Thermocline-associated habitat use and fight dynamics of swordfish during recreational angling events

Abstract Swordfish (Xiphias gladius) are increasingly targeted by recreational fisheries using deep-drop angling techniques that access fish at mesopelagic depths, including fisheries in southeast Australia. Despite rising fishing effort, little is known about swordfish behavioural and physiological responses to capture, limiting species-specific post-release mortality mitigation strategies. We conducted the first high-resolution analysis of swordfish behaviour during recreational angling using biologgers on terminal tackle to collect depth, temperature, and acceleration data from ten capture events. All fish exhibited a consistent sequence during capture events: bait interaction, a rapid voluntary ascent from mesopelagic depths (400–500 m), an extended fight, and either landing or disconnection. Initial ascents slowed upon reaching the thermocline and occurred largely without sustained line tension, indicating that vertical movements were primarily fish-driven during this stage. Throughout the subsequent fight, fish spent a majority (67%) of time above and often near the thermocline but crossed it intermittently in dives lasting 1–30 min (mean ± SD duration: 6 ± 7 min). We suggest this represents either an escape response or thermal habitat selection to maintain swimming performance and manage recovery from exhaustion. To quantify shifts in control during capture events, we developed an angler control index (ACI) derived from depth-time wavelet analysis that distinguished periods dominated by angler retrieval from fish-driven movement. High ACI values occurred primarily toward the end of fights, consistent with increasing angler control and fish fatigue. Although most (86%) landed fish showed signs of abdominal distention, presumably reflecting swim bladder barotrauma, ascent rates were consistent with those recorded from free-swimming swordfish, suggesting that this condition may not arise solely from rapid pressure change but also involve physiological disruption associated with exhaustive exercise. Our results demonstrate that swordfish experience and actively move within a heterogeneous vertical environment during capture, suggesting the possibility of behavioural thermoregulation. Characterizing these dynamics provides new insight into fight mechanics, exhaustion, and potential opportunities to reduce fight duration in recreational fisheries targeting this large pelagic predator.