Gear modifications and alternative baits that reduce bait scavenging and minimize by-catch on baited drum-lines used in the Queensland Shark Control Program

Abstract

Bycatch minimisation is an important issue for fisheries managers worldwide and is particularly important in the Queensland Shark Control Program (QSCP) since marine turtles, cetaceans and other marine life are incidentally caught along with the targeted shark species. Different hook modifications and baits were trialled on drum-lines used by the QSCP in an effort to reduce the incidental capture of turtles and to deter dolphins from scavenging drum-line baits. Double hook lines did not differ significantly (Wilcoxon matched pairs test T=8.5, n=8, P>0.05) in shark catch compared with single hook lines although they were more likely to catch green turtles ( Chelonia mydas (L.)). Plastic hook guards reduced the turtle bycatch by almost 70% but shark catch also declined significantly ( T=10, n=11, P<0.05) by 30%. Both mesh guards and hook shrouds dramatically reduced scavenging of baits by dolphins in the short-term but they may also increase the turtle catch and their impact on shark catch was inconclusive. None of the baits tested significantly reduced the incidental capture of marine turtles but several baits (particularly frozen shark flesh) reduced scavenging by dolphins. Mullet ( Mugil cephalus (L.)) baits on drum-lines were removed by dolphins within an average of nine seconds of being placed in the water in some areas, with scavenging being more prevalent in southern areas where dolphin/human interactions are more prevalent. Chicken frames and whole chicken were also scavenged by both dolphins and turtles although less intensively than either mullet or carp ( Cyprinus carpio (L.)) baits.