Occurrence of dolphins and seabirds and their consumption of by-catch during prawn trawling in Spencer Gulf, South Australia

Abstract

In the Spencer Gulf prawn fishery prawns are caught at night by trawling. Groups of dolphins, the short-beaked common dolphin ( Delphinus delphis) and the bottlenose dolphin ( Tursiops truncatus), and seabirds, the silver gull and the pacific gull ( Larus novaehollandiae, Larus pacificus), feed on discarded by-catch. The dolphins follow the cod ends as they appear close to or on the surface. Here they pick or bite fish protruding through the cod end mesh or forage on discards. Seabirds pick floating objects. In order to quantify the importance of dolphins and seabirds as consumers of discarded by-catch, four series of surveys were undertaken using prawn trawlers engaged in commercial fishing thereby obtaining a large number of observations simultaneously over a large area and at different seasons. On three successive nights, each night on three occasions, after the first trawl shot, at midnight and after the last shot of the night, the trawler skippers recorded the numbers of seabirds and dolphins feeding on discarded by-catch. The mean number of dolphins per boat and observation varied between seasons with 0.5–1.3 dolphins/boat/observation. At the northerly Site 1, 3.4 dolphins were recorded per boat and observation while at the more southerly sites (Sites 2–5), 0.5–0.8 dolphins were observed per boat and observation. No significant difference between observations at night was observed. The occurrence of seabirds varied between sites but not between season and time of night. The largest numbers of seabirds was observed at Sites 1 and 4 with a mean of 2.2 and 2.8 seabirds per observation. It was concluded that an estimated 18–183 t of discards are consumed per year by dolphins constituting 0.3–2.6% of the discarded by-catch. Seabirds potentially consume 1647–165 kg discards per year from the Spencer Gulf prawn fishery, which constitute less than 0.03% of the discarded by-catch.