Pairwise tag performance: testing the sensitivity of the tag detection index and the mortality of tagged fish index
Vessel-specific indices of tag detection rates and tagging mortality rates were developed in 2012 and further developed in 2013, using a methodology which controls for the spatial and temporal variability of fishing operations by pairing each individual tag release or recapture event with all other fishing events which occurred in the same time and place (i.e. within a specific distance and in the same fishing season). The method showed that when the confounding effects of variable fishing location and time were controlled there were nonetheless significant differences between vessels with respect to the tag detection index in the Ross Sea region.
Here we present the results of simulations which indicate that the index of tagging mortality rate obtained using this method is not as useful as is the index of tag detection rate. We recommend that in the absence of a better index of tagging mortality, the tag detection index be used to select data for use in the 2013 toothfish stock assessments for the Ross Sea and Subarea 88.2 fisheries, with respect to both tag releases and tag recaptures. This recommendation is in accordance with that of most of the participants at WG-SAM 2013 (paragraph 4.7).