Fate of discards from Prawn Trawlers in Torres Strait

Abstract

A study was made of the fate of teleosts, non-commercial crustaceans and cephalopods discarded from trawlers in Torres Strait. These groups make up about 80% of the discards by weight, have a high mortality rate and are therefore the most likely animals to be eaten by scavengers. The remaining 20% of discards consists of animals such as turtles, sharks, bivalves and sponges, which are caught in low numbers and appear to have a low mortality from trawling.

Fish made up 78%, non-commerical crustaceans 18%, and cephalopods 3% by weight of the material studied. Nearly all fish were dead when discarded, and about half sank. About half of the non-commercial crustaceans were alive when discarded and all sank when discarded. Few cephalopods (2%) were alive when discarded, and around 75% sank.

Sharks and dolphins were the most common scavengers of floating discards at night. Birds (common and crested terns, and lesser and greater frigates) scavenged only during the day. Discards that sank did so rapidly, taking less than 5 min to reach 25 m depth. A high rate of loss of baits set for 10 min in the water column (24% in trawled area at night) indicated significant scavenging in mid- water-probably by sharks. Observations of baits set on the bottom showed that teleosts (nemipterids) and sharks ate most of the material that reached the bottom; scavenging by invertebrates was negligible.

In an adjacent area that had not been trawled for 8 years, no dolphins and fewer birds were seen scavenging floating discards but there were more sharks. In this area, significantly fewer fish were attracted to a bait on the bottom at night compared with the trawled area. The cause of the difference in scavenging observed between the two areas is not known; while it may reflect learned behaviour by some scavengers such as birds and dolphins, there may also be intrinsic differences between the two areas unrelated to trawling.

Discarding from trawlers has the effect of transferring large quantities of biological material from the bottom to the surface. This makes available to surface scavengers food that would otherwise be inaccessible.