ESTIMATING DENSITY OF DOLPHIN SCHOOLS IN THE EASTERN 

 TROPICAL PACIFIC OCEAN BY LINE TRANSECT METHODS 



Rennie S. Holt 



ABSTRACT 



Data were collected from aerial and research ship surveys to estimate density of dolphin schools in 

 the eastern tropical Pacific using line transect (LTi theory. The surveys were conducted from 1977 

 through 1983. Several assumptions of LT theory were investigated for both aerial and ship data. 

 Factors were developed to alleviate eflects of suspected violations of the assumptions. I estimated 

 densities from data stratified into an inshore area surveyed by planes and an offshore area surveyed 

 by ships. The density estimate for the inshore area was 4.18 schools/1,000 km2 and 2.04 for the 

 offshore area. For the entire area, the density estimate was 2.71 schools/1,000 km^. Adjustments for 

 possible biases owing to adverse sea state and sun glare conditions increased the inshore estimate by 

 B'.v and the total area estimate by 4%. 



The National Marine Fisheries Service (NMFS) is 

 responsible for assessing the status of those dol- 

 phin stocks taken incidentally by tuna purse sein- 

 ers in the eastern tropical Pacific (ETP) Ocean. 

 Techniques used to assess these stocks (Smith 

 1979") require estimates of school density, so den- 

 sity estimates were made in 1975 (Smith 1975'^) 

 and in 1979 iHolt and Powers 19821. Since 1979, 

 NMFS has collected additional information to 

 test the assumptions of its statistical methods and 

 to further survey the areas inhabited by the dol- 

 phins. In this paper, I present analyses of data 

 collected from 1977 through 1983 to determine 

 density estimates of dolphin schools in the ETP. 

 In addition, I investigate several factors which 

 may bias the estimates. 



To obtain estimates of density of dolphins (indi- 

 viduals) it is further necessary to consider school 

 size, the proportions of various species in mixed 

 schools, and areas inhabited by the various 

 stocks. Estimation of these factors is complex; 

 they are to be dealt with elsewhere and are not 

 addressed in this paper. 



MATERIAL AND METHODS 

 Surveys 



Data used to calculate the density of dolphin 

 schools were collected during several years. 

 Aerial surveys were conducted in 1977 and 1979 

 (Fig. 1 ), and nine research ship cruises were made 

 during 1977, 1979, 1980, 1982, and 1983 (Fig. 1). 

 Most surveys were conducted between January 

 and early April; one of the 1977 ship cruises was 

 made in October and the two 1980 cruises were 

 made from May through August. 



A two-engine PBY amphibious patrol bomber 

 was used in the 1977 aerial survey (SWFC 1978^), 

 and a four-engine PBY bomber was used in the 

 1979 aerial survey (Jackson 1980^). Operating 

 and viewing conditions aboard the two aircrafts 

 were similar. Both planes cruised at 148-240 km/ 

 hour (80-130 kn) and had bubble-shaped waist 

 windows. The PBY used in 1977 had a flat bow 

 window which was shaped like an isosceles trape- 

 zoid. The 1979 PBY had a round bubble-shaped 

 bow window. The round bubble window allowed 



'Southwest Fisheries Center La Jolla Laboratory. National 

 Marine Fisheries Service. NOAA. P.O. Box 271. La Jolla. CA 

 92038. 



-Smith. T. 1979. Report of the status of porpoise stocks 

 workshop (.August 27-31. ]979i. Southwest Fish. Cent. Adm. 

 Rep. No. LJ-79-41. La Jolla, CA, 120 p. 



•'Smith. T. 197.5. Estimates of sizes of two populations of 

 porpoise iStenclla) in the eastern tropical Pacific 

 Ocean. Southwest Fish. Cent. Adm. Rep. No. LJ-7.5-65, La 

 Jolla. CA. 88 p. 



Manuscript accepted February 1987. 

 FISHERY BULLETLN: VOL." 85. NO 3. 1987 



■*SWFC iSouthwest Fisheries Centeri. 1978. Aerial survey 

 trip report, January-June 1977. Southwest Fish. Cent. Adm. 

 Rep. No. LJ-78-01, 73 p. National Marine Fisheries Service, 

 NOAA, P.O. Box 271, La Jolla, CA 92038. 



SJackson, T. 1980. Report: Porpoise population aerial sur- 

 vey of the eastern tropical Pacific Ocean, January 22-April 25, 

 1979. Southwest Fish Cent. Adm. Rep. No. LJ-80-01, 

 74 p. National Marine Fisheries Service, NOAA, P.O. Box 

 271, La Jolla, CA 92038. 



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