18 



The situation in Tampa Bay could improve in time. Stranding response teams 

 are becoming more active in Tampa Bay, 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 the dolphins in Tampa Bay would improve the 

 basis for identifying and evaluating disappearances of catalog members. 



Uneven stranding response effort in Tampa Bay over the six years of the 

 survey precluded trend analyses over the entire period of the project. The 

 unusually high numbers of strandings in 1991 and 1992, followed by a decline in 1993 

 (Figure 8) may be real. Dolphin strandings, both in Sarasota and more generally 

 along the central west coast of Florida, reached levels two to three times normal from 

 late 1991 through 1992 (unpublished data). The size of the Sarasota population was 

 estimated to have declined about 10% as a result of these unusual mortalities. The data 

 in Figure 7 hint at a similar decline in Tampa Bay, but no significant trend 

 (comparison of 95% CLs) was found. 



Immigration/Emigration/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 six- 

 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 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, a maximum of about 0.123 of the Tampa population was estimated to be 

 in flux each year, as immigrants, emigrants, or transients (Table 6). The low rates of 

 immigration, emigration and transience found for the dolphins in the Tampa Bay 

 study area in the six-year period suggest a relatively closed population. Resident 

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

 have extended home ranges. Based on the high proportion of marked animals (0.70) 

 that were only sighted once, Weigle (1990) concluded that a large number of 

 transients used Tampa Bay. Contrary to Weigle's findings, our results suggest there is 

 a high proportion of resident dolphins using Tampa Bay, some with extended home 

 ranges, and few transient animals. 



Summary of Population Rate Parameters for Tampa Bay 



Under stable circumstances during September - October, between 437 and 728 

 dolphins use the Tampa Bay study area. About 0.035 of these animals are young-of- 

 the-year, but this is likely an underestimate. At most, 0.055 of the dolphins present 

 are recent immigrants, but this value is elevated from the inclusion of dolphins that 

 have not immigrated, but have fins that have changed, or may have been present but 

 not photographed in previous years. About 0.023 of the dolphins will be considered 

 to be lost, through emigration, death, or because of undetected fin changes. 

 Transients account for 0.045 of the total population size. Immigration, emigration, 

 and transience are not major influences on the number of animals present at any 



