Estimation of tag-loss rates for tagged fish in the Patagonian toothfish (Dissostichus eleginoides) fisheries at Heard Island and McDonald Islands in Division 58.5.2

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.