Abstract.— Amendments enacted 

 in 1988 to the U.S. Marine Mammal 

 Protection Act of 1972, coupled with 

 changes during the 1980's in the 

 composition of the international 

 purse-seine fleet fishing "on dolphin" 

 for yellowfin tuna in the eastern 

 tropical Pacific (ETP). motivated a 

 simulation experiment to assess the 

 effects of fleet size and observer cov- 

 erage level on estimates of cumula- 

 tive annual mortality for dolphin 

 stocks, where data quality differed 

 greatly between stocks. Results from 

 this simulation study provided a ba- 

 sis for subsequent U.S. legislation 

 regarding criteria for comparing dol- 

 phin mortality rates of U.S. vs. non- 

 U.S. fleets fishing in the ETP, pur- 

 suant to regulations governing 

 import of tuna from this area. 



Accuracy and precision of mortal- 

 ity estimates were influenced very 

 strongly by data quality (dolphin 

 group type ) and moderately strongly 

 by fleet size and level of observer 

 coverage. Because neither fleet size 

 nor dolphin group type can be con- 

 trolled by a sampling program, 

 desired levels of accuracy and preci- 

 sion in estimates of dolphin mortal- 

 ity can be achieved only by manipu- 

 lating the level of observer coverage. 

 The unexpectedly strong effect of dol- 

 phin group type implies that to pro- 

 tect all groups equally, observer lev- 

 els should be chosen to accommodate 

 the dolphin group type exhibiting the 

 poorest-quality data. 



Effects of dolphin group type, 

 percent coverage, and fleet size on 

 estimates of annual dolphin 

 mortality derived from 1987 U.S. 

 tuna-vessel observer data 



Elizabeth F. Edwards 

 Christina Perrin 



Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA 

 PO Box 27 I , La Jolla, CA 92038 



U.S. -registered tuna purse-seiners in 

 the eastern tropical Pacific Ocean 

 (ETP) inadvertently kill dolphins dur- 

 ing fishing operations (DeMaster et 

 al., 1992). Since 1972 this kill has 

 been monitored by scientific observ- 

 ers accompanying seiners on routine 

 fishing trips. The kill data are moni- 

 tored to determine whether annual 

 quotas have been reached for dolphin 

 mortality overall, and for several spe- 

 cific stocks of dolphins. If any quota 

 is reached, fishing must cease on the 

 affected stock for the remainder of 

 the calendar year. Since 1989, the 

 data have also been used to determine 

 annual dolphin mortality rates for 

 comparison with non-U. S. fisheries 1 . 

 Prior to 1987, this kill monitoring 

 was accomplished by observers on 

 30-50% of the trips made by U.S. 

 purse-seiners during a given year. 

 Thus, mortality was estimated rather 

 than measured directly. In the ab- 

 sence of any alternative (i.e., higher 

 rates of observer coverage), the accu- 

 racy and precision of these estimates 

 were assumed to be adequate. This 

 assumption was acceptable at the 

 time because the U.S. fleet was large 

 and generated many data. About 100 

 vessels fished the eastern tropical 

 Pacific Ocean (ETP) each year, each 



Manuscript accepted 4 June 1993. 

 Fishery Bulletin 91:628-640 11993). 

 628 



1 The mortality estimator used in these simu- 

 lations ( kill/day) is not the same estimator 

 used currently (1993) to determine mortality 

 comparability (kill/set). The reasons for this 

 appear in the methods section. 



vessel making two to five trips of 

 about three-months duration each. 



The situation changed dramati- 

 cally in the 1980's when, for a vari- 

 ety of reasons (Sakagawa, 1991), the 

 U.S. fleet began to decrease 20-30% 

 per year, decreasing from over 90 ves- 

 sels in 1981 to about 10 vessels in 

 1992. This decrease in size of the U.S. 

 fleet was offset by a reciprocal in- 

 crease in number of non-U. S. vessels 

 fishing in the ETP. This increase has 

 been dominated by Mexican vessels 

 (fleet size exceeding 50 vessels by 

 1990) but also includes smaller fleets 

 from several other nations. These 

 smaller fleets range in size from 1 to 

 about 20 vessels (IATTC Annual Re- 

 ports, 1980-1991). In addition to 

 these changes in composition of the 

 international fleet, amendments 

 passed in 1988 to the U.S. Marine 

 Mammal Protection Act stipulated 

 that yellowfin tuna caught in the 

 ETP could be imported from non-U. S. 

 countries only if mortality rates for 

 those countries were comparable to 

 U.S. kill rates. 



These events created a strong in- 

 centive to evaluate the effects on 

 mortality estimates of varying ob- 

 server coverage levels and fleet sizes. 

 Because the new regulations per- 

 tained to some individual stocks in 

 addition to dolphin mortality over- 

 all, the previously uninvestigated ef- 

 fect of dolphin group type was also 

 of interest. 



