20 



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) and Method 3 (mark-resight method) can 

 accomplish this goal for Tampa Bay dolphins with annual surveys. The other methods 

 require additional assumptions about the 1988-1993 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 Desig n 



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 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 (eventually over 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 potential expense of not following a rigorous survey route or 

 sampling design), and an effort to cover the whole area in a short period of time with 

 repeatable survey routes. The first component allows the development of the photo- 

 ID catalog so that sufficient numbers of marked dolphins are identified to estimate 

 abundance precisely, while the second component would provide a standardized 

 effort each year so that annual comparisons can be made. 



Method 3 (mark-resight method) would provide satisfactory estimates from the 

 second component of such a survey because the statistical properties of the more- 

 traditional mark-recapture methods are well-known and the sampling units provided 

 adequate sample sizes of marked animals. In Tampa Bay, however, it proved difficult 

 to conduct "complete surveys" within the available survey window. Instead, we 

 could only survey regions repeatedly while conditions were favorable when other 

 regions were unworkable, and then shift our efforts opportunistically. If "complete 

 surveys" can not be conducted, then Method 2 (mark-proportion) provides an 

 acceptable alternative as long as the numbers of sightings and proportion of marked 

 dolphins are high, and the effort among different regions is not greatly biased. This 

 method is particularly useful because it can be more-readily calculated from the first 

 component of the survey design during which the largest numbers of groups would 

 be sighted. Methods 1 (catalog-size method) and 4 (resigh ting-rate method) provided 

 useful double-checks on the estimates of the other two methods. 



