Intrusions of surface sewage plumes into continental shelf waters: Interactions with larval and presettlement juvenile fishes

Abstract

Primary treated sewage (domestic and industrial) is discharged through several shoreline and deepwater outfalls into the coastal waters off Sydney, southeastern Australia. The effluent discharged from the shoreline outfalls formed highly visible surface plumes that intruded up to 5 km seaward and 8 km along the coast from their point of discharge. Each sewage plume was a lens (1 to 5 m deep) of low-salinity turbid water that overlay high-salinity clear shelf water, and distinct frontal regions usually developed between plume and shelf water. Large numbers of young fishes in surface waters were caught in and around the sewage plumes. In particular, many fish were concentrated at the fronts (e.g Carangidae, Engraulididae Sparidae), which was probably the result of advection at fronts as well as behavioural responses of fish. Surface sewage plumes therefore affected small-scale (< 1 km) patterns of distribution and density of young fishes. The oceanography of sewage plumes may increase and prolong exposure of fishes to pollutants during early development. Sewage plumes could affect young fishes spawned near to and at great distances from outfalls because of alongshore currents, and effects on fish populations could be manifest over large spatial scales. The influences of sewage plumes on larger-scale patterns of distribution, transport and survival of young fishes are discussed.