11 



of known and potential immigrants relative to the total catalog size. This 

 immigration rate should be considered an overestimate because it was not possible to 

 factor out additions to the catalog resulting from undetected changes to the fins of 

 existing residents, and animals present but not photographed during 1988-1990. 



Emigrants from the Tampa Bay study area were defined as: ( 1 ) dolphins 

 identified in the first three years of the surveys but not identified in the last three 

 years, and (2) dolphins identified in the first four years, but not identified in the last 

 two years. Potential emigrants were checked against known mortalities from 

 stranding records and photographs. Sighting records from the DBRI database were 

 examined to identify sightings of these individuals in other areas and years. An 

 emigration rate was calculated based on the proportion of the number of known and 

 potential emigrants relative to the total number of marked animals in the catalog. 

 The rate of emigration should be considered an overestimate because we were not 

 able to differentiate between disappearances due to emigration, mortality and 

 undetected changes to the dorsal fin, and animals present but not photographed 

 during the last two or three years. 



The incidence of transience was estimated by identifying individuals that were 

 sighted in only one year of the six-year survey period and had no other sighting 

 records in the DBRI database. To calculate a rate of transience, we selected the years 

 1990 and 1991 to minimize the probability that an animal might be an immigrant or 

 emigrant. The incidence of transience was estimated to be the proportion of 

 individuals that met the criteria above relative to the total catalog size for each 

 survey year. This rate is probably an overestimate because it may include dolphins 

 that in fact are not transients, but were missed during other surveys, died, or their 

 fins changed without being detected. 



The strict criteria used for defining immigrants, emigrants and transients 

 preclude calculating rates for more than the two years, 1990 and 1991. Therefore, 

 trend analyses were not possible for these parameters. 



Results 



Survey Effort 



Surveys were conducted during windows of 34-42 days each year (Table 1). 

 The size of the window each year depended on weather and the number of boats 

 available. Unseasonable cold fronts or tropical storms adversely affected survey 

 schedules in several years. During the first years of the project, only two boats were 

 used, but beginning in 1990 as many as three or four boats were used. Survey effort 

 was measured in several ways. One measure was a count of the number of boat days. 

 A boat day was scored when a boat left the dock to search for dolphins. On average, 

 42 boat days were spent in the study area each year (range = 30-54 days, Table 1). A 

 more refined measure of survey effort is provided by considering the numbers of 

 hours spent searching for dolphins within the survey area. The total number of 

 search hours (exclusive of time spent with each sighting) spent "on-effort" (under 

 excellent, good, or fair survey conditions, see appendix) is presented in Table 1. An 

 average of 113 hours of on-effort search time was spent each year (range = 85-141 

 hours). 



Another measure of effort is the number of linear kilometers covered by our 

 survey boats. These data are summarized in Table 1 , and are presented by region to 



