Updated abundance indicators for New Zealand blue, porbeagle and shortfin mako sharks

Recognizing the data-poor nature of many of the world’s shark fisheries, scientists have recently turned to alternative methods for assessing threats to the sustainable utilisation of chondrichthyan resources. These methods have the advantage of being more forgiving of data gaps, less reliant on assumptions structuring population dynamics, and more readily updated than traditional stock assessments. One approach is to develop a series of stock status indicators to assess the response of the population to fishing pressure. Such indicators are usually straightforward to compute (except for standardised CPUE) and track over time, thus providing the opportunity to observe trends that can serve as early signals of overexploitation. Indicators of stock status can be useful for initial assessments and/or for prioritising future data collection or analytical work (Clarke et al. 2013). The main shark species caught in New Zealand’s tuna longline fishery are blue shark (Prionace glauca), shortfin mako shark (hereafter abbreviated to ‘mako shark’; Isurus oxyrinchus), and porbeagle shark (Lamna nasus) (Francis 2013; Griggs & Baird 2013). In a previous study using New Zealand fishery data up to 2013, four types of indicators were developed for the three shark species: distribution, percentage catch composition, standardised catch per unit effort (CPUE), and median size/sex ratio (Francis et al. 2014). The present project extends that study by updating the distribution time series for all three species by two more years, and updating the CPUE time series for porbeagle sharks by two more years.