13 



assumptions associated with generating abundance estimates from mark-resighting 

 analyses. 



Abundanc e Estimates and Trends 



The catalog-size index (Method 1) resulted in minimum population estimates of 

 319 to 456 dolphins over the six years of the study, with an average of 386 (Table 2). 

 The Method- 1 estimates are known to be underestimates because they do not take into 

 account the unmarked dolphins. Methods 2, 3, and 4 attempted to correct for this 

 underestimation. 



Method 2 (mark-proportion method) calculated population-size estimates from 

 proportions of marked animals relative to revised minimum, revised maximum, and 

 final best group size estimates. The differences between minimum and maximum 

 population-size estimates were so small that we present only the estimates based on 

 the final best group size. The number of dolphins estimated by Method 2 ranged from 

 488 to 567, with an average of 524 (Table 2). 



Method 3 (mark-resight method) obtained point estimates for each of the one 

 to three "complete surveys" during each year. The estimates ranged from 479 to 675 

 across all years, with an average of 564 (Table 2). 



Method 4 (resighting-rate method) provided annual point estimates ranging 

 from 416 to 602 dolphins, with an average of 516 (Table 2). 



The abundance estimates were examined for trends across the six years of the 

 surveys. Population-size estimates varied from one year to the next independently of 

 effort (Figure 6), and therefore were considered to reflect accurately changes in 

 abundance. Comparison of 95% CL for Methods 2 and 3 (Figure 7) suggest that there 

 were no significant differences in the abundance estimates across all six years of the 

 survey. Additional support for this conclusion was derived from linear regression 

 analyses of the four abundance indices and estimates. These analyses indicated that 

 the slope of the regression lines of abundance vs. year did not significantly differ 

 from zero during 1988-1993 (p = 0.15 for Method 1; p = 0.84 for Method 2; p = 0.55 for 

 Method 3; p = 0.31 for Method 4). 



Power Analysis 



The catalog-size index (Method 1) used a regression analysis of the six annual 

 estimates to remove the effect of a potential trend and calculated a CV of 0.1 1 from the 

 residuals (although no trend was apparent, a test with only six data points would be 

 sensitive to outliers and would have low power). Given that alpha = 0.05, power = 0.80, 

 r - 1.00 or -0.50, and CV - 0.1 1, we can then calculate the minimum number of surveys 

 necessary to detect a trend. Three survey sessions would be required to detect either 

 an increasing or a decreasing trend. 



A bootstrap variance procedure applied to Method 2 (mark-proportion method) 

 yielded CVs ranging from 0.04 to 0.06, with an average CV of 0.05. This would allow 

 an increasing or a decreasing trend to be detected in two surveys. 



The CVs for the estimates from each "complete survey" for the mark-resight 

 method (Method 3) ranged from 0.03 to 0.07, with an average CV of 0.05 for 1988-1993. 

 This would allow an increasing or a decreasing trend to be detected in two surveys. 



