Vertical habitat use by black and striped marlin in the Western Indian Ocean

Abstract

Black marlin <i>Istiompax indica and striped marlin Kajikia audax are large, fast-swimming, oceanic apex predators. Both species are increasingly exploited by fisheries with varied gear encounter rates at different depths, causing concern for their status. Here, we examined vertical habitat use by 34 black and 39 striped marlin caught off Kenya, using pop-up satellite tags to compare their diving behaviours. Tags recorded depth and temperature time-series for a mean (±SD) of 43 ± 53 days per track. Marlin dived extensively moving up to ~14 vertical km in cumulative dives per day in addition to a daily mean of ~50 km in horizontal movements. Both species had similar maximum depths (460-470 m). Striped marlin dived deeper more frequently than black marlin, and also spent more of their time at the water surface (top 5 m: 50.7 vs. 32.3% in black marlin). Most striped marlin had a normal diel vertical migration dive pattern over their track (61.5% of individuals), while ~35% of black marlin showed a crepuscular pattern, diving particularly deep at dusk and dawn. Striped marlin spent almost twice as much time (7.4%) inside the oxygen minimum zone (<150 μmol kg^-1 dissolved oxygen) than black marlin (4%). The extensive use of surface waters by striped marlin may be a behavioural response to re-oxygenate and/or warm up after dives into cold or oxygen-poor waters. Two free-jumping events immediately before tag detachment demonstrated why it is challenging to keep tags attached to these highly active fishes. Their vertical habitat use shows that both species are highly susceptible to capture in regional drift gillnet and longline fisheries.