Effects of entanglement and escape 

 from high-seas driftnets on rates of 

 natural mortality of North Pacific 

 albacore, Thunnus alalunga 



Richard W. Brill 



Honolulu Laboratory, Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



2570 Dole Street, Honolulu. Hawaii 96822-2396 



David B. Holts 



La Jolla Laboratory, Southwest Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 RO. Box 271, La Jolla, California 92038-0271 



North Pacific albacore {Thunnus 

 alalunga ) are heavily fished and may 

 have experienced recent reductions 

 in recruitment (NOAA, 1991). U.S. 

 troll fishery landings and catch per 

 unit of effort (CPUE) have been de- 

 creasing since the 1960s and are cur- 

 rently at only about 50% of their 

 peak levels (Kleiber and Perrin, 

 1991; NOAA, 1991). Landings and 

 CPUE by the Japanese pole-and-line 

 fishery have also been falling con- 

 tinuously since the late 1970V. 

 North Pacific albacore are addition- 

 ally targeted by longline fleets, and 

 considerable numbers are taken by 

 high-seas large- and small-mesh 

 driftnet fleets. The small-mesh fleets 

 of Japan, Taiwan, and Korea target 

 neon flying squid (Ommastrephes 

 bartramii) and incidentally capture 

 albacore. The large-mesh fleets of 

 Japan and Taiwan target albacore, 

 skipjack tuna (Katsuwonus pelanus I, 

 and various billfishes (Istiophoridae 

 and Xiphiidae). 



Foreign driftnet fleets and U.S. 

 trailers taking albacore overlap geo- 

 graphically (Fig. 1). They also tar- 



get a common stock even when 

 fishing in widely separated areas 

 because of the trans-Pacific migra- 

 tion of albacore (Otsu and Uchida, 

 1963). Albacore that encounter drift 

 nets, and escape to survive long 

 enough to be recaptured by another 

 fishery can bear some external 

 marks- '. These marks provide di- 

 rect evidence of interactions among 

 fisheries. U.S. trailers operating in 

 the North Pacific have reported an 

 increased frequency of net-marked 

 fish with the expansion of the Tai- 

 wanese, Korean, and Japanese 

 high-seas driftnet fleets (NOAA, 

 1991;"). 



Albacore that become entangled 

 in drift nets face a number of pos- 

 sible fates (Fig. 2). The fraction that 

 become entangled and subsequently 

 drop out (alive or dead) is not 

 known. If it is significant, the North 

 Pacific albacore stock could be af- 



'Tsuji. S., H. Nakano, and N. Bartoo. 1992. 

 Report of the Twelfth North Pacific Albacore 

 Workshop. Dep. Commer., NOAA, Natl. Mar. 

 Fish. Serv., Southwest Fish. Sci. Cent., P.O. 

 Box 271, La Jolla, CA 92038. Admin. Rep. 

 LJ-92-04, 15 p. 



798 



-Bartoo, N., D. B. Holts, and C. Brown. 1991. 

 Report of the 1990 cooperative albacore ob- 

 server project. Dep. Commer, NOAA, Natl. 

 Mar. Fish. Serv., NOAA, Southwest Fish. Sci. 

 Cent.. B.O. Box 271. La Jolla. CA 92038. 

 Admin. Rep. LJ-91-09, 16 p. 



'Bartoo. N., D. B.. Holts, and L. Halko. 1992. 

 Report of the 1991 cooperative North Pacific 

 albacore observer project. Dep. Commer, 

 NOAA, Natl. Mar. Fish. Serv., Southwest 

 Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 

 92038. Admin. Rep. LJ-92-07. 



fected more seriously by high-seas 

 drift nets than landings alone indi- 

 cate. Our study, part of a larger ef- 

 fort to determine the impacts of 

 drift nets on the North Pacific alba- 

 core stock, was undertaken to de- 

 termine whether albacore that en- 

 counter drift nets, and subsequently 

 escape alive, suffer higher than nor- 

 mal rates of natural mortality. 



Our strategy was to look for dif- 

 ferences in measures of fitness, and 

 differences in short-term and long- 

 term growth rates between net- 

 marked (i.e., fish that have encoun- 

 tered drift nets and escaped) and 

 unmarked albacore. We also looked 

 for indications of bacterial infection 

 and long-term stress in net-marked 

 albacore. These parameters could 

 evince, although not quantify, dif- 

 ferences in rates of natural mortal- 

 ity We had to use indirect measures 

 because rates of natural mortality 

 (or physiological changes) could not 

 be directly observed in albacore en- 

 countering simulated drift nets in 

 shoreside tanks. Unlike skipjack 

 and yellowfin tuna {Thunnus 

 albacares) (Brill, 1992), albacore 

 have yet to be successfully main- 

 tained in captivity. 



Selection of specific parameters 

 was constrained by the necessity of 

 collecting measurements and 

 samples at sea aboard small com- 

 mercial fishing vessels. We could ac- 

 quire only measurements and 

 samples that could be obtained un- 

 der difficult conditions and that did 

 not interfere with normal fishing 

 operations. We were further limited 

 to collecting samples that could be 

 stored for later processing in the 

 laboratory and that would not re- 

 duce the market value of the catch. 

 Given these constraints, we chose 

 the following parameters. Weight to 

 length and maximum girth to 

 length ratios were used as indica- 

 tors of general fitness (Bolger and 



Manuscript accepted 14 May 1993. 

 Fishery Bulletin, U.S. 91:798-803 1 1993). 



