Bottlenose Dolphin Abundance, Distribution, Population Structure, and Health in the Charlotte Harbor Ecosystem: Update and Findings
By Kim Bassos-Hull, MSc

A dolphin leaps near Captiva Pass in Charlotte Harbor on 30 September 2004.  Note the hurricane-damaged mangroves in the background.

We began three years of seasonal surveys in Charlotte Harbor, an estuary on the southwest coast of Florida, in September of 2001 as part of an integrated study of the Harbor’s ecosystem by several research centers based at Mote Marine Lab.  Our program’s primary goal was to determine the abundance and distribution of bottlenose dolphins during summer and  winter and to better understand the health and population structure of these dolphins through biopsy darting efforts.  As of March 2004 we have completed our six seasonal surveys, and are now moving forward with data entry, photo-id, and analyses.  This work has established a unique baseline that can be used, among other things, to evaluate the environmental changes brought on by Hurricane Charley when it passed through the middle of our study area on 13 August 2004. 

We have spent 253 days on the water since 2001 and collected 1,685 group sighting records.  Two types of search effort were used to collect sighting information; (1) a 1 km grid transect design which included cross-harbor, edge, and contour transects that was replicated two times each season and (2) opportunistic transects both within the defined study area and in the Gulf coastal waters and inshore areas to the north and south of the study area.  With about 75% of the photo-analysis complete we have identified 676 different marked dolphins, each with between one and eighteen sightings since 2001.  Repeated sightings of at least 471different marked individuals show they are present year-round (sighted in at least one September and one February field season).  During 1990-96 we conducted photo-id surveys in this region and at least 390 dolphins that were identified then have been re-sighted since 2001 showing long-term site fidelity in this area. 

Ninety-three blubber/skin biopsy samples have been collected from 88 different dolphins (65 males and 23 females) within the estuary and nearshore Gulf waters outside the passes (thanks Patty Rosel and Anna Sellas for running the gender analyses).  Of the 88 sampled dolphins, 60 have been identified and catalogued and have from 1 to 25 sightings.  So far eleven samples have been examined for organochlorine contaminant concentrations, providing suggestions of lower levels than in Sarasota Bay .

When Hurricane Charley passed through Charlotte Harbor in August the storm caused major ecological disturbance throughout the ecosystem.  Now there is interest from government agencies in assessing the post-hurricane impacts on the health and abundance of bottlenose dolphins in the Harbor especially since we have several years of pre-hurricane data.  Therefore we may be conducting more abundance surveys and collecting more biopsy samples to detect potential changes in coming years.  As a first step we conducted a subset of our transect surveys from August 24-28^th to look at potential immediate impacts from the hurricane.  With photo-analysis still underway we resighted several of our catalogued individuals present in the estuary during these surveys.  One carcass of a male dolphin that we have known since 1996 was recovered within a week after the hurricane.  Funding for this project has been provided by the Mote Scientific Foundation and NOAA Fisheries.