Bottlenose dolphin abundance, distribution, seasonal and long-term site fidelity in the Charlotte Harbor Ecosystem
By Kim Bassos-Hull, MSc, SDRP Research Associaten

Long-term studies documenting abundance trends and site fidelity of bottlenose dolphins in coastal estuaries can provide clues to the health of the ecosystem and stocks for management decisions. Dolphin distribution within these estuaries can be indicative of environmental features and prey distribution. We completed multi-week photographic identification surveys during September 2001, 2002, and 2003 and February 2002, 2003, and 2004 to determine dolphin abundance and distribution within Charlotte Harbor, as a continuation of National Marine Fisheries Service sponsored surveys conducted from 1990-1994 and in 1996. Smaller scale opportunistic surveys were done on occasion in some spring and summer months, for example, after Hurricane Charley passed through the study area in August 2004. Since 2001 we have spent 255 days on the water and collected 1,710 group sighting records. Two types of search effort were used: (1) a 1 km randomized grid transect which included cross-harbor, edge, and contour transects and (2) opportunistic transects both within the defined study area and in surrounding waters. We attempted to collect dorsal fin identification photographs of all dolphins in each sighting along with information on location, group size, numbers of calves, activities, and environmental parameters.

With photographic identification analyses mostly complete we have identified 851 different dolphins that use the Charlotte Harbor estuary and/or Gulf of Mexico surrounding waters. Repeated sightings of at least 471 marked individuals show they are present year-round and at least 390 dolphins show long-term site fidelity of five years or longer. Preliminary mark-recapture analyses indicate comparable numbers of dolphins in the region during the 1990’s summer surveys and the more recent 2001-2003 summer surveys (ie. N=423 with 95% confidence interval 369-499 in September 2001), with a potential increase in numbers in winter (ie. N=548 with 95% confidence interval 435-722 in February 2002). Preliminary examination of the distribution of sightings show relatively low numbers of dolphins near the river mouths during the summer rainy season when hypoxia (very low dissolved oxygen concentration) was recorded and larger numbers in these areas during the drier winter months when waters had higher salinities and were well mixed, with higher oxygen levels. Though most of the identified dolphins demonstrated strong site fidelity as reported for other Gulf of Mexico estuaries, a few individuals were documented traveling between estuaries. At least 32 dolphins with 10 or more sightings have been observed before and after Hurricane Charley and there was no obvious change in the distribution of their sightings. Funding for this project has been provided by the Mote Scientific Foundation and NOAA Fisheries (NMFS).