Graduation Year

2012

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

David A. Mann

Keywords

abundance, acoustics, distribution, dolphin, oceanography

Abstract

This dissertation is an investigation of dolphin sound production and distribution off west central Florida. Although a wealth of information exists on the production of common sounds (whistles, echolocation) made by captive, trained dolphins, far less is known about free-ranging dolphin sound production and of unusual sounds. In addition, while inshore dolphin populations or communities are the subjects of research projects in many locations, dolphins in offshore waters are less commonly studied. The objectives of this dissertation were to contribute information on free-ranging dolphin sounds and continental shelf dolphin distribution.

While echolocation has been rigorously studied in captive, trained dolphins, there is far less known about how free-ranging dolphins use their echolocation. In order to investigate the use of echolocation by free-ranging dolphins, echolocation recordings from 14 groups of common bottlenose dolphins (Tursiops truncatus) were obtained during towed hydrophone cruises on the West Florida Shelf (WFS) and in Tampa Bay. The mean echolocation pulse rate was inversely related to water depth, suggesting echolocation pulse rate was a function of the two-way travel time of echolocation pulses, which was related to depth. Pulse rate modes were related to potential target distances, and indicated dolphins were commonly echolocating on targets up to at least 91.8 m away. The results of this study indicate that free-ranging bottlenose dolphins are using their echolocation in a manner similar to that found in studies with captive, trained dolphins.

Unusual low frequency sounds from bottlenose dolphins were found in the towed hydrophone recordings in Tampa Bay, and the acoustic properties and behavioral contexts of these sounds were investigated. Additional recordings were obtained from Sarasota Bay and Mississippi Sound. These low frequency narrow-band (LFN) sounds were tonal, had peak frequencies between 500 Hz and 1000 Hz, and were produced in trains. Inter-LFN intervals (the time duration between sequential LFN sounds) were significantly longer in recordings from Mississippi Sound. Sounds were correlated with social behavior, and were common during socio-sexual behavior. These sounds were found below optimal hearing range of bottlenose dolphins, and are prone to masking by boats.

A combination of autonomous acoustic recorders and visual surveys were used to determine the distribution and sound production patterns on the WFS. Visual surveys supported the results of previous studies indicating that bottlenose dolphins were more common in coastal areas and off of Tampa Bay, while Atlantic spotted dolphins (Stenella frontalis) were more common beyond the 20 m isobath. A single group of rough-toothed dolphins (Steno bredanensis) was observed. Overall, dolphin numbers decreased from inshore to offshore. Acoustic detections mirrored this distribution pattern, however acoustic detections were not as high in coastal regions as expected from the visual survey results, which suggests low sound production rates by coastal dolphins. Atlantic spotted dolphin numbers increased in more northern and inshore waters in spring, suggesting a seasonal migration pattern. Peaks in dolphin sounds in the coastal regions were commonly observed in daylight and evening hours, while in offshore areas sound production peaked at night. This pattern likely reflects foraging activity, and the diel activity cycles of common prey species. Coastal dolphins made proportionately more echolocation than whistles, while the opposite was true for deeper water dolphins.

In inshore waters (< 25 m depth), dolphin sound production was generally positively correlated to water temperature (bottom temperature and sea surface temperature) and negatively correlated with chlorophyll, while the opposite pattern tends to occur in deeper waters (> 35 m). This delineation roughly coincides with the distribution patterns of oceanographic properties, prey species distribution, and the distribution of Atlantic spotted dolphins and bottlenose dolphin ecotypes. These results suggest a shift from a benthic based ecosystem to a phytoplankton based ecosystem with increasing depth on the WFS.

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