By-catch of the Prawn Fishery of Torres Strait; Composition and Partitioning of the discards into components that float of sink

Abstract

The quantity and composition of the by-catch from commercial prawn trawling in Torres Strait, Australia, were assessed from the results of seven trawl surveys and the fishery effort from log-book records, over 2 years. The total weight of by-catch caught in the fishery was estimated at 6930 t (±900 t) in 1985 and 4630 t (±710 t) in 1986. A wide range of marine organisms including cephalo- pods, crabs, rock lobsters, scallops, sharks, rays, snakes and turtles were caught, but teleost fishes were the largest component: 5520 t (±970 t) in 1985 and 2910 t (±510 t) in 1986. Nemipteridae and Synodontidae, the main teleost families, made up 15% and 13% of the teleost catch by weight, respectively. Apart from small quantities of shovel-nose lobsters (Thenus orientalis), rock lobsters (Panulirus ornatus) and squid, all the by-catch (approximately 99%) was discarded. There was little change in the composition of the fish by-catch between the 2 years but there was a noticeable change in the fish : prawn ratio from 5.1 : 1 in 1985 to 3.3 : 1 in 1986.

Samples of the Torres Strait trawl by-catch from trawl surveys in 1988 were used to estimate the component of the by-catch which floated (available to surface predators) and that which sank (available to mid-water and demersal predators and scavengers). Most of the discards sank (-70%) but an estimated 40% of the teleost catch (1200-2200 t) floated. Of the teleosts, over 75% of the Apogonidae, Nemipteridae, and Monacanthidae floated, whereas all Synodontidae, Mullidae and Bothidae sank. Although for five fish families there was a difference in the proportion that floated when shallow water (= < 25 m) trawls were compared with deeper water (26-55 m) trawls, this did not affect the estimates of by-catch that floated or sank.