Distributions of sperm whales along the northwestern and central continental slope in the Gulf of Mexico as determined from an acoustic survey [abstract]

Recordings of sperm whales Physeter macrocephalus were collected via a towed passive hydrophone array. The study area ranged from the 100-2000m isobaths from the Florida-Alabama to the Texas-Mexico borders in the northwestern Gulf of Mexico, USA. The study area was divided into 14 north/ south transects at 74km intervals. Seven cruises were conducted on a seasonal basis from 1992-1994. Sperm whale vocalizations were identified based upon their unique spectral characteristics such as frequency range, duration, and temporal pattern. There were a total of 67 on-effort acoustic sperm whale contacts (a - contact is defined as an encounter with a vocal whale or whale group) during 1,055 hrs (11,997 km) of acoustic recording; corresponding to a rate of 0.064 acoustic contacts/hr (0.005 contacts/km). Chi- square analyses were conducted based upon acoustic level of effort. The average bottom depth per contact was 1,244m (sd=414). A Chi-square analysis of depth categories indicated that more sperm whales were observed than expected at a depth range of 711-1,190m (Chi-square=10.24, p=0.017) along the continental slope. Days were divided into six equal time of day (TOD) categories, and a Chi-square analysis of TOD categories indicated no significant difference in the number of sperm whale contacts across TOD (Chi-square=4.47, p=0.45). A Chi-square analysis of Beaufort wind speeds indicated no significant difference in the number of sperm whale contacts across Beaufort wind speed categories (Chi-square=8.01, p=0.91). A Chi- square analysis of seasons indicated no significant difference in the number of sperm whale contacts across seasons (Chi-square=1.91, p=0.59). A Chi- square analysis of ocean surface dynamic height data indicated that more sperm whales were observed than expected at a low dynamic height range of -30 to –10cm (Chi-square=4.81; p=0.00016). There were sperm whale concentrations in the Mississippi River Canyon and Western Gulf which may be associated with the Loop Current and eddy formations, and decay.