Swordfish horizontal movements in relation to stock structure within the Eastern North Pacific

Abstract This study reports on the use of electronic tag technology to better understand swordfish (Xiphias gladius) movements and stock structure in the North Pacific (NP). Long-term movement data are presented in relation to historical and recently modified stock-structure boundaries for swordfish tagged off California. Data show extensive movements across the eastern portion of the NP basin ranging from the equatorial waters up to Vancouver Island, Canada and out to the Hawaiian Island Archipelago. In addition to traditional tag and recapture information, high-resolution data on the seasonal movement patterns and migration routes were analyzed from 56 individuals outfitted with dorsal-fin mounted Argos transmitters. Track data suggests that swordfish tagged off California generally departed on offshore migrations toward the warmer subtropical spawning grounds during the winter and springtime before returning to west coast foraging grounds during the summer and fall months. Seasonal offshore movements were typically directed either to the southeast along the Baja California peninsula or toward the Hawaiian Islands on a west or southwest heading. Annual and multi-annual tracks (≤904 d) from swordfish tagged off California show that: (i) trans-equatorial movements are limited; (ii) movements are confined to the area inside of the Northwest Hawaiian Islands (166°W); and (iii) some individuals exhibit seasonal site fidelity toward Southern California foraging grounds. Tag data also show that swordfish regularly move across former management unit boundaries that were previously used to designate NP stocks (Eastern Pacific Ocean and Western and Central North Pacific). Collectively, these data were used to help refine the jurisdictional boundaries of NP swordfish by the International Scientific Committee Billfish Working Group in 2023. Given the importance of the swordfish resource to global economies, additional tagging data is needed to better determine stock structure across the NP basin.