This study used an operating model to simulate a Dissostichus eleginoides population in Research Block 5843a_1 with a fixed exploitation rate and various tagging rate scenarios to assess the impact on the numbers of tagged fish recaptures and accuracy of tag-based estimates using the Chapman estimator. The results showed that tag-based biomass estimates obtained from a low numbers of recaptures will have a high risk of over-estimating biomass and that for a given number of tagged fish available a minimum numbers of recaptures are required to ensure a low likelihood of biomass overestimation. For Chapman-estimates to achieve a 25% level of accuracy at least 50% of the time, required a minimum of 6 recaptures when there were approximately 270 tagged fish available for recapture. To increase the chance of achieving the same level of accuracy to 80%, a minimum of 20 tagged fish recaptures were required when approximately 815 tagged fish were available.
The minimum number of recaptures required to achieve a specified level of accuracy will vary depending on the initial population size, the exploitation and tagging rate and the consequent number of tagged fish available. Simulations of toothfish populations could be performed for all Research Blocks to assess the data requirements to obtain Chapman estimates with a high likelihood of being within a desired level of accuracy. Based on these simulations, the Working Group consider developing criteria such as a minimum number of annual tag-recaptures required in each Research Block to determine when it is appropriate to use a tag-based estimate of biomass.