SPC stock assessment scientists Shelton Harley and Joel Rice have just returned from a one week blue shark stock assessment workshop in San Diego, California. Here Shelton reports on what makes this assessment a little different to others typically undertaken by SPC.

The assessment for blue sharks in the North Pacific is the third shark stock assessment undertaken by SPC as part of our service agreement to the Western and Central Pacific Fisheries Commission (WCPFC). Two unique features of this assessment are: 1) that it covers an area outside of the mandate of the WCPFC and therefore requires close collaboration with the Inter-American Tropical Tuna Commission (IATTC) and the International Scientific Committee for tuna and tuna-like species (ISC); and 2) the SPC stock assessment is one of two that are to be undertaken for the stock.

Typically we undertake our stock assessments in Noumea and we have all the data at hand, but in this case we only have a small part of the data; we therefore rely heavily on data and prepared analyses provided by others. A real opportunity of this assessment is to be working with scientists from the United States, Japan, and Taiwan; drawing on their own unique insights into the data and approaches to data analysis.

For this assessment SPC is using the Stock Synthesis package which integrates catch, catch rate, size composition data, and age- and sex-based differences in growth, mortality, and vulnerability to fishing. The other assessment being used is a more simplistic production model which uses only catch and catch rate data and has very simplified biological assumptions. One further challenge for the workshop was how best to consider the results for each approach. In theory the more biologically realistic model would be preferred, but in practice data for sharks are often poor and a more complex model could be misleading. A positive outcome of the meeting was a collaboration between Dr Yasuko Semba, a top Japanese shark scientist, and Joel Rice, to estimate how natural mortality of blue sharks varies both with age and between males and females. The results from this analysis will be included in the Stock Synthesis assessment.

The working group will be meeting again in Taiwan in June to finalize the assessment. One of the key challenges it will face is how best to handle some of the disagreements that exist between important catch rate data for different fishing fleets. Depending on which is considered [more] correct, the implications for blue sharks and the fisheries that catch them are very different. The two assessments will be presented and reviewed by the Scientific Committee of the WCPFC in August this year in Majuro, Republic of the Marshall Islands.