Sarasota Dolphin Research Program

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Autonomous acoustic recordings of dolphins on the West Florida Shelf: Preliminary results and current research
By Peter Simard, PhD Student, Carrie Wall, MS, PhD Student, and David Mann, PhD, University of South Florida

DEVO hydrophone array deployed in June 2009.

In 2008, a small array of prototype autonomous acoustic recorders was deployed in the Gulf of Mexico offshore between Sarasota and Tampa Bay in 5 to 30 m depth. Nineteen of these recorders, developed by researchers and engineers at the University of South Florida, collected acoustic recordings for approximately three months. Fifteen recorders were recovered in late 2008 (four were lost), each containing hundreds of hours of recordings. One long-term goal of this research is to identify species through differences in frequency content of echolocation clicks, and peak frequency and bandwidth (among other parameters) of whistles. Through our own visual surveys and previous studies, we know that bottlenose dolphins and, to a lesser degree, Atlantic spotted dolphins are common in the area, and other cetacean species are very rare. Therefore, the results presented here are likely for both of these common species combined.

Preliminary analyses have been conducted for five recorders. Three recorders were located to the north of Tampa Bay; two were located south of Sarasota Bay. There was no significant relationship between whistles in the recordings and time of day. However, echolocation occurred significantly more often in daylight hours, suggesting that foraging in offshore areas tends to occur during the day. One potential explanation is that dolphins generally rest and socialize at night. Alternatively, many fish are more acoustically active at night, and passive listening may be used as a foraging technique by these dolphins at night, reducing the need for echolocation. Spatial patterns were also identified. Both echolocation and whistles were recorded more frequently by two recorders north of Tampa Bay than by the two recorders south of Sarasota Bay. In shallower water (~15m), whistles made up a lower proportion of recorded sounds than echolocation (< 25% of recordings were whistles). However, in deeper water (~30m), whistles were far more common (40% - 100% of recordings were whistles).

Since the initial 2008 deployment, a new generation of acoustic recorders were designed and constructed, and over 80 recorders at over 60 locations on the West Florida Shelf were deployed in June 2009. This array, the largest civilian hydrophone array in the world, extends from Tarpon Springs to Charlotte Harbor in near shore waters to the shelf break (~100m, with an additional station in 1500m depth), and will operate for a full year. With the development of automatic detection algorithms, we will soon be able to analyze the full data set from this deployment as well as the 2008 deployment, allowing a complete analysis of the spatial and temporal patterns of dolphin distribution based on sound production. Visual survey data conducted in the study area will also be used to determine distribution patterns, and photographic identification data will be compared with the Chicago Zoological Society Sarasota Dolphin Research Program catalog of Sarasota Bay dolphins to determine the distribution and habitat use of specific individuals. A goal is to explain these patterns in the context of oceanographic fronts identified from satellite ocean color data coupled with 3-dimensional physical oceanographic models. Understanding dolphin distribution on a large spatial and temporal scale in relation to easily identified oceanographic features will have a large impact on dolphin conservation on the West Florida Shelf. This is particularly important given the recent interest in petrochemical exploration in these waters. Funding for this project was provided by the National Oceanographic Partnership Program. Peter Simard conducted this work as part of his dissertation research; R. Wells serves on his graduate committee.