Characterisation of the fisheries catching South Pacific blue sharks (Prionace glauca) in the Western and Central Pacific Ocean

South Pacific blue sharks (Prionace glauca) are thought to consist of a single stock separated from blue sharks in the north Pacific at the Equator. This paper describes the fisheries catching blue sharks in the Southwestern Pacific Ocean as well as the reported and observed data potentially available for use in a stock assessment.

Currently it appears that there are a reasonable amount of data available for undertaking catch reconstructions and CPUE standardisations for the development of a stock assessment. Overall, the data will be confounded by reporting changes that have come about from regulatory changes and these are apparent throughout the results. The proportion of blue sharks in logsheets increased, and the proportion of logsheets with zero blue sharks has declined in recent years from around 80% prior to 2010 to around 35% currently. Few CCMs provided data prior to 2000 and most data were from Australia and New Zealand. The spatial extent of the data provisions has increased in the last two decades and is now broadly representative of the fishing effort. However, due to these changes, the catch history of blue shark is not consistently representative through time.

There is a general increase in the number of observer samples of all kinds over time, and these data are also more detailed in recent years. There are strong trends across most fleets for vessels to discard sharks as CCMs implement WCPFC regulations and some CCMs ban the retention of all sharks within their EEZs. There is also a propensity for vessels to cut sharks free before they are landed on the vessel, and recently a higher proportion of discards are reported as cut free. Depth of gear and latitude will impact the catch rates of blue sharks. Smaller blue sharks are found in the more southerly latitudes.

Longline gear attributes such as hooks between floats, hooks set, baskets set, bait used, branch line length and distance will likely be informative for CPUE standardisation. However, they are inconsistently reported, both among and within fleets. Generally, there is a trend for more hooks between floats, and a decreases in the hooks set and in the baskets set.

Blue sharks are wide ranging across the South Pacific Ocean, and display weak size and seasonal movement patterns which do not seem to cross the Equator into the north Pacific. Overall, there appears to be a reasonable amount of data from 1990-2019, but the data by fleet are incomplete and poorly reported throughout the history of the fishery for most fleets. Catch reporting has improved across all fleets over time, has resulted in more data being available in recent years. However, these trends are unlikely to be linked to changes in targeting or stock biomass, but are simply reflective of increased coverage rates.

The following recommendations are proposed for the Scientific Committee to consider:
1. The length data should only be used as fleet specific selectivity data and not used to interpret changes in length over time.
2. Aggregated data are submitted as annual totals for the WCPFC area only, making them uninformative for a stock specific assessment. Therefore, blue shark (and probably other Key Sharks) aggregated data should be reported by ocean area not simply as WCPO and, where possible, these data should be retrospectively corrected.
3. Observers (or the vessel) should record number of shark lines deployed or the number of floats with shark lines.