Bigeye Tuna behavior and physiology. Their relevance to stock assessments and fishery biology.

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.