A comparison of visual and acoustic marine mammal monitoring methods

Abstract

Concern about the potential effect of increased oceanic noise on marine mammals has led to the consideration of a variety of at-sea marine mammal monitoring methods. A recent marine mammal survey utilized both visual observation and passive acoustic monitoring. Visual observations were conducted using 7×50 binoculars during the day and generation III night vision devices (NVDs) at night. Acoustic data were collected with a towed hydrophone array and analyzed with a customized, PC-based acoustic workstation. Seventy-seven visual sightings were made. Forty sightings were made at night and 37 were made in daylight. The effective range of the NVDs was estimated through observations of a target at known distances; 50% detection rating was achieved at 130 m. There were 98 acoustic detections. Of the 77 visual sightings, 42 were also detected acoustically. Six sightings were made without comparable acoustic detection, and 19 sightings were made when there was no acoustic monitoring. Conversely, 56 acoustic detections were made without visual sightings. These data suggest that, for species that commonly vocalize, a combined acoustic/visual survey will increase detection probabilities. Furthermore, night vision devices have the potential to be an effective observation tool for marine mammals. [Work sponsored by ONR.]