Field validation of morphological identification keys for Antarctic grenadiers (Macrourus spp.) with additional externally visible characters in Subareas 88.1 and 88.3

Grenadiers (Macrouridae) are abundant bycatch in the Southern Ocean toothfish fishery and represent an ecologically important prey group for Dissostichus mawsoni. In Subareas 88.1 and 88.3, Macrourus caml (QMC) and M. whitsoni (WGR) dominate, but their morphological similarity complicates rapid and accurate identification by observers onboard. This study evaluated a practical morphological key for field use, based on pelvic-fin ray counts and lower-jaw tooth rows, as proposed in WG-FSA-2023/32 Rev. 1. A total of 300 specimens collected over three consecutive fishing seasons (2022/23–2024/25) were independently identified by observers at sea and re-examined in the laboratory. Observer–laboratory agreement was quantified using confusion matrices, Cohen’s kappa, accuracy, balanced accuracy, macro-F1, and species-level precision and recall. Observer performance varied: one observer showed slight agreement beyond chance under prevalence-skewed samples, another achieved substantial agreement, and a third reached almost perfect agreement. The dominant error pathway was M. caml misidentified as M. whitsoni, with significant differences among observers confirmed by chi-square tests. Inclusion or exclusion of M. carinatus, which was rare, did not affect conclusions. In light of the observed error structure, two externally visible characters for on-deck use are proposed for prospective application: the anterior shape of the snout (assessed in strict ventral view) and body and fin colors. These characters will be incorporated starting next season. The morphological key proved to be field-usable across observers, though targeted refinement and training are recommended. These results provide the first quantitative evaluation of grenadier identification in Subareas 88.1 and 88.3, support improved bycatch statistics and ecosystem assessments within CCAMLR, and highlight operational pathways for enhancing observer training and standardization in future monitoring programs.