The horizontal and vertical dynamics of swordfish in the South Pacific Ocean

Abstract

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 <100m during the night and >400m 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.