Updated longline bycatch estimates in the WCPO

This report updates regional estimates of longline catches, covering the full range of finfish, billfish, shark and ray, marine mammal and sea turtle species that have been recorded in longline observer data. The estimates do not cover domestic longline fisheries in the west-tropical sector of the WCPFC-CA, as SPC holds little representative observer data for these fisheries. Reported catches were used where available, i.e. for bigeye, yellowfin, albacore and skipjack tuna, and billfish species.
It is difficult to obtain reliable estimates of WCPO longline catches from observer data, given the low levels and imbalanced nature of observer coverage, and additionally the low coverage of available aggregate effort data disaggregated by hooks between float in the mid-2000s. Observer coverage has been particularly low in the north west Pacific. As such, the catch estimates for the region north of 10°N, and consequently the catch estimates for the WCPFC Convention Area as a whole, are unlikely to be reliable and should be viewed in that context.
Introduction of flag effects to the catch rate models improved the model fits. However, the catch rate models do not appear to adequately capture targeting behaviour, or spatial variation in catch rates more generally. There may be sufficient observer data to consider explicitly capturing spatial variation in catch rate models in the next iteration of this work, given the recent increases in spatial coverage of available observer data.
A simulation exercise was undertaken to explore how electronic and/or observer monitoring coverage rates, and the approach used to spread this coverage within fleets, may impact the precision of estimated catch rates. More precise estimates of catch rates were generally obtained with partial coverage of all trips (e.g. 10 % of sets per trip) compared to full coverage of sets for a subset of trips (e.g. 10 % of trips), all else being equal. However, the increase in precision was highest for species that are frequently caught, and weakest for rarely caught species. This has implications on how best to allocate resources to collect and process monitoring from longline vessels in the region, particularly any procedures for applying electronic monitoring.