Squarenotch was one of the 466 dolphins that Wells documented as part of his University of California, Santa Cruz Ph.D. dissertation “Structural Aspects of Dolphin Societies,” which he completed in 1986. As part of that work, Wells examined the population structure and social units of dolphins in Southwest Florida. His studies further expanded knowledge of the Sarasota Bay dolphin community, building on research that began in 1970 with the seminal finding that the Bay’s dolphins were long-term residents across decades and generations. Today, the Bay’s dolphins are the focus of the world’s longest-running study of a wild dolphin population.
Finding that identifiable individual dolphins could be studied for extended periods of time in their own natural habitat has allowed researchers to gain insight into dolphin biology and physiology and delve deeply into the dolphins’ day-to-day lives of complex social interactions as a society. It has also improved understanding about their needs in the face of increased human activity in coastal waters.
“It’s exciting to document new calves each year as one measure of the health of the Bay’s dolphin population and the health of the Bay itself,” Wells said. “But the long-term nature of our research allows us to drill deeper and consider the question of why we had a record number of births. It appears that the red tide that reached Sarasota Bay in 2018 may have played a role.”
The red tide that entered Sarasota Bay in the summer of 2018 was unusually strong and lasted through the winter of 2018-2019, killing marine life and causing millions of dollars in losses to coastal economies. During and following the bloom, SDRP researchers:
- Caught fewer than half the average number of stingrays than expected during prey fish surveys conducted during the summers of 2019 and 2020. SDRP has been conducting catch-and-release purse seine fish surveys in shallow seagrass meadows since 2004. These surveys provide information on the relative abundance of dolphin prey and other fish in Sarasota Bay.
- Used photographic identification surveys to determine that Sarasota Bay dolphins were bitten by sharks in record numbers in 2019 and 2020.
- Documented that 45 percent of the females that gave birth to calves in 2021 had lost dependent calves during or since the red tide. On average, 31 percent of Sarasota calves do not survive more than two years, so there were increased losses of calves during the red tide period.
“Adding these facts together allows us to make some tentative inferences,” Wells said. “We know that stingrays are a primary prey item for sharks. When a preferred prey is unavailable, they’ll look to alternatives.
“The loss of typical shark prey may have led to the increased interactions we documented between sharks and dolphins as alternative prey, which allows us to infer that increased shark predation accounts for at least some of the losses of dependent calves during that period. When we document decreased ray catches, we tend to see increased disappearances of young dolphin calves.”
Since Sarasota dolphins typically rear their young for about four years, the loss of dependent calves before that age during and following the red tide (2018-2019) meant that more females were available to reproduce in 2020, contributing to the increase in the number of births this year, Wells said.
“The record number of births is a wonderful story in itself, but thanks to our long-term data, we’re able to develop hypotheses about some of the factors that may have led to this result, which should lead to a better understanding of what can happen to an animal population when an environmental anomaly occurs,” Wells said.
Primary funding for the photographic identification and prey fish surveys was provided by the Charles & Margery Barancik Foundation, which has a long-term commitment to supporting Sarasota Bay dolphin research.
“We appreciate the work of Dr. Wells and his team who track our dolphins, water-based ecological sentinels, that help us identify whether our local environment is healthy and sustainable,” said Teri A Hansen, President|CEO of Barancik Foundation.