Movements, recapture patterns, and factors affecting the return rate of carcharhinid and other sharks tagged off northern Australia

Abstract

Between February 1983 and May 1985, ~10 500 sharks of 23 species were fin-tagged off northern Australia. Tagging concentrated on the commercially important Carcharhinus tilstoni and C. sorrah. Most recaptures were made in 1984 and 1985, but returns continued until May 1997. In all, 579 tags (5.5%) were recovered. Tag shedding was estimated to be low (0.025 year ^–1 for C. tilstoni) and tagging mortality was significantly lower for sharks caught by hand-line than by gill-net. Australian gill-netters, Taiwanese gill-netters (fishing in the Australian Fishing Zone) and Australian prawn trawlers accounted for most of the returns. The maximum distance between the release and recapture positions was >1100 km, but most returns were made within 50 km of the tagging site. Nearly all the releases were in inshore waters fished by Australian vessels. Although many recaptures were made by the offshore Taiwanese fishery, the Taiwanese fishing effort was much higher than for the inshore Australian fishery, so that relative to fishing effort, relatively few sharks moved from inshore to offshore waters.