22 



The primary methodological differences involved level of effort. We had 

 fewer boat-days each year for the Charlotte Harbor surveys than for the Tampa Bay 

 surveys due to budgetary limitations. Though the Charlotte Harbor study area was 

 82% as large as the Tampa Bay study area, we had only 56% as many within-study- 

 area boat-days each year compared to Tampa Bay. Fewer boat-days translated into 

 fewer kilometers of survey transects, which meant less intensive photographic 

 coverage of dolphins in the study area than was accomplished in Tampa Bay. This 

 in turn might have affected the development of the identification catalog, resulting 

 in an artificially low M in some cases. Differences in weather conditions from year 

 to year resulted in varying geographical coverage within the study area, which may 

 also have affected the size of M, and may have influenced m/n as well. Each of 

 these factors is critical to the calculation of abundance estimates. 



Each of the abundance estimation procedures assumed that M accurately 

 represented the pool of marked dolphins in the study area during the survey period, 

 and was independent of level of effort. The high proportion of marked dolphins 

 (m/n), the relatively consistent values for M from year to year, and the numbers of 

 resightings of marked individuals over the course of each survey suggested that we 

 had obtained reasonable coverage and established a representative identification 

 catalog in Tampa Bay (Wells et al. 1995). In Charlotte Harbor, however, m/n 

 declined over time, the numbers of resightings per individual were smaller than 

 Tampa Bay (Figure 6), and M fluctuated across years. 



One way in which effort might influence M would be through uneven 

 geographical distribution of surveys resulting in differential exposure to marked 

 individuals. Given the existence of individual ranging patterns as proposed earlier 

 in this report, decreased survey coverage of portions of the study area might mean 

 fewer opportunities to photograph residents of those regions, resulting in a smaller 

 and inaccurate M. Effort was not uniform across regions from year to year (Table 2). 

 Adverse weather conditions made it difficult to reach the more distant regions, 

 including Region 4 (Charlotte Harbor North) and Region 5 (northern Pine Island 

 Sound, Figure 1), during some years. Our survey coverage of these two regions in 

 1994 was approximately double the coverage during the early years, and M was 

 greater than in any previous year. 



Region 5 was a potential source of complications regarding M both because 

 coverage was variable from year to year, and also because it opened into greater Pine 

 Island Sound to the south, a potential source of new dolphins or destination for 

 previously identified dolphins, outside of our study area. We attempted to control 

 for these complications by recalculating abundance estimates without including 

 Region 5 sightings, or the marked dolphins sighted only in Region 5. This analysis 

 showed that Region 5 had little effect on M or on the abundance trend. 



We considered the possibility that uneven geographical coverage could result 

 in a biased m/n. If this ratio varies from region to region, then differential coverage 

 could result in a biased overall ratio, as applied in Method 2. We found that the 



