Effects of Nonrandomness on Line Transect Estimates 

 of Dolphin School Abundance 



Elizabeth F. Edwards and Pien-e M. Kleiber 



ABSTRACT: Line transect analysis is a census 

 method that has been used to derive estimates of 

 dolphin school abundance from sightings data col- 

 lected by obseners on tuna purse seine vessels. The 

 method is based on the assumption that movements 

 of the sighting platform (tuna vessel) and sighted 

 objects (dolphin schools) are random with respect 

 to each other. In practice, neither schools nor ves- 

 sels move randomly. Stratification of sightings 

 data has been used to alleviate partially the effects 

 of this nonrandomness, but the effectiveness of this 

 stratification cannot be tested with data from com- 

 mercial vessels because the movements of the ves- 

 sels cannot be controlled. 



As an alternative, we have used a relatively simple 

 mathematical simulation model to investigate the 

 severity of bias introduced into school abundance 

 estimates by nonrandom movements of schools and 

 vessels, and by the data stratification procedure. 

 Simulations show that nonrandom movements on a 

 scale of a few hundred miles, coupled with the data 

 stratification procedure, can lead to overestimates 

 of dolphin school abundance by as much as a factor 

 of two. These results focus attention on the need to 

 understand patterns of dolphin school distribution 

 in smaller scales of space and time than have been 

 studied previously, and to develop data stratifica- 

 tion methods more robust against the effects of 

 small-scale nonrandomness. 



The National Marine Fisheries Service (NMFS) 

 monitors mortality of dolphins involved in fish- 

 ing operations by the United States purse seine 

 fleet for yellowfin tuna, Thunnus albacares, in 

 the eastern tropical Pacific Ocean (ETP), to de- 

 termine whether mortality has exceeded an 

 annual quota implemented by an act of the U.S. 

 Congress. The quota levels depend upon 

 whether dolphin populations are thought to be 

 increasing or decreasing in number, relative to 

 population levels during previous years. 



The most effective method currently available 

 for detecting trends in relative abundance is an- 

 alysis of population abundance estimates collect- 



ed over a period of 5-15 years. The most effec- 

 tive method currently available for making these 

 abundance estimates is hne transect analysis of 

 dolphin school sightings data (Holt 1987; Buck- 

 land and Anganuzzi 1988). Two data sources are 

 available for these line transect estimates of 

 abundance: 1) data collected by observers dur- 

 ing research surveys (RSOD — Research Survey 

 Observer Data) and 2) data collected by obser- 

 vers during commercial fishing operations 

 (TVOD— Tuna Vessel Observer Data). NMFS 

 has used RSOD because research surveys can be 

 designed specifically to satisfy the assumptions 

 required by line transect analysis (Smith^). 

 However, research surveys are very expensive 

 and are becoming more so. This expense causes 

 RSOD to be sparse relative to TVOD and possi- 

 bly unavailable in the future. 



TVOD are a potential solution to these prob- 

 lems, having three significant advantages over 

 RSOD: TVOD are much more abundant, are rel- 

 atively inexpensive, and are likely to continue 

 being collected as long as fishermen set on and 

 kill dolphins. Observer-days from tuna vessels 

 account for roughly 95'7f of the annual observer 

 effort in the ETP, while observer-days from re- 

 search vessels account for only ¥/c. TVOD are 

 inexpensive relative to RSOD because TVOD 

 are collected by the observers in addition to 

 monitoring dolphin mortality, the latter being 

 the main reason the observer progi'am was ini- 

 tiated. This monitoring program has been in 

 operation for the past 14 years, will continue into 

 the foreseeable future, and monitors about 30% 

 of trips by purse seiners (both U.S. and non- 

 U.S. vessels) each year in the ETP". Ideally, 

 TVOD could be used in place of RSOD to moni- 

 tor changes in abundance of dolphins. 



Elizabeth F. Edwards and Pierre M. Kleiber, Southwest 

 Fisheries Center, National Marine Fisheries Service, 

 NOAA, P.O. Bo.x 271, La Jolla. CA 92038. 



'Smith, T. D. 1975. Estimates of sizes of two popula- 

 tions of poipoise (Stenella) in the eastern tropical Pacific 

 Ocean. Admin. Rep. No. LJ-75-67. Southwest Fish. 

 Cent., Natl. Mar. Fish. Serv., NOAA, La Jolla, CA. 



"Inter-American Tropical Tuna Commission, Annual Re- 

 ports 1980-1988. Scripps Institution of Oceanography, La 

 Jolla, CA 92038. 



Manuscript accepted April 1989. 

 Fisliery Bulletin, U.S. 87: 859-876. 



859 



