Estimating the biosonar detection range of mesopelagic patches by spinner dolphins

Spinner dolphins (Stenella longirostris) in the near-shore waters of the Hawaiian islands forage on the mesopelagic boundary community (mbc) of organisms consisting of myctophids, mid-water shrimp and small squids. They forage at night and supposedly in a coordinated fashion and in groups between 16 and 24 animals that are divided into pairs. In the search phase of the foraging process, the dolphins are thought to be spaced in a V-shape formation with the tip of the “V” at the deepest depth, and swim parallel to shore hunting for patches of prey that they can encircle and herd into a tight three-dimensional patch. A profiler housing a broadband echo-ranger that projected dolphin-like biosonar signals was used to measure the target strength of the mbc based on a dolphin's integration window of 264 μs. The bottlenose dolphin, Tursiops truncatus, was used as a proxy to estimate the biosonar detection ranges of Stenella longirostris searching for mbc patches because only limited acoustic research has been performed with spinner dolphins. Using the sonar equation, the biosonar threshold detection range of spinner dolphins was estimated to be approximately 50 to 64 m, more than sufficient range for the animals to formulate their prey herding behavior.