Arrow squid represent an important fishery resource, but their fast growth and relatively short lifespan make stocks difficult to assess and manage. This study focused on developing a novel method to distinguish different squid cohorts based on size and catch-per-unit-effort (CPUE), across different areas and over time. Using information from trawl fisheries on the Stewart-Snares-shelf and at Auckland Islands, the assessment assumed two dominant cohorts in each area, occurring in autumn (early season) and spring (late season). This assumption was used to develop a mixture model (i.e., a probabilistic model) to determine the proportional contribution of each cohort to a given fishing event, which, in turn, allowed simultaneous estimation of CPUE for each cohort. The model indicated consistent depletion (i.e., decreases in squid over time) for the late-season cohort, but there was no consistent trend for the early-season cohort. This proof of concept highlights the potential to further develop this approach for providing stock assessments for squid in these trawl fisheries.
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