The deployment of electronic data storage tags that are surgically implanted or satellite-linked provides marine researchers with new ways to examine the movements, environmental preferences, and physiology of pelagic vertebrates. We report the results obtained from tagging of Atlantic bluefin tuna with implantable archival and pop-up satellite archival tags. The electronic tagging data provide insights into the seasonal movements and environmental preferences of this species. Bluefin tuna dive to depths of >1000 meters and maintain a warm body temperature. Western-tagged bluefin tuna make trans-Atlantic migrations and they frequent spawning grounds in the Gulf of Mexico and eastern Mediterranean. These data are critical for the future management and conservation of bluefin tuna in the Atlantic.

Habitat compression and associated potential habitat loss was validated using electronic tagging data from 47 blue marlin. This phenomenon increases vulnerability to surface fishing gear for billfishes and tunas 8,9 , and may be associated with a 10-50% worldwide decline of pelagic predator diversity 10 . Further expansion of the Atlantic OMZ along with overfishing may threaten the sustainability of these valuable pelagic fisheries and marine ecosystems.Dissolved oxygen is critical for sustaining most marine animal life. When dissolved oxygen is minimized, widespread mortality 11,12 or avoidance 13 of affected areas can result. OMZs in the eastern tropical seas represent the largest contiguous areas of naturally occurring hypoxia 9 in the world's oceans. In the present climate change cycle, characterized by anthropogenic CO 2 emissions 2 and global warming, these areas are expanding and shoaling 3,12,14 . Possible consequences of OMZ expansion to the marine ecosystem 14 include loss of vertical habitat for high-oxygen-demand tropical pelagic billfishes and tunas and the associated increased risk of overfishing of these species by surface fishing gear 8,9 .Large-scale expansion of OMZs over the past 50 years 3 poses a challenge for predicting impacts to pelagic fish stocks and their ecosystem. Although oceanographic modelling and ocean observations for retrospective analyses are useful for examining past trends, understanding future OMZ expansions and the concurrent impacts on billfish and tuna populations is essential for preventing overfishing. We analysed recent hypoxia data associated with OMZ expansion in the eastern tropical Atlantic (ETA) to examine possible habitat loss of the near-surface layer. We also present vertical habitat use data of Atlantic blue marlin (Makaira nigricans) monitored with electronic tags (Fig. 1). Changes in habitat use were validated by maximum daily depths

Large areas of cold hypoxic water occur as distinct strata in the eastern tropical Pacific (ETP) and Atlantic oceans as a result of high productivity initiated by intense nutrient upwelling. We show that this stratum restricts the depth distribution of tropical pelagic marlins, sailfish, and tunas by compressing the acceptable physical habitat into a narrow surface layer. This layer extends downward to a variable boundary defined by a shallow thermocline, often at 25 m, above a barrier of cold hypoxic water. The depth distributions of marlin and sailfish monitored with electronic tags and average dissolved oxygen (DO) and temperature profiles show that this cold hypoxic environment constitutes a lower habitat boundary in the ETP, but not in the western North Atlantic (WNA), where DO is not limiting. Eastern Pacific and eastern Atlantic sailfish are larger than those in WNA, where the hypoxic zone is much deeper or absent. Larger sizes may reflect enhanced foraging opportunities afforded by the closer proximity of predator and prey in compressed habitat, as well as by the higher productivity. The shallow band of acceptable habitat restricts these fishes to a very narrow surface layer and makes them more vulnerable to over-exploitation by surface gears. Predictably, the long-term landings of tropical pelagic tunas from areas of habitat compression have been far greater than in surrounding areas. Many tropical pelagic species in the Atlantic Ocean are currently either fully exploited or overfished and their future status could be quite sensitive to increased fishing pressures, particularly in areas of habitat compression.