Descriptive analysis of the Antarctic toothfish
(Dissostichus mawsoni) tagging scheme in the Ross Sea
for the years 1997/98 to 2003/04

A descriptive analysis of the toothfish tagging programme carried out in the Ross Sea since 2001 is presented for the first time. Tag-release and tag-recapture data are presented for both toothfish species for Subareas 88.1 and 88.2 for New Zealand vessels only. This is because data from non-New Zealand vessels were unavailable at the time of the analysis. A total of 4903 Antarctic toothfish have been released and 89 recaptured, and 443 Patagonian toothfish released and 9 recaptured. For the last two years, when tagging has been part of the Conservation Measure, New Zealand vessels have tagged between 1.0 and 1.37 toothfish per tonne of catch. Tagging rates by area over the past three years have been in the same proportion as the catch by area. However, recapture rates have tended to be higher in the northern and eastern SSRUs 88.1C and 88.2E.
The maximum movement of Antarctic toothfish from the New Zealand data set has been about 200 km. However, most (80%) Antarctic toothfish have moved less than 50 km. Consequently, nearly all fish have been recaptured from the same SSRU where they were released. The mean size of tagged Antarctic toothfish has increased since 2001, but is still smaller than the mean size of fish taken in the commercial catch. Larger toothfish (>35 kg) are difficult to tag without significant damage to fish, and there appears to be a trade-off between maximising size of released fish and minimising tagging mortality. Growth rates of Antarctic toothfish that have been at liberty for 2–3 years have averaged 5– 7 cm per year, which is consistent with growth rates predicted from the von Bertalanffy growth curve. The preliminary estimate of tag loss from double tagging experiments is 13% per year.