Spatio-temporal patterns in movement, behaviour and habitat use by a pelagic predator, the blue shark (Prionace glauca), assessed using satellite tags

Abstract

The blue shark (Prionace glauca) is the most abundant and globally distributed pelagic shark species, but also the most exploited, with historic annual mortality estimated at 10–20 million individuals per year. Inherent biases and flaws in fishery derived data used for stock management may, in part, explain population declines described for Atlantic, central Pacific and north Pacific Oceans. Satellite tagging has improved data quality in these regions, by providing fishery independent data, however, data is lacking for the southwest Pacific. Yet, in this region, New Zealand surface-longline fisheries alone, have 31% of catch being blue sharks, and peak mortality from shark finning reaching 158,000 individuals a year. To improve management data, fifteen blue sharks were satellite tagged in northeastern New Zealand between 2012–2015. Horizontal tracks spanned temporal periods and spatial scales that were greater on average than most other blue shark studies. Age and sex segregation, correlated with gestation period, suggested the New Zealand region is utilised as a birthing and breeding ground. Satellite tracks supported international consensus that each major oceanic basin contains a single stock. Juveniles showed preference for habitat within continental shelf waters, while mature animals primarily utilised shelf and pelagic waters. For the first time, SPLASH tags were used on blue sharks, providing dive profile data from tracks of up to 14,559 km and 343 days. One of the deepest dives for this species was recorded at 1364 m, along with quantification of a repetitive diel vertical migration pattern. Long term monitoring of dive profiles enabled the inference of behavioural states, which with location and time stamps allowed a behavioural state prediction model to be validated in a novel way. The scale and spatio-temporal trends of blue sharks’ vertical movement suggest that this species may provide important ecological services with respect to nutrient recycling, carbon export and stimulation of primary production. Data provided by this study furthers our understanding of blue shark biology, suggests ecosystem services which support the sustainable management of this species, provides fisheries independent data that can inform management strategies, and has already directly contributed to a New Zealand shark fin ban introduced in 2014, which reduced bycatch mortality by 83–89% per annum.