RESEARCH VESSEL SURVEY DESIGN FOR MONITORING DOLPHIN 

 ABUNDANCE IN THE EASTERN TROPICAL PACIFIC 



Rennie S. Holt.' Tim Gerrodette/ and John B. Cologne' 



ABSTRACT 



During 1986 the National Marine Fisheries Service began conducting long-term research ship sur- 

 veys to determine status of spotted dolphin, Stenella attenuata, stocks in the eastern tropical Pacific. 

 This is the main dolphin species taken incidentally by the yellowfin tuna, Thunnus albacares, purse 

 seine fishery. We use research vessel survey data collected from 1977 to 1983 to investigate the 

 annual changes in spotted dolphin population size that could be detected given various levels of 

 research vessel survey effort during specified time periods for several levels of statistical error. 



We find that two research vessels each operating for 120 days per year for 5 years (six surveys) 

 could detect a lO'J annual rate of decrease in dolphin abundance (a total 41% decrease over 5 years) 

 with alpha and beta error levels of lO'^'f. Adding a third vessel would provide better coverage of the 

 dolphins' range, but would allow only a slightly lower rate of decrease to be detected (an 11% annual 

 rate, for a total decrea-se of 44%). These numbers point out the difficulty of detecting even major 

 changes in spotted dolphin population size with present survey methods. Alternatives are discussed, 

 but all either cost more money, require a longer time to detect a decline, or accept higher levels of 

 statistical error. 



The National Marine Fisheries Service (NMFS) 

 has the responsibility of determining the status of 

 (dolphin stocks which are taken incicientally by 

 the yellowfin tuna, Thunnus albacares, purse 

 seine fishery in the eastern tropical Pacific (ETP) 

 (Richey 1976'*). The status of spotted dolphins, 

 Stenella attenuata , is of special concern since it is 

 the major species taken by the fishery (Smith 

 1979^). Of the spotted dolphins, the northern off- 

 shore stock is of more concern since it has been 

 fished more frequently than the southern offshore 

 stock. The spinner dolphin, S. longirostris , and 

 the common dolphin, Delphinus delphis, are also 

 taken. In addition, the striped dolphin, S. 

 coeruleoalba, and the Eraser's dolphin, Lageno- 

 delphis hosei , are occasionally caught but are dif- 

 ficult to distinguish from the other three species 



^Southwest Fisheries Center La Jolla Laboratory, National 

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

 92038. 



^Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, 

 HI 96822-2396. 



■^Department of Biostatistics, SC-32, University of Washing- 

 ton, Seattle, WA 9819.5. 



^Richey, C. R. 1976. Memorandum of opinion. CA NO. 

 74-1465 and CA NO. 75-0227 U.S. District Court, District of 

 Columbia, May 11, 1976. 



SSmith, T. D. 1979. Report of the status of the porpoise 

 stock workshop (August 27-31, 1979, La Jolla, Califor- 

 nia). Southwest Fish. Cent. Adm. Rep. No. LJ-79-41, 120 

 p. SWFC La Jolla Laboratory, National Marine Fisheries 

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



at a distance (Holt and Powers 1982). These 

 5 species are herein termed target species. 



The NMFS conducted assessments of popula- 

 tion status in 1976 (SWFC 1976^) and again in 

 1979 (Smith fn. 5) based on estimates of absolute 

 stock abundance. The validity of the absolute es- 

 timates depended on several assumptions being 

 met. Unfortunately, some assumptions, such as 

 not allowing systematic errors in data recording 

 or the assumption that dolphin schools do not 

 move prior to being detected by shipboard observ- 

 ers, may not have been met and thus the assess- 

 ments were not entirely satisfactory. An alterna- 

 tive approach for assessing stock status, 

 therefore, is to use relative population estimates 

 to detect trends in stock sizes over a long time 

 period. Relative estimates can provide an assess- 

 ment of stock condition as long as the biases in 

 the abundance estimates are consistent over the 

 sampling period. Therefore, the NMFS is 

 presently considering using annual estimates of 

 population abundance as relative estimates to de- 

 tect declines in population size of spotted dolphins 

 during a sampling period of at least 5 years. 



6SWFC (Southwest Fisheries Center). 1976. Report of the 

 workshop on stock assessment of porpoises involved in the east- 

 ern tropical Pacific yellowfin tuna fishery. Southwest Fish. 

 Cent. Adm. Rep. No. LJ-76-29, 60 p. SWFC La Jolla Labora- 

 tory, National Marine Fisheries Service, NOAA, P.O. Box 271, 

 La Jolla, CA 92038. 



Manuscript accepted February 1987. 

 FISHERY BULLETIN; VOL 85, NO 3. 1987. 



435 



