Yellowfin tuna (Thunnus albacares) are known to preferentially occupy
the surface mixed layer above the thermocline and it has been suggested that they are
physiologically restricted to water temperatures no more than 8 °C colder
than surface waters. However, we here report for dive data acquired from a
large yellowfin tuna which demonstrate for the first time that this species
is indeed capable of making prolonged dives into deep cold waters. A
yellowfin tuna (134 cm fork length) caught near an anchored fish aggregating
device (FAD) in the Seychelles (Western Indian Ocean) was equipped with an
internally implanted archival tag and released. The fish was recaptured 98 days
later. As predicted for this species, this fish spent 85% of its
time shallower than 75 m (maximum thermocline depth experienced by the
fish) but, over the course of the track, it performed three deep dives to
578 m, 982 m and 1160 m. Minimum ambient water temperatures recorded at
these depths were 8.6 °C, 7.4 °C and 5.8 °C respectively and varied
by up to 23.3 °C from surface temperatures. The fish spent 8.3% of its
time in waters more than 8 °C colder than the surface layer and daily
experienced a wide range of sea temperatures (mode at 15-16 °C) and of
temperatures of the gut cavity (mode at 6 °C). The reason for these dives
can not be known. These depths and temperatures significantly exceed those
reported in the literature so far and clearly demonstrate that this species
has the physiological and behavioral ability to penetrate deep cold
sections of the ocean.