ENSO drives near-surface oxygen and vertical habitat variability in the tropical Pacific

El Nino-Southern oscillation (ENSO) is the leading cause of sea surface temperature variability in the tropical Pacific with known impacts on tuna geographic range, but its effects on oxygen and available oxygenated habitat space are less clear. Variations in oxygenated vertical habitat space in the upper-ocean can alter interactions between predator and prey, as well as drive changes in the vulnerability of economically important tuna and other pelagic fish to surface fishing gear. Using in situ measurements, we show that ENSO is the primary driver of upper-ocean oxygen partial pressure (pO(2)) variability on year-to-year time scales in the tropical Pacific. Mechanistically, these pO(2) variations are primarily caused by vertical shifts in thermocline depth, which alternately elevate and depress cold, hypoxic waters from the ocean interior depending on the ENSO phase and location. Transport-driven, isopycnal pO(2) variations within the thermocline also play an important but secondary role. In the western tropical Pacific, waters within the exclusive economic zones of Palau, Micronesia, Nauru, and the Marshall Islands undergo the greatest variations in oxygenated tuna vertical habitat extent: approximately 19.5 m, 23.9 m, 19.5 m, and 19.3 m, respectively, between El Nino and La Nina phases. Oxygen thus plays an important role in altering available tuna vertical habitat space between different phases of ENSO.