CHANGES IN SIZE OF THREE DOLPHIN (STENELLA SPP.) 

 POPULATIONS IN THE EASTERN TROPICAL PACIFIC 



Tim D. Smith 1 



ABSTRACT 



Dolphins from three populations, one of Stenella attenuata and two of S. longirostris, have been 

 killed incidentally in the yellowfin tuna purse seine fishery in the eastern tropical Pacific, two 

 populations since about 1959 and the other since about 1969. Size changes in these populations are 

 estimated from numbers killed each year, population size estimates in 1979, and net recruitment 

 rates. Ranges of values for some parameters are considered, accounting for some uncertainties. 

 Assuming central values of the ranges of maximum net recruitment rate (3%) and the population 

 level giving maximum net productivity (65%), one S. longirostris population, the eastern spinner 

 dolphin, is near 20% of pre-exploitation levels; the S. attenuata population, the northern offshore 

 spotted dolphin, is between 35 and 50%; and the second S. longirostris population, the whitebelly 

 spinner dolphin, is between 58 and 72% of pre-exploitation levels. 



Purse seine fishing for tuna in the eastern tropi- 

 cal Pacific often involves dolphins found in asso- 

 ciation with yellowfin tuna. Tuna fishermen 

 pursue and capture the dolphin-yellowfin tuna 

 complex, releasing the dolphins from the net 

 while retaining the tuna (Green et al. 1971). Mor- 

 tality of dolphins occurs incidental to this fishing 

 process. 



Purse seine fishermen were using dolphin 

 schools to catch tuna by 1959; there is anecdotal 

 information suggesting limited use as early as 

 the 1940's (anonymous reviewer). Starting in the 

 mid-1960's the Bureau of Commercial Fisheries, 

 predecessor of the National Marine Fisheries 

 Service (NMFS), conducted limited research to 

 document the situation and to collect data on 

 numbers and kinds of dolphins killed. This re- 

 search expanded in the 1970's, especially after 

 passage of the Marine Mammal Protection Act 

 (MMPA) of the United States in 1972, and con- 

 tinues. Substantial research efforts were mount- 

 ed to assess the status of the dolphin stocks and to 

 develop procedures for reducing incidental mor- 

 tality and injury. 



Two assessments of the condition of dolphin 

 populations involved in the yellowfin tuna purse 

 seine fishery have been completed in recent 

 years. 2,3 1 describe the results of the latest assess- 



ment of the three populations most affected by 

 the fishery; calculation of population sizes from 

 1959 through 1978 is emphasized, based on esti- 

 mates of the population size in 1979, on annual 

 numbers killed from 1959 through 1978, and on 

 net recruitment rates. These results, based on 

 data available through the end of 1980, do not 

 necessarily represent NMFS policy, which in- 

 volves additional considerations. A third assess- 

 ment of these populations is scheduled for 1984 

 and will include information since 1980. 



POPULATION MODEL 



Methods developed in 1976 (footnote 2) for esti- 

 mating pre-exploitation abundance are based on 

 a simple recursive relationship 



Nm = N t - K t + R t (N t -%K t ), 



where t denotes the year; N, the abundance; K, 

 the number of animals killed; and R, the net re- 

 cruitment rate. This model assumes that the 

 population size in the next year is simply the 

 present population size, minus the present inci- 

 dental kill, plus the net number of individuals re- 

 cruited to the population during the year. This 

 latter quantity is taken to be the net recruitment 



'Southwest Fisheries Center, National Marine Fisheries 

 Service, NOAA, P.O. Box 271, LaJolla, Calif.; present address: 

 Northeast Fisheries Center, National Marine Fisheries Ser- 

 vice, NOAA, Woods Hole, MA 02543. 



2 Southwest Fisheries Center La Jolla Laboratory. 1976. 

 Report of the workshop on stock assessment of porpoises in- 



volved in the eastern Pacific yellowfin tuna fishery. Natl. 

 Mar. Fish. Serv., NOAA, Admin. Rep. LJ-76-29, 53 p. 



3 Smith, T. D. (editor). 1979. Report of the workshop on 

 status of porpoise stocks, La Jolla, Calif., 27-31 August 1979. 

 Southwest Fish. Cent. La Jolla Lab., Natl. Mar. Fish. Serv., 

 NOAA, Admin. Rep. LJ-70-41, 120 p. 



Manuscript accepted June 1982. 



FISHERY BULLETIN: VOL. 81, NO. 1. 1983. 



