Post-release behaviour of black marlin, Makaira indica, caught off the Great Barrier Reef with sportfishing gear

The post-release behaviour of eight black marlin (Makaira indica), caught b y standard sportfishing techniques off the Great Barrier Reef, Australia, w as investigated using ultrasonic telemetry. Five marlin between 100 and 420 kg were successfully tracked for periods of 8 to 27 h. Of the three others tagged, one was killed by a shark and two shed their tags, probably as the result of poor attachment. The black marlin spent most of their time withi n 10 m of the surface, both day and night. During the day, however, they al so spent some time between 40 and 140 m depth. They rarely penetrated the t hermocline, and then only briefly, remaining at temperatures no more than 8 C degrees below that of surface waters. The deepest dives were to 178 m. F our of the five marlin tracked, initially moved offshore before heading par allel to the shore, whereas the other marlin stayed close to the reef edge. The average mean swimming speeds over the ground for entire tracks ranged from 0.7 to 1.02 m s(-1).