106 



Fishery Bulletin 



1990 



LONGITUDE 



Figure 1 



Tracklines traversed by the NOAA 

 RV Darid Starr Jordiin (solid) and 

 McA rthur (dash) during the 198G dol- 

 phin survey, eastern tropical Pacific. 

 Tracklines generated using noontime 

 positions. 



Materials and methods 



Study area and survey coverage 



The study area was described by Au et al. (1979) (Fig. 

 1). We partitioned the area into four strata: inshore, 

 middle, and west located north of 1°S, and a south 

 stratum. These strata were selected based upon pre- 

 liminary examinations of historical distributions of 

 dolphin stocks and oceanographic features. 



The NOAA research vessels David Starr Jordan and 

 Mc Arthur traversed predetermined tracklines in the 

 ETP during 29 July-5 December and 6 December, 

 respectively (Fig. 1). Each vessel spent approximate- 

 ly 120 days at sea. Detailed operations, survey pro- 

 cedures and preliminary data summaries for each 

 vessel are presented elsewhere (Holt and Sexton 1987, 

 Holt and Jackson 1987). 



On each ship, two observers used 25 x l)inoculars 

 located on each side of the ship to search from directly 

 ahead to abeam of their respective sides of the ship. 

 A third observer served as data recorder and searched 

 directly ahead of the ship when not recording data. Two 

 teams of three observers each alternately occupied the 

 three duty stations. Each team was on duty for 2-hour 

 shifts. During each shift, members spent approximately 

 equal time occupying each duty station. Whenever 

 possible, schools were approached and observers re- 

 corded independent "best" estimates of school size. If 

 an observer could not obtain a best estimate, a "mini- 

 mum" estimate was made. Independent estimates were 

 averaged to obtain mean minimum and best estimates. 



Abundance estimation 



Estimates of population abundance of the target spe- 

 cies (N,j) are computed as (Holt and Powers 1982): 



N,j = 1 \D,SaP,kP,kA,IA,,][A,j, 



^P\,A',,,] 



A-=l 



where 



Dj; = estimate of density of all dolphin schools, 

 both identified and unidentified to species, 

 in area A', 



S,i; = estimate of mean size of target schools in 

 area A', 



Pi I; = estimate of proportion of dolphin schools 

 which are target schools in area A', 



P,i; = estimate of proportion of individuals of 

 species i in target schools in area A-, 



P' II = estimate of proportion of individuals of 

 stock j of species i in target schools in 

 overlap region containing two stocks of 

 species i (overlap region discussed in text), 



Al- = total area inhabited by the target species 

 in area A, 



A, I; = area inhabited by species i in area A, 



Ajjt = area inhabited by species ?', stock,/, in area 

 A-, and 



A'lji; = area inhabited by species i, stocky, in over- 

 lap region of area k. 



The variance of N,j was calculated using boot- 

 strapped methods. For each stratum, the number 

 of legs (segment of time during which all sighting 



