The IHA’s January–May restriction avoids 77% of annual risk. The restriction is based on whale migration patterns, not acoustics. The Level B harassment zone is nearly identical year-round (~103 km²). All the seasonal variation comes from biology — where the whales are, not how far the sound travels.
NARW density: Roberts et al. 2024, Vineyard Wind lease area, monthly mean. Zone area: 103 km² (Level B harassment zone radius of 5.7 km per IHA, held constant across months as a simplification — RAM PE shows <10% seasonal variation in zone radius due to sound speed changes). Expected takes = density (animals/100 km²) × zone area (km²) / 100. Risk levels are illustrative thresholds for visual comparison: HIGH (>0.5 expected takes/pile), MEDIUM (0.2–0.5), LOW (<0.2). These are not regulatory standards.