Quantifying distinctions in the otolith shape of morphologically similar Sub-Antarctic grenadier species (Macrourus) to assess fishery observer identifications

Accurate bycatch species identification is crucial in commercial fisheries, as species vary in vulnerability to fishing pressure. Fishery observer experience also varies, which can form inaccurate commercial fishing datasets and compromise species-level identifications. At the sub-Antarctic Heard Island and McDonald Islands (HIMI),
four grenadier (Macrourus) species represent a high bycatch percentage in the Patagonian toothfish (Dissostichus eleginoides) demersal longline fishery. Current fisheries bycatch limits for Macrourus species are categorised into two species pair groups: shallow-water living (Macrourus caml, M. whitsoni) and deep-water living (M. holotrachys, M. carinatus). However, misidentification by fishery observers due to similar characteristics limits high taxonomic resolution within catch records. Therefore, improving observer accuracy of bycatch stock identification from pooled Macrourus Genus level to species level is required within HIMI fisheries. We photographed 800 otoliths across the four Macrourus species collected from HIMI’s longline fishery bycatch (2015–2021). Macrourus otolith shape and outline analysis was conducted using shapeR, while random forest models (RFM) coupled initial observer identifications with otolith morphometrics to predict species identification within each bycatch pair. RFM prediction accuracy varied from > 95 % for M. holotrachys and M. caml to 70 % and 60 % for M. carinatus and M. whitsoni respectively, highlighting otolith width, perimeter, and length as important predictor morphometrics. Otolith elliptical Fourier constructions of M. whitsoni and M. carinatus displayed dorsal margins with prominent outward shifts, which can visually assist onboard identification of these species. Overall, our research establishes baseline otolith morphometric data for four Macrourus species, supporting the refinement of historical Genus-level bycatch identifications to each species pair group at HIMI’s commercial fishery.