Characterisation of the fisheries catching South Pacific shortfin mako sharks (Isurus oxyrinchus) in the Western and Central Pacific Ocean

South Pacific shortfin mako sharks (Isurus oxyrinchus) are thought to consist of two stocks a southwest and southeastern stock which are both separated from those in the north Pacific at the Equator. This paper describes the fisheries catching shortfin mako 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, and possibly some misidentification of porbeagle sharks. 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 shortfin mako shark is short and not consistently representative through time. In addition, aggregated data are submitted as annual totals for the entire WCPO, and they are not stock specific. These data should in future be separated at the equator.

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 tendancy 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 shortfin mako sharks.

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. The number of shark lines deployed or the number of floats with shark lines should be recorded as currently it is not possible to assess the proportion of shark lines deployed per set. Some observers and fleets are still recording mako sharks to the generic mako code and not specifying these as shortfin or longfin mako sharks. This should be rectified in future.

Shortfin mako sharks are wide ranging across the South Pacific Ocean, and display weak size and seasonal movement patterns, there appears to be a reasonable amount of data from 1990-2020, 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, and 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 data are limited as a result an integrated stock assessment for shortfin mako sharks will be challenging and increase the uncertainty in the outputs. This is likely to be the case for most shark assessments. Added to this is the impact of regulatory changes on fishery dependent data, and generally low observer coverage in longline fleets. Some focused work on how to get better CPUE data for sharks in general would be useful and it is recommended that a workshop to review the data, data handling protocols and ways to improve data collection.

The following recommendations are proposed for the Scientific Committee to consider:

• Aggregated data are submitted as annual totals for the WCPFC area only, making them uninformative for a stock specific assessment. Therefore, shortfin mako shark aggregated data (and probably other key sharks) should be reported by ocean area not simply as WCPO and, where possible, these data should be retrospectively corrected. As such paragraph 1 bullet point 3 of the scientific data to be provided to the commission should include the following: ”For key sharks estimates of annual catch should be separated into catch north and south of the Equator. The WCPFC secretariat should work with CCMs to get these data retrospectively corrected where possible.”

• Observers (or the vessels crew) should record number of shark lines deployed or the number of floats with shark lines.

• Observers and vessel crew should be encouraged to use mako species specific codes rather than the generic code MAK.

• Convene a workshop to assess how to get better CPUE data for sharks in general assess the data, data handling protocols and ways to improve data collection.