12 



allow a comparison of within-region effort across years. Differences across years 

 reflect the effects of weather, variable numbers of boats, and the use of different 

 field stations that facilitated access to different regions. 



Dolphins were seen throughout the study area, but they were not uniformly 

 distributed. Larger groups tended to be found in the more open and deeper waters 

 (Figures 2a-e). The total number of sightings and dolphins seen each year closely 

 track the level of survey effort (Figure 3). The number of photographs taken was 

 related to the number of dolphins. On average, 5-6 photographs per dolphin were 

 taken each year. 



Photo-ID Catalog Development 



The level of survey effort was considered sufficient to warrant generation of 

 abundance estimates based on mark-resighting analyses. This conclusion was 

 supported by the high proportion of identifiable dolphins in the population (62% to 

 82%, Table 2), and the frequency distribution of resightings of identifiable dolphins 

 within survey years (Figures 4a-f). One third to one half of the dolphins were 

 sighted at least twice during a given survey year, up to a maximum of 13 times each. 

 A low number of resightings would have suggested insufficient coverage of the pool 

 of marked animals, resulting in population estimates that varied with the level of 

 survey effort rather than being independent of effort. 



Our Tampa Bay catalog for 1988-1993 included 858 different dolphins. The 

 catalog size provides a minimum population estimate for the Tampa Bay study area 

 ranging from 319 identifications in 1990 to 456 in 1992. On average, 57% of the 

 dolphins in an annual catalog were also seen in either the previous or subsequent 

 year, 52% were seen two years earlier or later, 47% were seen three years earlier or 

 later, 44% were seen four years earlier or later, and 35% were seen five years earlier 

 or later (Table 3). 



Photographs taken during the 1988-1993 NMFS surveys built upon an existing 

 Tampa Bay catalog of 150 animals identified during 1975-1987 (Figure 5; Wells 1986). 

 As expected, during the initial years of the surveys a large number of identifications 

 were added to the catalog. New fins were added to the catalog at a slower rate during 

 subsequent years (Figure 5). The proportion of first-time identifications comprising 

 the total catalog each year declined from 74% in 1988 to 14% in 1993. These results 

 are comparable to those from the Sarasota community (Wells and Scott 1990), 

 suggesting a relatively closed population for the Tampa Bay study area. 

 Identifications added to the catalog over the years may represent changes to the fins 

 of known animals, non-distinctive calves acquiring new markings (only a small 

 number of calves are in our catalog), or animals that may have been missed in 

 previous years. We found that overall there were few changes to fin markings 

 throughout the surveys, and minor changes could be detected by a skilled observer 

 familiar with the catalog. However, dramatic changes to fin markings could easily be 

 undetected and could result in a previously identified animal being entered twice in 

 the catalog. 



The stability of fin markings over time enhances the probability of resighting 

 individuals. The high frequency of resighting individuals and the long-term 

 sighting histories suggest a high degree of residency for some animals in the Tampa 

 Bay study area during the survey period (Figure 4a-f). The consistency of the catalog 

 and stability of fin markings over time contribute to our confidence in meeting the 



