Anthropogenic Noise and Behavioral Plasticity in Marine Mammals

Abstract

Anthropogenic underwater noise \u2014 from shipping, seismic surveys, sonar, and offshore construction \u2014 has increased by approximately 3 dB per decade in major ocean shipping lanes since the 1960s, creating a pervasive and growing stressor for acoustically dependent marine mammals. This study quantified the behavioural plasticity responses of four cetacean species \u2014 common bottlenose dolphin (Tursiops truncatus), harbour porpoise (Phocoena phocoena), fin whale (Balaenoptera physalus), and sperm whale (Physeter macrocephalus) \u2014 to controlled noise exposure and natural shipping noise gradients across three European sea areas (North Sea, Baltic Sea, Mediterranean Sea) using passive acoustic monitoring (n = 64 hydrophone stations; 284,628 hours of recordings) and GPS-tagged individual tracking (n = 86 GPS-tagged individuals). Five behavioural response categories were quantified: vocalisation rate adjustment, frequency shift in calls, dive depth/duration modification, horizontal avoidance distance, and group cohesion change. All four species showed significant behavioural responses to noise exceeding 120 dB re 1 \u03bcPa (RMS), with response thresholds differing by species and noise type. Harbour porpoise showed the lowest response threshold (108 dB) and strongest behavioural disruption index (BDI = 0.84 \u00b1 0.08), while fin whales showed the highest threshold (128 dB) and lowest BDI (0.42 \u00b1 0.06). Vocal plasticity \u2014 measured as the frequency upshift and amplitude increase in calls during noise exposure \u2014 was detected in all four species (Lombard effect: 1.2\u20132.8 dB/dB noise increase). Shipping lane proximity was the strongest predictor of habitat use reduction in GPS-tracked individuals (r\u209b = \u22120.78, p < 0.001). Behavioural plasticity capacity was positively correlated with population stability across monitored populations (r\u209b = +0.72, p < 0.001), identifying vocal plasticity as a key resilience trait. These findings provide empirically grounded noise exposure thresholds and behavioural impact metrics for EU Marine Strategy Framework Directive Good Environmental Status assessment of cetacean populations