Influence of Central Pacific Oceanographic Conditions on the Potential Vertical Habitat of Four Tropical Tuna Species

Climate change has resulted in the geographic and vertical expansion of oxygen minimum zones but their impact on the vertical distribution of commercially important species, such as tunas, is not well understood. Although La Nina events are characterized by increased upwelling along the equator, the increased primary productivity and bacterial proliferation drive the expansion of oxygen minimum zones. Vertical habitat of four tropical tuna species were characterized using direct observations of the oceanographic conditions of the Central Pacific Ocean during the 2008 La Nina event and existing primary literature on temperature and dissolved oxygen physiological tolerances for these tunas. Concentrations of potential prey were estimated using Acoustic Doppler Current Profiler raw backscatter and surface zooplankton tows. Based on the oceanographic conditions observed from February to Tune, low dissolved oxygen levels, more so than low temperatures, were inferred to restrict the predicted vertical habitat of four commercially important tuna species (bigeye, yellowfin, skipjack, and albacore). During peak La Nina conditions, temperature and dissolved oxygen tolerance limits of all four tuna species were reached at approximately 200 m. Zooplankton and myctophid fish densities peaked in the upper 200 in between 0 degrees N and 5 degrees N, which corresponded to a region with a shallow thermochne (150 m). Our findings suggest the possibility that competition and susceptibility to surface fishing gears may be increased for tropical tunas during a strong La Nina event due to vertical habitat restrictions.