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Francis M.P.,NIWA - National Institute of Water and Atmospheric Research | Holdsworth J.C.,Blue Water Marine Research | Block B.A.,Stanford University
Marine Biology | Year: 2015

A wide range of pelagic predators survive in the open ocean, yet little is known about how they use that environment. Much of the current information on these species comes from studies in shelf waters, and it is not clear how representative the results are of their oceanic lifestyle. We used pop-up satellite tags to explore the horizontal and vertical use of the open ocean by porbeagle sharks (Lamna nasus) in the south-west Pacific Ocean and to identify possible mitigation measures to reduce their bycatch in tuna longline fisheries. Ten porbeagle sharks were tracked in waters around New Zealand for 72–300 days (median 221 days). Sharks made horizontal movements of hundreds to thousands of kilometres, with a maximum estimated track length of almost 10,000 km. Mature females made seasonal latitudinal migrations from ~46–48°S in summer to ~35–38°S during winter–spring, where they are hypothesised to give birth to pups. Porbeagle sharks exhibited diel vertical migration, diving deeper during the day than at night. Dives generally began at dawn and finished at dusk and lasted 11–15 h depending on day length. Porbeagles feed mainly on mesopelagic fish and squid and appear to forage in the vertically migrating deep scattering layer. All sharks dived to at least 600 m with a maximum recorded depth of 1024 m. During the day, most of their time was spent at depths of 200–600 m in the open ocean. Porbeagle shark bycatch could be substantially reduced by limiting longline fishing to daylight hours when they are too deep to be caught. However, longliners currently set mainly at night to avoid seabird bycatch, which would increase unless other mitigation measures were implemented to protect them. A review of factors affecting the bycatch of various species, and the impact of mitigation measures on catches of target species, is required to identify an appropriate suite of management measures. © 2015, Springer-Verlag Berlin Heidelberg. Source

Evans K.,CSIRO | Abascal F.,Spanish Institute of Oceanography | Kolody D.,CSIRO | Sippel T.,Blue Water Marine Research | And 4 more authors.
Journal of Experimental Marine Biology and Ecology | Year: 2014

The movement patterns of broadbill swordfish (Xiphias gladius) in the South Pacific Ocean are largely unknown. Understanding the connectivity of the species across the Pacific and any variability in diving behaviour as it relates to fisheries availability/catchability are of particular relevance. Here, we present an electronic tagging dataset spanning the western and eastern South Pacific Ocean regions. Movements observed suggest a lack of connectivity between the southern and northern regions of the western and central Pacific Ocean (WCPO) and limited connectivity between the eastern and western parts of the Tasman and Coral Seas in the south-western Pacific Ocean. At least some swordfish appear to undertake movements between tropical waters extending from around Vanuatu to French Polynesia to waters around New Zealand, indicating greater connectivity than previously thought. Observations indicate no movement between the WCPO and the eastern Pacific Ocean (EPO), although data from boundary areas are lacking. Swordfish demonstrated a mixture of diel vertical distributions and daytime surface behaviour, spending time mostly in waters <. 100. m during the night and >. 400. m during the day. Diel vertical movements resulted in movement through water temperatures that varied on the order of 15-20°. C with temperatures at depth as low as 2.4°. C and those at the surface as high as 31.4°. C. Vertical distributions of swordfish varied both spatially and temporally with swordfish in the Tasman/Coral Seas demonstrating the least variability. Spatio-temporal variability in vertical distributions is likely driven by variability in environmental conditions and associated prey distributions. Swordfish tagged in the Tasman/Coral Seas and in the EPO interrupted deeper daytime distributions with two distinct types of surfacing behaviour: temporally associated and temporally isolated. Temporally isolated surface behaviour occurred throughout the year and in association with on average lower sea surface temperatures. Temporally associated surface behaviour was restricted to austral summer months only and in association with on average higher sea surface temperatures. Our results represent a major step towards reducing uncertainty about the spatial dynamics of swordfish in the South Pacific Ocean. At the same time, questions as to the extent of connectivity of swordfish throughout the south Pacific and the linkages between spawning ground and foraging ground locations are raised. Further investigation of the movements of swordfish from the central southern Pacific Ocean is required to determine what linkages there may be between the WCPO and the EPO and whether connectivity suggested by genetic studies is supported. © 2013 Elsevier B.V. Source

Sippel T.,University of Auckland | Sippel T.,Blue Water Marine Research | Sippel T.,University of Hawaii at Manoa | Holdsworth J.,Blue Water Marine Research | And 2 more authors.
PLoS ONE | Year: 2011

Behaviour and distribution of striped marlin within the southwest Pacific Ocean were investigated using electronic tagging data collected from 2005-2008. A continuous-time correlated random-walk Kalman filter was used to integrate double-tagging data exhibiting variable error structures into movement trajectories composed of regular time-steps. This state-space trajectory integration approach improved longitude and latitude error distributions by 38.5 km and 22.2 km respectively. Using these trajectories as inputs, a behavioural classification model was developed to infer when, and where, 'transiting' and 'area-restricted' (ARB) pseudo-behavioural states occurred. ARB tended to occur at shallower depths (108±49 m) than did transiting behaviours (127±57 m). A 16 day post-release period of diminished ARB activity suggests that patterns of behaviour were affected by the capture and/or tagging events, implying that tagged animals may exhibit atypical behaviour upon release. The striped marlin in this study dove deeper and spent greater time at ≥200 m depth than those in the central and eastern Pacific Ocean. As marlin reached tropical latitudes (20-21°S) they consistently reversed directions, increased swimming speed and shifted to transiting behaviour. Reversals in the tropics also coincided with increases in swimming depth, including increased time ≥250 m. Our research provides enhanced understanding of the behavioural ecology of striped marlin. This has implications for the effectiveness of spatially explicit population models and we demonstrate the need to consider geographic variation when standardizing CPUE by depth, and provide data to inform natural and recreational fishing mortality parameters. © 2011 Sippel et al. Source

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