Population structure of bottlenose dolphins in and around St. Joseph Bay, Florida
By Brian Balmer, MSc Candidate

Bottlenose dolphins along the Florida panhandle experienced three unusual mortality events in 1999, 2004, and 2006, in which over 320 dolphins died. A large number of these strandings were located near St. Joseph Bay, FL. However, it is not known which stock(s) were impacted. This project represents the first effort to identify dolphin ranging patterns in this region. We used three methods to determine these patterns: photo-identification/biopsy darting surveys, mark-recapture abundance estimates using photo-identification, and radio tracking of individuals.

Photo-identification/biopsy darting surveys were undertaken during April—May 2004, February—July 2005, and February 2006. Over 250 individuals have been identified, and half of these have been identified in multiple seasons. One hundred and one biopsy samples have also been collected from individuals along the Florida Panhandle. These samples are currently being analyzed by Sue Carney (Mote Marine Laboratory) and Patty Rosel (National Marine Fisheries Service).

Mark-recapture photo-identification surveys were conducted February, March, April, May, and July 2005, as well as February and September 2006. Preliminary data from these mark-recapture abundance estimates suggests a two- to three-fold increase in dolphin numbers during the spring and fall seasons. Summer and winter seasons appear to have a smaller, yet more stable number of animals within the St. Joseph Bay region.

In April 2005 and July 2006, NOAA sponsored two capture-release health assessments of dolphins in the region; twenty four individuals were tagged with VHF radio transmitters. These dolphins were monitored daily through boat, aerial, and/or vehicle tracking for over 109 days. Dolphin utilization areas were estimated for each radio tagged animal by measuring the distance of shoreline between the farthest east and west sighting location, and multiplying that distance by the maximum distance the dolphin was located offshore. In comparing the tagging results from the April 2005 and July 2006 St. Joseph Bay health assessments, there are some interesting trends observed. The average utilization areas for animals tagged in April 2005 and July 2006 are very similar, suggesting similar range size between the animals tagged in both health assessments. However, the distribution of animals during these two time periods is notably different. The ranges of all animals from July 2006 covered approximately 65 km of coastline, predominantly within the St. Joseph Bay region. However, four of the tagged animals from April 2005 had ranges extending well past the St. Joseph Bay region, covering approximately 165 km of coastline. These data, along with ongoing photographic identification and genetic analyses, are beginning to suggest the presence of animals with differing utilization area patterns within the same geographic region, possibly with a seasonal component to the variation.

These results, although preliminary, suggest that the 1999, 2004, and 2006 unusual mortality events may have impacted multiple stocks of dolphins along the Florida panhandle, and/or that dolphins originating in regions to the east and west may now be utilizing habitat vacated by dolphins that died during the UMEs.

This research would not be possible without funding from NOAA Fisheries and the Disney Wildlife Conservation Fund.