Tag-loss or tag-shedding rates for tagged fish in the Patagonian toothfish (Dissostichus eleginoides) fisheries at Heard Island and McDonald Islands (HIMI) in Division 58.5.2 were estimated following the method proposed by Adam & Kirkwood (2001). Tag-loss rates were generally low, with longline-caught and recaptured fish losing their tags quicker than trawl-caught and recaptured fish. With an initial tag-loss proportion of 2.8% and ongoing tag-loss rate of 0.056 in the longline fishery, 99.3% of all double-tagged fish retain at least one tag after one year, and 80.2% after ten years. The parameter of annual tag-loss rate in CASAL’s single-tag model was approximated, for a maximum time at liberty of 6 years, as l = 0.014 for longline and l = 0.007 for trawl. However, this single tag-loss rate for longline varied strongly between time periods, with l = 0.007 for 2003-2006, l = 0.021 for 2007-2011, and l = 0.006 for 2012-2015. We recommend that for longline releases the parameter estimates for these time periods be used in the stock assessment for Patagonian toothfish in Division 58.5.2.
Dr Philippe Ziegler (Australia)
Dr Dirk Welsford (Australia)