26 



identification and the original identification as a loss. Unidentified or missed 

 mortalities obscure actual emigration rates by counting them as losses instead of as 

 known mortalities. It is also possible animals were in the study area but not sighted, 

 or were photographed but not identified because of inadequate photographic quality 

 or coverage (Slooten et al. 1992). 



Overall, about 9% of the Charlotte Harbor population was estimated to be 

 transient, whereas an average of 53% of the identifiable dolphins was known from 

 multiple years. The low incidence of immigration, emigration and transience 

 found for the dolphins in the Charlotte Harbor study area in the five-year period 

 suggest a relatively closed population, at least during the August survey period. 

 Resident dolphins have a greater chance of being resighted than do animals that are 

 known to have extended home ranges. Several individuals have been resighted in 

 the study area opportunistically during different seasons. 



The apparent increase in abundance over the five years, and the dramatic 

 seasonal increase reported from the aerial surveys suggested that Charlotte Harbor 

 may not be as closed a unit as Sarasota or Tampa Bays. Seasonal increases from 

 summer to winter of 176% and 223% reported by Thompson (1981) and for Charlotte 

 Harbor and Pine Island Sound are much greater than the 25% seasonal increase 

 reported for Tampa Bay (summer to autumn, Scott et al. 1989). Shane (1987) 

 reported seasonal changes in patterns of occurrence in Pine Island Sound, but did 

 not present estimates of change in abundance. No significant seasonal changes in 

 abundance have been noted for Sarasota Bay, although seasonal changes in habitat 

 use were evident (Wells 1993). Assuming that the seasonal variations in Charlotte 

 Harbor reported from the aerial surveys reflect a true increase in abundance, then 

 photographic identification surveys during the season of greatest abundance mav 

 shed light on the potential source of some of the increase in abundance reported 

 from our August surveys. 



Summary of Population Parameters for Charlotte Harbor 



During August of each year from 1990 through 1994, an average of about 308 

 dolphins used the Charlotte Harbor study area (average of Methods 2 and 3). The 

 abundance apparently increased from 198 - 369 (95% CLs, Methods 2 and 3) in 1990 - 

 1992 to 315 - 463 in 1993 - 1994. Part of this increase appeared to be due to an increase 

 in reproduction. The average natality across the study years was 0.034, but a peak of 

 0.05 was reached in 1993. The increase in the proportion of calves from 0.12 in 1990 

 to 0.21 in 1993 and 1994 suggests the successful recruitment of many of the young-of- 

 the year. It was not possible to calculate rates of immigration or emigration. 

 Evidence from the high proportion of animals present in multiple years and the 

 absence of documentation of unidirectional movements between Charlotte Harbor 

 and other adjacent and distant contiguous study areas along the central west coast of 

 Florida indicate that permanent immigration and emigration appear to be rare 

 events. About 9% of the dolphins appeared to be transients. Immigration, 

 emigration, and transience are not major influences on the number of animals 

 present at any given time, but they may be important ecologically by providing a 



