27 



means of genetic exchange between populations, as demonstrated for the Sarasota 

 dolphin community and for Tampa Bay (Duffield and Wells 1991, Wells and Scott 

 1990, Wells et id. 1995). It was not possible to calculate a meaningful mortality rate, 

 but even though there was no indication from stranding data of catastrophic losses 

 from the population during the survey period, the data mirrored patterns of 

 mortality reported from other parts of the central west coast of Florida during the 

 same period. 



We attempted to summarize the components of the interannual differences 

 in abundance estimates in Table 8. It appears that the increase in abundance from 

 1992 and 1993 may be attributed to a return to presumably normal mortality after 

 high mortality the previous year, a higher-than-normal number of young-of-the- 

 year recorded, a higher-than-normal number of calves recorded after a relatively 

 low number recorded the previous year, and a higher-than-normal number of 

 residents recorded in the area (due to increased movement into the area or more 

 effective photographic effort). These data suggest that conditions in the area 

 improved in 1993, particularly in comparison to 1992, with relatively high 

 recruitment and possibly site fidelity, and improved survivorship. 



Comparison of Abundance Estimation Methods 



Methods 2, 3, and 4 produced similar estimates of population size (Table 3) 

 even though the sampling units and calculations differed. All three of these 

 methods have similar assumptions: a closed population, an equal probability of 

 sighting all animals, random samples of dolphins resighted, and permanent and 

 reliable marks on the dolphins. 



To detect a trend in abundance, the method with the lowest bias, greatest 

 precision, and easiest implementation in the field would be preferred. The accuracy 

 of the estimates depends greatly on the adherence to the assumptions above. The 

 problem of heterogeneity of sighting probabilities can cause a negative bias in the 

 estimate of N (e.g., Hammond 1986), and has been shown to occur in mark-resight 

 studies on bottlenose dolphins in Sarasota Bay (Wells and Scott 1990). To examine 

 the effects of heterogeneity on the different methods, a greater understanding of the 

 community structure of the area is necessary. Method 3, the mark-resight method, 

 attempted to reduce the potential effect of heterogeneity by balancing the coverage of 

 the regions within the study area, under the assumption that multiple communities 

 oi dolphins having restricted home ranges could be over- or under-sampled if 

 coverage is not equal for all regions. Piecing together segments surveyed over a 

 period of several weeks, however, could lead to biases if the assumption of 

 population closure was violated. This assumption, based on the dolphin 

 communities of Sarasota Bay, could be tested when the movements and ranges of 

 Charlotte Harbor dolphins are better known. 



The precision of the estimates is largely a result of the size and number of the 

 samples and the proportion of marked dolphins in the population (M/N). Three of 

 the above methods illustrate a range of compromises that can be made between the 



