Investigating potential hurricane and red tide related impacts on bottlenose dolphin abundance, reproductive rates, distribution, and site fidelity in Charlotte Harbor and Pine Island Sound.
By Kim Bassos-Hull, MSc

Some of the Charlotte Harbor survey team at Demere Key. From left to right, Rubai and Elisabeth Fahrni Mansur, Laura Markley, Aaron Barleycorn, Robin Perrtree, Goldie Phillips, Katie McHugh, Vanessa Greenwood, and Kim Bassos-Hull.

On August 13, 2004, Hurricane Charley, a major Category Four storm, swept a path through Charlotte Harbor and Pine Island Sound on the west coast of Florida causing widespread ecological damage. Subsequently, Charlotte Harbor and Pine Island Sound were affected by prolonged red tide episodes at fish kill levels throughout 2004 and 2005. These combined events may have affected habitat health, including prey availability, in ways that could have short or long-term implications for the bottlenose dolphins that use this estuary. Our program was in a unique position to evaluate potential impacts from these natural events because of existing baseline data. Dolphin abundance, reproductive rates, distribution, and site fidelity was collected in this region during intensive seasonal surveys immediately prior to the 2004 hurricane. Longer-term data on residency was collected over the last 24 years. Knowledge of the status of the dolphin population units inhabiting Charlotte Harbor and Pine Island Sound is important for management, and follow-up surveys would provide a unique opportunity to investigate the adaptability of these animals to large scale disturbances.

With funding from Harbor Branch Oceanographic Institution's Protect Wild Dolphins program, we conducted a comprehensive, multi-week photographic identification survey in Charlotte Harbor and Pine Island Sound in September 2006 at the same scale as our 2001-2003 September surveys. Using 3 boats we surveyed two complete sets of transects between September 5-22nd, for a total of 46 boat-days. Overall, we had 246 sightings with 1,222 dolphins, 225 calves, and 71 young-of-the-year. We took 10,318 photographs and are currently in the grading and photo-identification stage of analysis. As of November 15^th, we have matched 186 individuals to our catalog. Prior to these September 2006 surveys we completed two smaller scale (3-4 day) surveys after Hurricane Charley in August and September 2004 with emergency funding provided by Mote Marine Laboratory. From the August and September 2004 surveys we identified 32 dolphins that had at least 10 sightings. These dolphins had been observed both before and after Hurricane Charley with no obvious change in their sighting distributions. With the completion of these larger scale September 2006, surveys we will be able to look at abundance trends and reproductive rates while amassing a larger set of animals to compare pre- and post-Hurricane Charley sighting distributions. Preliminary reproductive rates from these September 2006 surveys were within the range of the September 2001-2003 values (mean young-of-the-year proportion in 2006 = 0.042 ± 0.103 SD) compared to 2001 (0.020 ± 0.069 SD), 2002 (0.037 ± 0.097 SD), and 2003 (0.047 ± 0.110 SD). We are also interested in looking at the health of these animals to see if there are any effects from red tide or other habitat changes. We have worked with Mote’s Stranding Investigations Program to identify carcasses and measure brevetoxin levels when possible. In addition we have observed lesions on several individuals and will compare current sightings to those from the last few years. Over the next six months we hope to complete analyses and determine whether these dolphin communities in Charlotte Harbor and Pine Island Sound were affected by these large scale ecological disturbances