421 



Abstract.— North Pacific albacore tag- 

 ging data from a tag-release program 

 conducted from 1971 to 1989 were ana- 

 lyzed to obtain estimates of exploitation 

 rates and related parameters. The ma- 

 jor albacore fishing fleets in the North 

 Pacific, the U.S. baitboat, Japan 

 baitboat, troll and longline fleets were 

 used in the analysis. Another category 

 of fleet ("other") that combined remain- 

 ing miscellaneous recapture sources 

 was also used. Tag-attrition models in- 

 corporating variable availability of 

 tagged albacore to the various fleets, 

 seasonal catchability, and multiyear 

 effects on catchability were developed 

 and applied. The incorporation of all 

 three effects was found to improve 

 model fit significantly. If exploitation 

 of the tagged population is representa- 

 tive of the North Pacific albacore popu- 

 lation as a whole and if tag reporting 

 rates were high, the results would sug- 

 gest that the exploitation rate has been 

 less than 10<7r per year since the early 

 1970s. However, a deficit of returns 

 from the troll fleet in comparison with 

 its catch suggested that the pattern of 

 exploitation of the tagged population, 

 by this fleet at least, was different from 

 that for the untagged albacore popula- 

 tion. After compensating for assumed 

 depressed availability of tagged alba- 

 core to the troll fleet, annual exploita- 

 tion rates were estimated to have de- 

 clined from a high of 40*?^ in the niid- 

 1970s to <10'7r since the early 1980s. 



Estimates of exploitation rates for north 

 Pacific albacore, Thunnus alalunga, 

 from tagging data 



Michel Bertignac 



John Hampton 



Secretariat of the Pacific Community 



B P D5 



98848 Noumea Cedex, New Caledonia 



Email address (for M Bertignac)) mictielbeiaispc org nc 



Atilio L. Coan Jr. 



Southwest Fisheries Science Center 



PO Box 271 



La Jolla, California 92038 



Manuscript accepted 17 August 1998. 

 Fish. Bufl. 97:421-433 ( 1999). 



Albacore tuna, Thunnus alalunga, 

 occur in the tropical and subtropi- 

 cal waters of the Pacific Ocean, 

 where they comprise separate 

 stocks north and south of the equa- 

 tor (Murray, 1994). In the north 

 Pacific, albacore spawn predomi- 

 nantly in the spring and summer 

 over a wide area bounded approxi- 

 mately by 10°-30°N (Nishikawa et 

 al., 1985). Juvenile albacore appear 

 in the North Pacific Transition Zone 

 (35°-45°N), where they are ex- 

 ploited by the surface fishery, com- 

 prising mainly Japanese and U.S. 

 baitboat, troll, and until the early 

 1990s, Japanese and Taiwanese 

 driftnet fleets (Fig. 1). Adult alba- 

 core are found at lower latitudes, 

 where they are caught by the sub- 

 surface longline fishery, comprising 

 Japanese, Korean, and Taiwanese 

 fleets (Fig. 1). Longline catches have 

 been fairly stable at 10,000-20,000 t 

 annually. Total surface fishery 

 catches (which have been domi- 

 nated by Japanese baitboat catches ) 

 have declined from a peak of around 

 100,000 t in the mid-1970s to 

 20,000-50,000 1 in recent years (Liu 

 and Bartoo, 19951. 



Since 1971, 23,780 albacore have 

 been tagged in the North Pacific in 

 a program conducted jointly by the 

 U.S. National Marine Fisheries Ser- 



vice (NMFSl and the American 

 Fishermen's Research Foundation. 

 The tag recoveries have been used 

 in studies of albacore movements 

 (Laurs and Lynn, 1977), growth 

 (Laurs and Wetherall, 1981), and 

 fishery interaction (Kleiber and 

 Baker, 1987). However, to date no 

 formal analysis of the data has been 

 undertaken to estimate exploitation 

 rates and related parameters. In 

 this paper, we develop tag-attrition 

 models (Kleiber et al., 1987), incor- 

 porating different structural as- 

 sumptions concerning availability 

 and catchability of the tagged popu- 

 lation. The models are fitted to the 

 albacore tagging data to obtain es- 

 timates of the instantaneous rate of 

 natural mortality and fleet-specific 

 catchability coefficients. These es- 

 timates are used to construct time 

 series of estimated exploitation rates. 

 The relative merits of the alternative 

 models and applicability of the esti- 

 mates to the North Pacific albacore 

 population in general are discussed. 



Materials and methods 



Tagging data 



Albacore tagging occurred from 1971 

 to 1996, mainly during April-Sep- 



