14 



Method 4 (resighting-rate method) used the regression analysis described in 

 Method 1 to yield a CV of 0.1 1. Three field seasons would be required to detect either 

 an increasing trend or a decreasing trend. 



Natality 



The natality rate, the proportion of dolphins considered young-of-the-year, 

 varied little during the course of the surveys, ranging from 0.028 to 0.040 (Table 4). 

 If these rates are applied to the population size estimates derived by Method 2 (mark- 

 proportion method), then annual estimates of 14 to 20 young-of-the-year are derived 

 for the Tampa Bay study area. The mark- proportion estimates are used here because 

 the variances were low, and the estimates for population size and natality were 

 calculated in a similar manner, i.e. on a proportion-of-school basis. 



Mortality 



There were 314 records of stranded animals from Hillsborough, Pinellas and 

 Manatee counties from 1977-1993; 238 of these records were from 1988 to 1993 (Table 

 5, Figure 8). We were unable to calculate a mortality rate due to the bias associated 

 with an increase in stranding response effort since the mid-1980s. Coastal 

 development and boating activity on Tampa Bay waters have also increased 

 dramatically, possibly contributing to the discovery of carcasses in previously 

 isolated areas. However, there are still many remote and inaccessible areas within 

 Tampa Bay where carcasses are unlikely to be found. All these factors confound 

 determination of the actual number of strandings and make it impractical to 

 calculate a mortality rate based on stranding records alone. 



In an attempt to distinguish between mortalities and other kinds of losses from 

 the population, photographs of stranded dolphins were examined. A total of 47 

 photographs were available to compare with the photo-ID catalog. Dorsal fins in 

 photographs of 30 animals were deemed non-distinctive, i.e., they belonged to 

 neonates, calves or otherwise had no diagnostic markings, they were too decomposed 

 to be used for matching or had obvious signs of post-mortem changes. Seventeen 

 animals were considered distinctive and were used to compare with the photo-ID 

 catalog (Table 5). We identified seven of the stranded animals: five were Sarasota 

 dolphin community members, and two were from Tampa Bay. One of the Tampa Bay 

 animals was not seen during our surveys, but had a sighting history dating back to 

 1983 and died in 1991. The other was first identified in 1984 and died in 1990. 



Of the 858 dolphins in the 1988-1993 Tampa Bay catalog, 459 were not seen 

 during the last year of the study. Six of these (0.013) were confirmed as mortalities 

 based on fin identifications. 



Immigration 



Fourteen dolphins were identified first in 1990, and were seen in each year 

 thereafter, resulting in their consideration as potential immigrants. Six of these 

 dolphins were sighted in 1990 in months other than September and October, but 

 within the same general areas as during the surveys. Four of these dolphins were 

 identified for the first time during surveys in 1991, but were initially seen outside of 

 the survey period in 1990. 



Six of the 14 dolphins considered immigrants had subtle features and may have 

 been seen in previous years before acquiring distinctive markings. Eight dolphins 

 were rated as distinctive with multiple diagnostic features that would have been 

 difficult to miss if the dolphins had been present in a sighting. 



