25 



Mortality 



Measurements of dolphin mortality rates for Charlotte Harbor proved to be 

 difficult to obtain during our survey period. In most cases we were unable to 

 distinguish between mortalities, emigrations, undetected fin changes, and animals 

 missed during the Charlotte Harbor surveys. In Sarasota, it has been possible to 

 evaluate losses from the population from two directions, through the collection and 

 examination of carcasses of identifiable individuals, and through records of 

 disappearances of known individuals (Wells and Scott 1990). Mortality estimates 

 are facilitated in Sarasota as compared to the Charlotte Harbor project because 

 Sarasota involves a smaller number of dolphins with a higher proportion of them 

 being identifiable, a smaller study area, a more-intensive, year-round monitoring 

 effort, and more-complete and consistent stranding response effort. 



The number of strandings reported during the Charlotte Harbor survey may, 

 however, provide a relative index for comparison of mortality patterns. Dolphin 

 strandings in Sarasota Bay, Tampa Bay and more generally along the central west 

 coast of Florida followed the Charlotte Harbor pattern of dramatic increase from 

 1990 to 1991-1992, with a decline in 1993 (Wells et al. 1995). In Sarasota, strandings 

 reached levels two to three times normal from late 1991 through 1992 resulting in a 

 10% decrease in the size of the Sarasota population (unpublished data). No such 

 decline was observed in Charlotte Harbor, however. Severe red tides from blooms 

 of the dinoflagellate Gymnodinium breve occurred along the central west coast of 

 Florida during 1991, 1992, and 1994, the years of greatest numbers of strandings. 

 Though no direct cause-effect relationships between red tide outbreaks and dolphin 

 mortalities have yet been identified conclusively, the correlation noted here and 

 elsewhere (Geraci 1989) suggests that further investigation may be warranted. 



Uneven stranding response effort in Charlotte Harbor over the five years of 

 the survey precluded quantitative trend analyses over the entire period of the 

 project. The situation in Charlotte Harbor could improve in time. Stranding 

 response teams are becoming more active in Charlotte Harbor, and communication 

 between teams is improving. We know that good photographs of fresh carcasses can 

 provide the basis for identifications (Urian and Wells 1993). These identifications 

 are important not only for monitoring the population, but also because knowing the 

 origin of a carcass can provide information that may aid in understanding cause of 

 death or interpreting levels of environmental contaminants in tissues. Long-term 

 and more frequent photographic monitoring of Itie dolphins in Charlotte Harbor 

 would improve the basis for identifying and evaluating disappearances of catalog 

 members. 



Immigration /Emigration/Residency /Transience 



Both immigration and emigration rates are difficult to interpret because of a 

 number of potentially confounding factors. The survey effort was limited to a two- 

 to three-week period, thereby minimizing the opportunity to identify dolphins in 

 other times of the year and other areas. Changes to the fins may hinder our ability 

 to identify individuals, resulting in the scoring of the changed fin as a new 



