21 



The number of calves of all ages observed per kilometer of survey transect increased 

 from 1990 values by 20% in 1991 and 1992, 40% in 1993, and 30% in 1994 (Table 7). 

 Thus, it seems reasonable to conclude that at least a portion of the apparent increase 

 in abundance of dolphins in Charlotte Harbor is the result of increases in 

 reproduction during the course of our project. 



If reproduction accounts for only a portion of the increase in abundance, then 

 the balance must come from an influx of non-calves, either new to the area, or 

 residents that had not been identified in the middle years of the study. As described 

 above, non-calves would be expected to have acquired markings over time. Thus, 

 an influx of new animals should be reflected in an increase in the annual catalog 

 size in later years. Such an increase was apparent, but not dramatic (Figure 5). The 

 number of new animals added to the catalog each year declined from 1990 - 1991 

 through 1993 - 1994, however, indicating that many, but not all, of the non-calves 

 identified in later years were re-identifications of animals originally added to the 

 catalog in earlier years. In addition, the average proportion of dolphins in the 

 catalog in a given year that were identified in previous or subsequent years 

 increased in 1993 - 1994 (Table 4). 



This increase may be explained partially by fluctuations in the timing of 

 seasonal increases in abundance. Aerial surveys by Thompson (1981) and Scott et al. 

 (1989) have shown summer-to-winter increases of 176-223% in Charlotte Harbor and 

 Pine Island Sound. If the main reason for the increased abundance was an influx of 

 non-calves, then we would expect the proportion m/n to remain relatively constant 

 over the five years. The fact that the proportion declined over the years suggests 

 that more of the increase is due to reproduction than to an influx of older, better- 

 marked animals (Table 3). The source of additional non-calves in Charlotte Harbor 

 was not the contiguous coastal waters to the north, based on the results of censuses 

 in Sarasota and Tampa Bays. It seems likely that any additional dolphins would 

 have originated in the Gulf of Mexico or Pine Island Sound. 



Thus we are left with a series of potential explanations for the apparent 

 increase, none of which alone seems sufficient to explain the entire increase. In 

 terms of relative contributions to the increase, it seems that recruitment of young 

 had a greater potential effect than did ^identifications of earlier catalog members, 

 and each of these accounted for more of the increase than did an influx of new non- 

 calves. 



We examined the possibility that the increase was at least in part a result of 

 methodological complications, perhaps exaggerating a smaller real increase in 

 numbers of dolphins. The low CVs, only slightly larger than those obtained by 

 Wells et al. (1995) for our first application of these estimation techniques, during the 

 Tampa Bay surveys, argued against methodological problems. We explored them, 

 however, because of several differences in methods between the two studies. 



