Movement behavior of swordfish provisions connectivity between the temperate and tropical southwest Pacific Ocean

Swordfish (Xiphias gladius) are a widely distributed (45°N-45°S) large pelagic fish targeted by fisheries worldwide. Swordfish that occur at high latitudes tend to disproportionately be large adults, so their movements have implications for population dynamics and fisheries management. In the southwest Pacific, little is known about this subset of the stock and existing evidence suggests limited movement from the subtropics into cooler high latitude waters. Here, we capitalize on the recent emergence of a recreational swordfish fishery off temperate southeast Australia to characterize movements of swordfish caught in the fishery with pop-up satellite archival transmitting tags. Data were recovered from tags deployed for 56-250 days on 11 swordfish (50-350 kg) tagged between 38 and 43°S in the western Tasman Sea. Five swordfish entered the Coral Sea (< 30°S), with four reaching north to 11-24°S, up to 3275 km away from location of capture. Behavior modelling suggests these four individuals rapidly transited north until encountering 23-27 °C water, at which point they lingered in the area for several months, consistent with spawning-related partial migration. One migrating swordfish still carrying a tag after the spawning season returned to ~ 120 km of its release location, suggesting site fidelity. Movements toward the central south Pacific were confined to two individuals crossing 165°E. Swordfish predominantly underwent normal diel vertical migration, descending into the mesopelagic zone at dawn (median daytime depth 494.9 m, 95% CI 460.4-529.5 m). Light attenuation predicted daytime depth, with swordfish rising by up to 195 m in turbid water. At night, swordfish were deeper during the full moon, median night-time depth 45.8 m (37.8-55.5) m versus 18.0 m (14.9-21.8) m at new moon. Modelling fine-scale (10 min^-1) swordfish depth revealed dynamic effects of moon phase varying predictably across time of night with implications for fisheries interactions. Studying highly migratory fishes near distribution limits allows characterization of the full range of movement phenotypes within a population, a key consideration for important fish stocks in changing oceans.