SMITH: SIZE CHANGES OF THREE DOLPHIN POPULATIONS 



belly spinner dolphin population in 1969 num- 

 bered between 400,000 and 500,000. The offshore 

 spotted and eastern spinner dolphin populations 

 declined rapidly in the 1960's and early 1970's in 

 the face of kills which were, for example, on the 

 order of 7-12% of the 1965 population sizes. The 

 whitebelly spinner dolphin population declined 

 most rapidly in 1974 when the kill was between 

 11 and 16% of its population size. 



These estimates of absolute population sizes 

 are shown in Figures 1 and 2 relative to the equi- 

 librium population size (N t /N,,), so that the trend 

 in abundance of these populations can be exam- 

 ined. For all of the parameter values considered, 

 these dolphin populations have declined substan- 

 tially relative to their pre-exploitation sizes. 



The ratio of 1979 to pre-exploitation popula- 

 tion sizes for different values of R,„ and MNPL 

 (and hence Z) shows the sensitivity of the calcula- 

 tions to changes in parameter estimates (Table 7; 

 Figs. 3,4). The value of MNPL when R,„ is zero 



is not meaningful, as the estimate of pre-exploi- 

 tation population size (Equation (2)) collapses to 

 the sum of the present population size estimate 

 and the total numbers killed over all years. This 

 is reflected in Figures 3 and 4 in the convergence 

 of the lines when R m is zero. 



Table 7.— Estimates of 1979 relative population sizes of off- 

 shore spotted, eastern spinner, and whitebelly spinner dolphin 

 populations, using two estimates which differ in species pro- 

 portions from (FR) combined fishing and research vessel data 

 and from (R) research vessel data alone, for ranges of maxi- 

 mum net recruitment rate (R,„ ) and maximum net productiv- 

 ity level (MNPL). 



1.0 



0.9 - 



0.2 



0.1 



0.0 





0.00 



0.03 

 R m 



0.06 



1.0 i- 



0.9 - 



B 0.8 



i °' 7 



H 



< 0.6 - 



Q. 



O 0.5 

 Q. 



LU 



> 0.4 



I- 

 < 



uj 0.3 h 

 K 



0.2 - 

 0.1 - 



0.0 



m 5>- 



ef*a •-- 





=B) 



Easternjg-^^ESS 



0.00 



0.03 



'm 



0.06 



Figure 3.— Population size of offshore spotted dolphins in 1979 

 relative to 1959 (Nn/Nss) as a function of maximum recruit- 

 ment rate (i?,,, = 0, 3, 6%) using two current population estimates 

 which differ in species proportions from (FR) combined fish- 

 ing and research vessel data and from (R) research vessel data 

 alone. MNPL values of 50% (dashed lines), 65% ( solid lines), and 

 80% (dot-dashed lines) are shown. 



Figure 4.— Population sizes of eastern spinner and whitebelly 

 spinner dolphins in 1979 relative to 1959 (Ni 9 /N 59 ) as a function 

 of maximum recruitment rate(.R„, =0, 3, 6%) using two current 

 population estimates which differ in species proportions from 

 (FR) combined fishing and research vessel data and from (R) 

 research vessel data alone. MNPL values of 50% (dashed lines), 

 65% (solid lines), and 80% (dot-dashed lines) are shown. 



11 



