16 



enables researchers to monitor not only population parameters but habitat use, social 

 association and distribution patterns. 



The high proportion of marked dolphins and the high frequency of 

 resightings underscores the importance of including only excellent quality images 

 of distinctively marked individuals in the photo-ID catalog. This minimizes 

 subjectivity in the matching process and reduces the chance of making incorrect 

 identifications or missing them altogether. 



Abundance Estimates and Trends 



Comparison of the point abundance estimates from Methods 2, 3, and 4 

 indicates striking consistency across methods, and lack of change across the six 

 years of the study (Figure 6). In all cases the lower 95% CLs were greater than or 

 equal to the minimum count provided by the catalog-size method. Thus, if we 

 consider the most extreme 95% CL values to be the limits to our estimates, the number 

 of dolphins using the Tampa Bay study area during the surveys was between 437 and 

 728. 



Our estimates are considerably larger than the aerial survey estimate of 148 - 

 348 (95% CL) reported by Scott et al. (1989) for the same months in 1985. In most 

 cases the numbers of dolphins from the catalog-size method exceed the aerial survey 

 estimates as well. It seems unlikely that the differences in the estimates over the 

 three years from 1985 to 1988 are due to dramatic changes in abundance, given the 

 lack of change in abundance over the six year period from 1988 through 1993. A 

 more likely explanation may be the differences in survey methods. 



A similar conclusion was reached by Scott et al. (1989) when they compared 

 their 1985 aerial survey maximum estimate of 23 (95% CL = 12 - 34) dolphins in 

 Sarasota Bay to published population size estimates of about 100 individuals. Aerial 

 surveys may tend to substantially underestimate the numbers of bottlenose dolphins 

 present, especially where there is high turbidity and/or low contrast between 

 dolphin coloration and water color, as is often the case in Sarasota. The Sarasota Bay 

 comparison may also exaggerate the differences resulting from survey methodology 

 because the study areas did not exactly coincide. The Scott er al. (1989) aerial surveys 

 did not include the entire home range occupied by the 91 known members of the 

 Sarasota dolphin community in 1985 (Wells and Scott 1990), and therefore may not 

 have included some resident dolphins in their estimate. Scott er al. (1989) also 

 suggested that the estimated resident abundance may not accurately reflect the 

 average daily abundance for the Sarasota dolphin community. While it is true that 

 some Sarasota residents may not be present in the home range every day, non- 

 residents passing through Sarasota may at least partially compensate for this 

 decrease in daily abundance (Wells and Scott 1990). Thus, short-term movements 

 alone probably do not adequately explain the fact that the aerial survey estimates 

 were only 25% of the known 1985 Sarasota population. We are left with 

 methodological rather than biological differences to account for much of the 

 difference in estimates. 



The estimates we have derived reflect the numbers of dolphins found in the 

 Tampa Bay study area at least once during a six-week period in September and 

 October of each year. The estimates are based on a catalog that includes all of those 

 dolphins for which satisfactory identification photographs were obtained during the 

 survey period, without distinguishing between differences in the degree of use of 

 the study area waters by different dolphins. 



