Abstract.- We summarize the 

 methods for estimating relative abun- 

 dance of seven dolphin stocks in the 

 eastern tropical Pacific Ocean using 

 sightings data collected on commer- 

 cial tuna vessels by trained observ- 

 ers, developed by Buckland and 

 Anganuzzi (1988a) and Anganuzzi 

 and Buckland (1989). Their estimates 

 of relative abundance, which may 

 show large year-to-year fluctuations, 

 are smoothed to provide estimates of 

 the underlying trend in dolphin abun- 

 dance between 1976 and 1988. The 

 bootstrap method provides estima- 

 tion of precision in a way that allows 

 trend estimates to be used for man- 

 agement purposes, without the need 

 to assume that trends in abundance 

 are linear. Concerns about the valid- 

 ity of the estimates are addressed. 



Estimating trends in abundance 

 of dolphins associated witli tuna in 

 the eastern tropical Pacific Ocean, 

 using sightings data collected 

 on commercial tuna vessels 



Stephen T. Buckland 

 Karen L. Cattanach 



SASS Environmental Modelling Unit, MLURI 

 Craigiebuckler, Aberdeen AB9 2QJ. United Kingdom 



Alejandro A. Anganuzzi 



Inter-American Tropical Tuna Commission 



8604 La Jolla Shores Drive, La Jolla, California 92093 



Manuscript accepted 27 November 1991. 

 Fishery Bulletin, U.S. 90:1-12 (1992). 



Incidental mortality of dolphins in 

 the tuna fishery in the eastern trop- 

 ical Pacific since 1959 has been suf- 

 ficient to affect abundance of stocks 

 of at least two species of dolphin: the 

 spotted dolphin Stenella attenuata 

 and the spinner dolphin S. longiros- 

 tris (Smith 1983). Although there is 

 less information available on stocks 

 of the common dolphin Delphinus 

 delphis, mortality estimates (e.g., 

 Hall and Boyer 1988) suggest that 

 abundance of stocks of this species 

 may also have been reduced. To mon- 

 itor possible effects of incidental mor- 

 tality on the size of dolphin stocks, 

 several attempts to estimate abun- 

 dance have been made, usually apply- 

 ing line-transect methodology to data 

 collected on either commercial tuna 

 vessels ("tuna vessel data") or re- 

 search vessels ("research vessel 

 data") or both. Holt and Powers 

 (1982) and Holt (1985, 1987) consid- 

 ered analyses of research vessel data 

 alone, and of tuna vessel data com- 

 bined with research vessel data. 

 More recently. Holt and Sexton 

 (1989, 1990a, b) analyzed data from 

 research vessels alone. Tuna vessel 

 data alone were analyzed by Ham- 

 mond and Laake (1983), by Polacheck 

 (1987), by Buckland and Anganuzzi 



(1988a), and by Anganuzzi and Buck- 

 land (1989). 



The tuna vessel data are collected 

 by scientific technicians placed by 

 two organizations onboard commer- 

 cial tuna purse seiners. The Inter- 

 American Tropical Tuna Commission 

 (lATTC) places technicians on vessels 

 of the international fleet (including 

 U.S. -registered vessels), and the 

 National Marine Fisheries Service 

 (NMFS) of the United States places 

 technicians on U.S. -registered ves- 

 sels only. Data were first collected by 

 NMFS in 1974, and by lATTC in 

 1979. 



Tuna vessel data provide a large 

 database, with regular coverage of a 

 substantial portion of the area oc- 

 cupied by the dolphin stocks. How- 

 ever, due to the nature of the fishery 

 operations, the assumptions neces- 

 sary for line-transect sampling to 

 yield unbiased estimates of absolute 

 abundance are often violated. There- 

 fore, analytic procedures should as 

 far as possible be insensitive to those 

 violations. We summarize here the 

 procedures of Buckland and Anga- 

 nuzzi (1988a), as modified by Anga- 

 nuzzi and Buckland (1989). Since 

 these procedures are unlikely to re- 

 move all biases, the estimates should 



