SMITH: SIZE CHANGES OF THREE DOLPHIN POPULATIONS 



indicate that through 1978, a total of 22 sets have 

 been observed on coastal spotted dolphins, out of 

 a total 9,672 observed overall (about 0.2%). 



The yellowfin tuna purse seine fishery was 

 concentrated nearshore in the early 1960's, and 

 many sets were made in the area probably occu- 

 pied by both coastal and offshore spotted dol- 

 phins. Direct observations in the 1960's distin- 

 guishing these forms of the spotted dolphin are 

 not available. Based on observations made in the 

 1970's where these forms were distinguished, 

 however, it appears that the coastal form has 

 never been significantly involved in this fishery. 

 Since 1978, sighting data collected by scientific 

 observers aboard tuna vessels have been edited, 

 using consistent criteria of school size, body size, 

 and coloration for distinguishing coastal and off- 

 shore spotted dolphins. In 1979, for example, 

 within about 50 km of the coast there were 46 

 sightings of coastal spotted schools, 160 sightings 

 of offshore spotted schools, and 25 sightings of 

 spotted dolphin schools which could not be dis- 

 tinguished to form with the available data. These 

 three school types were subsequently set on in 2, 

 73, and 6 instances, respectively. Even assuming 

 that all schools not identified to form were coastal 

 spotted dolphins, the proportion of sighted coast- 

 al spotted dolphin schools subsequently set on is 

 much smaller than that of the offshore form (0.11 

 vs. 0.46, P<0.001). This differential selection 

 exists even though the catch of yellowfin tuna in 

 sets on coastal spotted dolphins has been approxi- 

 mately twice that on offshore spotted dolphins. If 

 coastal spotted dolphins were a significant part 

 of this fishery, one would expect their involve- 

 ment in sets to be proportional to the rate at 

 which they are encountered. 



In addition, 18 of the 22 sets on coastal spotted 

 dolphins occurred in 1973, and, except for one 

 set, these were made by two vessels in the Gulf of 

 Nicoya, a small area off the Costa Rican coast. 

 Based on this information, I have assumed that 

 the coastal spotted dolphin has been involved 

 only rarely in this fishery. 



Two spinner dolphin populations, referred to 

 as the "eastern" and "whitebelly" forms, are in- 

 volved in the yellowfin tuna purse seine fishery. 

 A third form, termed the Costa Rican spinner, 

 occurs near the coast from Mexico to Panama, 

 but is not involved in the fishery. The eastern and 

 whitebelly forms overlap broadly in range, with 

 the whitebelly spinner dolphin generally occur- 

 ring more seaward. The eastern form has been 



involved with this fishery since 1959, whereas 

 the whitebelly spinner dolphin population ap- 

 parently became increasingly involved as the 

 fishery expanded seaward in the 1960's. 



The whitebelly spinner and the offshore spotted 

 forms have Southern Hemisphere populations 

 (Perrin et al. 1979). These populations have been 

 involved only recently with the yellowfin tuna 

 purse seine fishery, as it has expanded south- 

 ward, and are only lightly exploited. Data on 

 reproductive condition of these southern popula- 

 tions are used as estimates of reproductive rates 

 for unexploited or equilibrium populations. 



1979 POPULATION SIZE 

 ESTIMATES 



Holt and Powers (1982) gave estimates of abun- 

 dance based on aerial and research-vessel sight- 

 ing surveys and data from scientific observers 

 aboard fishing vessels. Estimates of the size, N,, 

 of the i th population in their survey area are 

 based on the equation 



N, = P t S t D P, A, 



(5) 



where P, denotes the proportion of dolphin 

 schools containing dolphin of the genera Stenella, 

 Delphinus, and Lagenodelphis; S t , the mean size 

 of these schools; D, the estimated density of all 

 dolphin schools sighted; P,, the fraction of schools 

 containing dolphins of the ith population; and A, 

 the area inhabited. This equation is applied to 1) 

 a nearshore stratum, surveyed using both an air- 

 plane and research vessels, and 2) an offshore 

 stratum, surveyed only by research vessels. The 

 nearshore stratum extends seaward from the 

 coastline about 800 km, and from lat. 22°N to 

 12°S. The offshore stratum extends from the 

 outer edge of the nearshore stratum to the bound- 

 ary of the dolphin range. 



Approximate areas of the maximum historical 

 range of the three dolphin populations are used 

 for the area inhabited, A in Equation (5). These 

 are estimates of the area enclosed by a smooth 

 curve which includes most locations where dol- 

 phins of different species have been reported by 

 both fishing vessels and research vessels, as de- 

 scribed in Holt and Powers (1982). 



While occasional sightings of dolphin schools 

 have been reported outside these areas, the areas 

 are overestimated in that ". . . at any point in time 

 it is likely that each of the various dolphin species 



