23 



ratio m/n was smaller in Regions 4 and 5 than in the other regions, and these 

 regions were over-represented in the survey efforts of later years as compared to the 

 other regions. This provided one potential explanation for the decline in the 

 overall m/n in later years, and may have contributed to the apparent increase in 

 abundance as evident from the results of Method 2. The "complete survey days" of 

 Method 3 control for survey effort, however, and the general level of agreement 

 between the results of Methods 2 and 3 suggest that a potentially biased m/n was not 

 a major contributor to the increase in abundance. 



The level of effort in Tampa Bay was greater and more consistent from year to 

 year than in Charlotte Harbor. For example, due to Hurricane Andrew coverage of 

 all regions in 1992 decreased to 51% - 65% of the kilometers surveyed in other years, 

 with a concomitant decline in M to 68% to 93% of the levels from the other years. 

 We examined the data for a direct relationship between survey effort and catalog 

 size, by regressing M against number of boat-days and numbers of kilometers 

 surveyed. No strong linear relationships were found, but M vs. boat-days 

 approached statistical significance (r 2 = 0.74, p = 0.06), hinting at the role of effort in 

 the development of an adequate catalog. Our findings suggest that an optimal level 

 of effort exists between that expended in Tampa Bay and that in Charlotte Harbor. 

 Empirical studies designed to identify the appropriate level of effort for mark- 

 recapture surveys would be helpful. 



Thus, methodological problems did not appear to be the primary factor in the 

 increase in the abundance of dolphins in Charlotte Harbor. Though the reasons for 

 the increase can not be fully explained with the information available, the increase 

 appears to be real, and appeared to be contributed to by several factors. The low CVs 

 associated with the abundance estimates provide additional confidence in the trends 

 that are evident. It is recommended that future surveys attempt to eliminate some 

 of the variables considered in the discussion above by striving for more intensive, 

 uniform effort throughout the study area. 



It is difficult to interpret comparisons of our abundance estimates to those 

 reported from aerial surveys of Charlotte Harbor, because of methodological 

 differences, and because of differences in the areas surveyed. The aerial surveys 

 typically reported abundance estimates from Charlotte Harbor and Pine Island 

 Sound combined, whereas our vessel surveys only included the northernmost 

 portion of Pine Island Sound, due to logistical constraints. Our average abundance 

 estimate from Method 2 (mark-proportion) for our limited survey area was 

 comparable to the upper 95% CLs reported from the same season by Thompson 

 (1981) and Scott et al. (1989) for their larger study area. As has been noted in other 

 comparisons of vessel vs. aerial surveys (Scott et al. 1989; Wells et al. 1995), the aerial 

 surveys appeared to have underestimated the numbers of dolphins in Charlotte 

 Harbor. 



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

 Charlotte Harbor study area at least once during a two- to three— week period in 



