392 



Different residence times of yellowfm tuna, 

 Thunnus albacares, and bigeye tuna, 

 T. obesus, found in mixed aggregations 

 over a seamount 



and-recapture program was con- 

 ducted to elucidate the dynamics of 

 the tuna populations associated with 

 the seamount and to document the 

 movements of individual fish form- 

 ing those aggregations. 



Kim N. Holland 



Hawaii Institute of Marine Biology 

 Coconut Island, Kaneohe, Hawaii 96744 

 E-mail address, khollandifthawaii.edu 



Pierre Kleiber 



National Marine Fisheries Service 



Southwest Fisheries Center, Honolulu Laboratory 



2570 Dole Street, Honolulu, Hawaii 96822 



Stephen M. Kajiura 



Department of Zoology 

 University of Hawaii at Manoa 

 Honolulu, Hawaii 96822 



Aggregations of tuna can be found 

 in association with seamounts 

 throughout all the tropical oceans 

 and these aggregations are often 

 exploited by tuna fishing fleets 

 (Fonteneau, 1991). In Hawaii, a 

 fishery has developed over the last 

 decade that targets the mixed spe- 

 cies aggregations of predominantly 

 subadult tuna found in association 

 with the Cross Seamount located 

 approximately 160 nmi south of 

 Honolulu and Oahu and 150 miles 

 east of South Point on the island of 

 Hawaii (Fig. 1). Its shallowest 

 depth is about 330 meters. 



The Cross Seamount fishery is a 

 hybrid troll, jig, and handline fish- 

 ery that augments slow trolling 

 methods by using frozen, cut and 

 whole bait to induce feeding behav- 

 ior around the fishing boat. Initially 

 described as a yellowfin tuna 

 ( Thunnus albacares I fishery, closer 

 scrutiny has revealed that the catch 

 is dominated by juvenile bigeye 

 tuna iT. obesus). Schools of skipjack 

 tuna (Katsuwonus pelamis) are also 

 found over the seamount but these 



are avoided by fishermen and, 

 when skipjack tuna occur in mixed 

 schools with the other species, the 

 quite large size of the baits deters 

 the capture of skipjack tuna. The 

 fishery occurs year-round but fish- 

 ing effort tends to subside periodi- 

 cally when larger tuna become 

 available closer to shore — primarily 

 during summer months. 



Two concerns have arisen within 

 the fishing community. First, fish- 

 ermen exploiting the Cross Sea- 

 mount resource were concerned 

 that increasing fishing effort at the 

 seamount by additional vessels en- 

 tering the fishery would overexploit 

 the schools of tuna associated with 

 the seamount. Second, there was 

 concern among the broader commu- 

 nity that heavy fishing effort at the 

 seamount might reduce the num- 

 ber of fish that were available to 

 other gear types operating closer to 

 shore. That is, the seamount might 

 be a major "staging point" for fish 

 that subsequently move into the 

 coastal handline and troll fisheries. 

 In response to these concerns, a tag- 



Materials and methods 



Tag-and-release operations were 

 conducted by trained tagging tech- 

 nicians placed onboard collaborat- 

 ing commercial vessels. Tuna were 

 caught by crew members using 

 handlines and pole-and-line meth- 

 ods, and the fish were then passed 

 to the tagging technician for evalu- 

 ation, identification, measurement, 

 and tagging. Standard, serially 

 numbered 11-cm nylon-tipped dart 

 tags (Hallprint Pty, Australia) car- 

 ried a message (stating a reward) 

 and a toll-free phone number for 

 reporting recaptured fish. Also, be- 

 cause the commercial fishing fleet 

 in the seamount fishery consists of 

 fewer than ten boats, close liaison 

 was established with these boats to 

 ensure maximum reporting of fish 

 recaptured at the release site. 



Residence times for tuna at the 

 seamount were calculated by con- 

 structing tag-recapture attrition 

 cui-ves (Kleiber et al., 1987) that 

 plotted the number of tagged fish 

 recaptured against time at liberty. 

 The elapsed time at which 50% of 

 the releases had been recaptured 

 at the point of release (Cross Sea- 

 mount) represents the "half-life" of 

 resident tuna within that area. 

 That is, for a group of animals 

 tagged at Cross Seamount, the resi- 

 dence time was defined as the 

 elapsed time at which only half 

 those animals remained at that lo- 

 cation. In order to avoid dispropor- 

 tionate influence offish recaptured 

 immediately after release and be- 



Manuscript accepted 5 May 1998. 

 Fisli. Bull. 97:392-39.5 (1999 (. 



