FISHERY BULLETIN: VOL. 81, NO. 1 



only occupies a portion of its historical range." 4 

 Overlap between coastal and offshore forms of 

 spotted dolphin is not reflected in the population 

 estimates given by Holt and Powers (1982). Due 

 to the large differences in areas inhabited, how- 

 ever, adjustments to account for the unknown 

 degree of overlap would increase the offshore 

 spotted dolphin population estimate by 3% at 

 most, which is insignificant for the general re- 

 sults being presented here. 



The density estimate for the nearshore area is 

 obtained from line transect theory applied to 

 aerial survey sighting data. This follows earlier 

 applications (Smith 1981), but with several im- 

 provements. For instance, the aircraft we used 

 had superior downward visibility; right-angle 

 distance from the aircraft trackline to the sighted 

 dolphin schools was determined directly, either 

 by electronic navigation equipment or visually 

 for shorter distances, rather than being calcu- 

 lated from visual estimates of range and bearing; 

 and the originally used negative-exponential 

 sighting model was replaced with the superior 

 Fourier series model (Burnham et al. 1980). The 

 density estimate for the offshore area, which 

 could not be surveyed by air, is obtained by com- 

 paring relative dolphin school sighting rates 

 from research vessel surveys in nearshore and 

 offshore areas with absolute density estimates 

 from the nearshore area. The resulting density 

 estimate of all dolphin schools of >15 animals in 

 the nearshore area is about 3.6 schools/ 1,000 

 km 2 , while the density estimate in the offshore 

 area is about one-half that value. 



The school size estimate is about 200 animals, 

 based on visual estimates of the size of schools 

 seen during the aerial survey. The accuracy of 

 these visual estimates has been confirmed by 

 counts of individual dolphins from aerial photo- 

 graphs, and the accuracy of the counts from 

 these photographs has been confirmed by counts 

 of dolphins released from a purse seine (Allen et 

 al. 1980). This estimated school size also includes 

 an adjustment for the tendency of larger schools 

 to be more readily visible at greater distances 

 from the aircraft, and hence to be overrepresent- 

 ed in the sample. 



Allen et al. (1980) also demonstrated that accu- 

 rate school size estimates could be made from 



ships. Although not used by Holt and Powers 

 (1982), the mean school size estimated from re- 

 search vessel sighting data was about 180, not 

 significantly different from the value derived 

 from aerial data described above. In contrast, 

 the mean school size estimated from tuna vessel 

 sighting data collected by scientific technicians 

 was about 580, significantly higher (P<0.001) 

 than the other two values. This difference im- 

 plies either nonrandomness of the sample of dol- 

 phin schools encountered by the tuna vessels, or 

 biases in the estimation techniques used by the 

 technicians. 



P ( for each of the 22 populations involved in the 

 yellowfin tuna purse seine fishery can be esti- 

 mated from data collected aboard either tuna 

 vessels or research vessels. Fishing vessels en- 

 counter significantly more schools composed pri- 

 marily of spotted and spinner dolphins than do 

 research vessels. The reason for this difference is 

 not known, but it is possible that fishing vessels 

 encounter spotted and spinner dolpin schools 

 more frequently than would be expected under 

 random search because they are searching for 

 tuna, which occur with these two schools more 

 frequently than with other species of dolphins. 

 Studies of the searching process of tuna fishing 

 vessels are being conducted which should help 

 resolve this question. Because the proportions P, 

 are different for unknown reasons, Holt and 

 Powers (1982) gave several sets of estimates of 

 total abundance, depending on the estimates of 

 Pi from different combinations of the research 

 vessel and tuna vessel data. Two sets of estimates 

 are considered here (Table 1), one using research 

 vessel data alone and the other using combined 

 tuna vessel and research vessel data. 



Aerial survey procedures used in the present 

 population-size estimates are still being refined. 

 For instance, a field study was completed in mid- 



Table 1.— Population size estimates (thousands) at 

 the beginning of 1979 for three populations of dol- 

 phins in the eastern tropical Pacific, using estimates 

 of the species mix from research vessel data alone, 

 and from combined tuna vessel data and research 

 vessel data, with standard deviations in parentheses 

 (Holt and Powers 1982). 



Population 



Research vessel 

 data only 



Fishing and research 

 vessel data 



4 Hammond, P. S. (editor). 1981. Report of the Workshop 

 on Tuna-Dolphin Interactions, Managua, Nicaragua, April 

 1981, p. 5. IATTC Spec. Rep. 4, Inter-Am. Trop. Tuna Comm., 

 c/o Scripps Inst. Oceanogr., La Jolla, CA 92093. 



