Download FAR 2026/05 A mixture model approach to derive within-season, cohort-specific CPUE for arrow squid (Nototodarus sloanii)

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.