Catch Evaluation of Target, By-product and By-catch Species Taken by Gillnets and Longlines in the Shark Fishery of South-eastern Australia

摘要

Experimental demersal gillnets and demersal longlines were deployed from research vessels on grounds of Mustelus antarcticus during 1973-76. Gillnet mesh-size had major effects on catch composition and catch rate, whereas gillnet hanging ratio, hook-size, hook shank-length and hook-spacing had minor effects. The gillnets and longlines were much more effective at catching chondrichthyans than teleosts, and catches of species of cephalopoda, bivalvia, gastropoda, mammalia, aves and reptilia were negligible. Any reduction in the present legal minimum mesh-size of 6 inch the shark fishery would markedly increase by-catch. In gillnets monitored by scientific observers aboard commercial vessels during 1998-01, the ratio of the number of chondrichthyan toteleost animals was -24:1 in Bass Strait and -5:1 in South Australia. In Bass Strait between 1973-76 and 1998-2001, the catch rate by 6 inch mesh gillnets of chondrichthyans declined by one-third, whereas a change in the catch rate of teleosts was not statistically detectable. Most of this decline is explained by reductions of 54% by Cephaloscylliwn laticeps and of 87% by Galeorhinus galeus. The retained commercial catch was 74% of the chondrichthyan animals and 58% of the teleosts caught; only 3% of the chondrichthyans and 2% of the teleosts were discarded dead. There are occasional interactions with protected species (marine mammals and Carcharodon carcharias).