Depth and muscle temperature of Pacific bluefin tuna examined with acoustic and pop-up satellite archival tags

Six Pacific bluefin tuna were tracked with ultrasonic telemetry and two wit h pop-up satellite archival tags (PSATs) in the eastern Pacific Ocean in 19 97, 1998, and 1999. Both pressure and temperature ultrasonic transmitters w ere used to examine the behavior of the 2- to 4-year-old bluefin tuna. The bluefin spent over 80% of their time in the top 40 m of the water column an d made occasional dives into deeper, cooler water. The mean slow-oxidative muscle temperatures of three fish instrumented with pressure and temperatur e transmitters were 22.0-26.1 degreesC in water temperatures that averaged 15.7-17.5 degreesC. The thermal excesses in slow-oxidative muscle averaged 6.2-8.6 degreesC. Variation in the temperature of the slow-oxidative muscle in the bluefin was not correlated with water temperature or swimming speed s. For comparison with the acoustic tracking data we examined the depth and ambient temperature of two Pacific bluefin tagged with pop-up satellite ar chival tags for 24 and 52 days. The PSAT data sets show depth and temperatu re distributions of the bluefin tuna similar to the acoustic data set. Swim ming speeds calculated from horizontal distances with the acoustic data ind icate the fish mean speeds were 1.1-1.4 fork lengths/s (FL s(-1)). These Pa cific bluefin spent the majority of their time in the top parts of the wate r column in the eastern Pacific Ocean in a pattern similar to that observed for yellowfin tuna.