinity at a level intermediate of the tolerances of both marine and estuarine fishes.