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first two factors. The mark-proportion method (Method 2) sampled individual 

 dolphin schools as units; this led to a large number of replicates, for which a 

 bootstrap resampling method for estimating variance works well. Alternatively, the 

 resighting-rate method (Method 4) used the entire survey season as a sampling unit, 

 yielding large sample sizes per season (139-381 dolphins), but at the expense of 

 replicate sampling. The mark-resight method (Method 3) used two or three 

 "complete surveys" of the area as a sampling unit, and about 43-170 dolphins per 

 field season, with sample sizes of about 2-88 dolphins per survey. The CVs 

 calculated from Methods 2 and 3 were both acceptably low, although they cannot be 

 compared directly because of the difference in variance-calculation methods 

 (Method 2 = non-parametric bootstrap; Method 3 = binomial). 



All of these methods may be prone to a negative bias due to heterogeneity of 

 sighting probabilities, but this would be particularly true for Methods 2 and 4 if care 

 was not taken to survey all areas at least some time during the field season. 

 Estimates from Methods 2 and 4 averaged 4.9% and 20.1% lower than those of 

 Method 3. 



Power Analyses 



The power analysis has proved to be a useful tool for survey design and 

 management decisions. One can make a priori management decisions about the 

 duration, sampling intensity, and statistical certainty of survey programs if one can 

 estimate the CV of the methods being contemplated. Given the objectives to detect 

 a halving or doubling in the population from one year to the next, it appears that 

 Method 2 (mark-proportion method) can accomplish this goal for Charlotte Harbor 

 dolphins with annual surveys. Method 3 (mark-resight method) would require up 

 to three annual surveys, although it detected a significant increase of 56% between 

 1992 and 1993. The other methods require additional assumptions about the 1990- 

 1994 abundance stability and are thus less useful. CVs can be obtained or improved, 

 however, by sampling more often than the annual surveys chosen for this study, 

 although care must be taken that additional variation due to seasonal differences in 

 dolphin abundance, movements, and behavior is taken into account. 



Survey Design 



Selection of a survey technique for detecting trends in dolphin population- 

 rate parameters should take into account the relative accuracy, precision, 

 repeatability, and efficiency of the available methodology. Our findings from 

 Charlotte Harbor and Tampa Bay indicate that coastal aerial surveys, while more 

 efficient than photo-ID surveys at covering large areas, provide estimates that are 

 less accurate and less precise. 



The main reason for the close agreement among the estimates calculated 

 from the different methods and the precision of the CVs was the high percentage of 

 marked dolphins identified each year (58% to 80%). A large amount of survey effort 

 is required to maintain such a high percentage. Ideally, the surveys should have 

 two components: an intensive effort to photograph and identify dolphins (at the 



