Bigeye Tuna (Thunnus obesus) behaviour and phisiology and their relevance to stock assessments and fishery biology

SUMMARY Bigeye tuna (Thunnus obesus) have distinctive depth distributions and vertical movement patterns. They remain in the uniformed temperature surface layer at night and can descend to greater than 500 m depth at dawn. They thus mirror the vertical migrations of the small nektonic organisms of the deep sound scattering layer and extensively exploit these as a food resource. At their maximum depths, bigeye tuna frequently experience prolonged exposure to ambient temperatures (.5 EC) that are up to 20EC colder than surface layer temperature, and oxygen concentrations less than 1.5 ml O2 l -1 . In contrast, skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (T. albacares) generally limit their forays to depths where water temperatures are no more than 8EC below surface layer temperatures, and ambient oxygen levels are above 3.5 ml O2 l -1 . Understanding the vertical movements and depth distribution of bigeye tuna, as well as the physiological abilities/tolerances and oceanographic conditions controlling them, has been shown key for improving longline catch-per-unit effort analysis and long-term populations assessments in the Pacific. Similar work is needed in the Atlantic. RESUME