Safety in the shallows: nearshore coastal habitats can provide physical and thermal features that optimize escape performance in newborn blacktip reef sharks (Carcharhinus melanopterus).

Abstract

The prevailing shark nursery paradigm suggests that high survival in these habitats is primarily driven by reduced predator encounters: so-called pre-encounter risk. In this study, we propose an alternative or complementary mechanism: that some nurseries may lower post-encounter risk by providing environmental conditions that maximize escape performance. To test this hypothesis, we examined how temperature, depth and habitat complexity influence the escape performance of newborn blacktip reef sharks (Carcharhinus melanopterus) in Mo'orea, French Polynesia. In a controlled setting, we exposed 48 newborn sharks to four temperature treatments (25, 27, 29 and 31°C) and measured fast-start acceleration, turning rate and latency to respond to a stimulus. We also calculated aerobic scope at 27, 29 and 31°C, as greater aerobic scope is associated with faster recovery from burst swimming. Our results show that warmer temperatures improve escape performance, with 29% higher acceleration, 9% faster turning rates and 48% shorter reaction times at elevated temperatures. Furthermore, aerobic scope remained ≥80% of its maximum capacity between 27.5 and 30.8°C, suggesting that newborn sharks can sustain high metabolic performance within this thermal window. Field measurements at nursery habitats revealed that daily thermal fluctuations generally remained within this optimal aerobic scope range, meaning that newborns can maintain high escape performance for most of the day. Additionally, high-resolution mapping confirmed that previously reported home ranges were associated with shallow (median depth = 0.74 m), structurally complex reef flats dominated by coral substrate. The combination of reduced hydrodynamic drag in shallow water and increased manoeuvrability in complex habitats likely enhances predator evasion. However, extreme warming events that exceed critical thermal limits may trigger behavioural trade-offs that compromise escape performance and elevate predation risk. Our findings suggest that these nurseries provide habitat-specific advantages for predator evasion, reinforcing their critical role in the survival of newborn sharks.