| Odontocete cetaceans possess a highly sophisticated sonar system capable of detecting prey at long ranges in turbid environments. The prey of dolphins likely experience selection pressure on the ability to detect dolphins acoustically, meaning that echolocation may incur ecological costs for dolphins. Therefore, one might expect dolphins to use sensory modes other than echolocation to detect prey in some situations. In this study of bottlenose dolphin ( Tursiops truncatus) foraging ecology, I quantified the species composition of the diet; characterized acoustic behavior of an important prey, Atlantic croaker (Micropogonias undulatus); examined factors that may influence habitat selection decisions of dolphins in the Neuse River estuary; tested the hypothesis that dolphins locate prey by passive listening; and investigated croakers' responses to sounds produced by bottlenose dolphins.; Croaker, weakfish (Cynoscion regalis), and spot ( Leiostomus xanthaarus), all from the family Sciaenidae, were the most important prey. Fifty-one percent of the prey items from dolphins inside estuaries were croaker. Juvenile and adult croaker produced broadband acoustic calls that spanned a range of over 7 kHz, with an average peak frequency of 1.3 kHz. This calling behavior was not associated with spawning but appeared to function as a means of maintaining school cohesion. Maximum source levels of these calls were approximately 145 dB re 1 μPa @ 1 m (maximum RMS). Croaker calling rates peaked at night, but they dominated the soundscape of the Neuse River estuary during the day. Individual fish were likely detectable from a distance of 50 m and large aggregations from 630 m.; Dolphins selected tributary creeks and the edges of the Neuse River (<3.5 m), and selection was correlated more strongly with the rate of croaker calling than with prey density. Using acoustic playback experiments, I demonstrated that bottlenose dolphins locate prey by passively listening for their calls. Thus, dolphins appear to use passive listening at multiple spatial scales during the foraging process: they may select habitat within an estuary based on the occurrence of fish calls (kilometers to tens of kilometers) and they listen for individual fish calls to locate specific prey (tens to hundreds of meters). Croaker respond to lower frequency components of dolphin echolocation clicks by reducing their turning rate, a strategy that could allow them to avoid detection.; Dolphin echolocation is highly sophisticated, but is used judiciously during foraging. This paradox may result from ecological costs associated with echolocation. Bottlenose dolphins and their prey may be engaged in an evolutionary arms race, similar to that described between bats and insects, in which improvements in prey detection by predators drive improvements in countermeasures employed by prey, and vice versa. |
