Southern Hemisphere porbeagle shark (Lamna nasus) stock status assessment

This report presents the results of a Southern Hemisphere status assessment of porbeagle shark. The study was a collaborative one involving many countries with Southern Hemisphere fisheries that catch porbeagles. Participating scientists from Argentina, Chile, Japan, New Zealand and Uruguay contributed data analyses and abundance indices. Our approach combined indicator analyses and a spatially-explicit sustainability risk assessment. Indicator analyses were performed independently for different Southern Hemisphere fisheries and served to characterise local trends in relative abundance based on commercial catch per unit effort (CPUE) data, and trends in size and sex ratio based on biological data.
We limited our analyses to the region south of 30 oS which provided most of the available data, although the porbeagle shark’s range extends slightly north of this latitude. Porbeagle sharks are taken in fisheries at least as far south as 56 oS. Southern Hemisphere population structure is not well understood, and we considered it unlikely that the population comprises a single well-mixed stock for management purposes. We subdivided the spatial domain of the assessment into five subpopulations or regions by longitude: 1) Western Atlantic Ocean; 2) Western Indian/Eastern Atlantic Ocean; 3) Eastern Indian Ocean; 4) Western Pacific Ocean; and 5) Eastern Pacific Ocean.
We applied different assessment methods by region, depending on data availability and quality. In the Western Indian/Eastern Atlantic Ocean, Eastern Indian Ocean, and Western Pacific regions, stock status assessment was performed using a spatially-explicit risk assessment. Indicator-based analyses were used to assess stock condition in the Eastern Pacific and the Western Atlantic, where there was limited information. We compared results from areas with varying levels of information, for greater insight into the status of the stock, levels of uncertainty, and data requirements for future studies.