Fishes of Dee Why lagoon: Species composition and factors affecting distribution

Dee Why Lagoon (33o45’S, 151 o19’E) is one of several small coastal lagoons in the Sydney area. It has a maximum surface area of 30 ha and is situated in a heavily urbanised catchment of 500 ha. In 1973 the lagoon and its surroundings were declared a wildlife refuge. The lagoon is closed about 70% of the time by a sand bar across its entrance (NSW Public Works Department, unpublished data). The bar washes away when heavy rain in the catchment raises the water level in the lagoon. On other occasions, the lagoon is deliberately opened to alleviate flooding of property in its catchment (Gordon and Cooke, 1977). Following an ‘opening’, the lagoon’s surface area is reduced to less than 12 ha. The bar is gradually rebuilt by sand transported in the ocean beach ‘swash’ zone. This results in partial flushing of the lagoon by marine waters over extended periods. Opening and flushing subject the lagoon to drastic salinity changes, which together with contaminated storm-water runoff, create an environment characterised by rapid changes in water quality.