Sperm whales ability to avoid approaching vessels is affected by sound reception in stratified waters

Collision with vessels is a major cause of whale mortality in the Mediterranean Sea. The effect of non-spherical sound propagation effects on received levels (RL) was investigated for the sperm whale (Physeter macrocephalus). Relevant dive patterns were considered in each case and the RL were compared for two periods using a ray tracing software, the winter conditions and the summer stratified situation. RL were plotted as a function of time in a simulated collision case for two vessel speeds representative of a conventional merchant ship (15 knots) and a fast-ferry (37 knots). In almost all simulated cases, RL featured a brutal 23-31 dB re 1 μPa rise from below 100 dB while the vessel approached the whale at close range. Summer situations were worse because this transition occurred at closer ranges, resulting in acoustic warning times of less than 30 s in the fast ferry case. These results suggested that sperm whales could not be able to achieve an escape manoeuvre in a critical situation such as a fast vessel approaching under stratified waters conditions.