Otolith morphology and population dynamics of grenadier bycatch to improve fishery observer species identification and fisheries management at the Heard and McDonald Islands

<p dir="ltr">Species identification accuracy is crucial within fisheries management as species contain different fishery sensitivities. Observer’s onboard fishery vessels identify target and bycatch species while recording haul depth, time, and catch length and weight data. However, the acknowledged yet understudied issue of species identification is reliant upon fisheries observers and their training. Variable observer experience and challenging working environments stimulate species misidentification, potentially presenting data bias. When commercial fisheries catch morphologically similar species, observer misidentification can lead to incorrect species within historical catch records and species population management. <br>The Patagonian toothfish bottom longline fishery of the Heard and McDonald Islands (HIMI) has four <i>Macrourus</i> grenadier species known as bycatch. Morphological and distributional differences have split the four grenadier’s bycatch limits from a pooled genus group into pairs ‘<i>Macrourus caml</i> and <i>M. whitsoni</i>’ and ‘<i>M. holotrachys</i> and <i>M.</i> <i>carinatus</i>’ respectively, however, similar characteristics between paired species contribute to fishery observer misidentification. Understanding how to improve observer accuracy is important for accurate <i>Macrourus</i> stock information at HIMI. <br>This thesis provides a literature review supporting the research manuscript examining baseline descriptions of <i>Macrourus</i> species biological data and otolith shape morphology, to improve fishery observer identification accuracy. We studied a total of 800 otoliths covering all four Macrourus species collected from HIMI's longline fishery (2015–2021). Otolith shape features were analysed to discern the four species, while random forest models were conducted utilising otolith morphometrics to assess accuracy between species pairs and identify important otolith shapes differentiating species. Age structures were constructed to potentially indicate longline, depth and location impacts. Our research presents baseline knowledge valuable for current and future observers, provides a novel tool that highlights species identification accuracy to determine training for individual observers, and forms the basis of regional impact on HIMI <i>Macrourus</i> species stock. <br>This thesis provides two chapters. Chapter one presents an extensive background literature review to the study field, rationale for research manuscript, research scope and limitations. Chapter two presents a manuscript prepared for submission to the Journal ‘<i>Fisheries Research</i>’ and has been formatted following the journal’s guidelines. Therefore, the manuscript and chapter one literature review contain content overlap. References are provided separately for chapter one and chapter two following identical formats of ‘<i>Fisheries Research</i>’ journal.</p>