215 



Abstract— Tag release and recapture 

 data of bigeye [Thunnus obesus) and 

 yellowfin tuna {T. albacares) from the 

 Hawaii Tuna Tagging Project (HTTP) 

 were analyzed with a bulk transfer 

 model incorporating size-specific attri- 

 tion to infer population dynamics 

 and transfer rates between various 

 fishery components. For both species, 

 the transfer rate estimates from the 

 offshore handline fishery areas to the 

 longline fishery area were higher than 

 the estimates of transfer from those 

 same areas into the inshore fishery 

 areas. Natural and fishing mortality 

 rates were estimated over three size 

 classes: yellowfin 20-45, 46-55, and 

 >56 cm and bigeye 29-55, 56-70, and 

 >71 cm. For both species, the estimates 

 of natural mortality were highest in the 

 smallest size class. For bigeye tuna, the 

 estimates decreased with increasing 

 size and for yellowfin tuna there was a 

 slight increase in the largest size class. 

 In the Cross Seamount fishery, the 

 fishing mortality rate of bigeye tuna 

 was similar for all three size classes 

 and represented roughly 12% of the 

 gross attrition rate (includes fishing 

 and natural mortality and emigra- 

 tion rates). For yellowfin tuna, fishing 

 mortality ranged between T7c and .30%, 

 the highest being in the medium size 

 class. For both species, the overall attri- 

 tion rate from the entire fishery area 

 was nearly the same. However, in the 

 specific case of the Cross Seamount 

 fishery, the attrition rate for yellowfin 

 tuna was roughly twice that for bigeye. 

 This result indicates that bigeye tuna 

 are more resident at the Seamount 

 than yellowfin tuna, and larger bigeye 

 tunas tend to reside longer than smaller 

 individuals. This may result in larger 

 fish being more vulnerable to capture 

 in the Seamount fishery. The relatively 

 low level of exchange between the Sea- 

 mount and the inshore and longline 

 fisheries suggests that the fishing 

 activity at the Seamount need not be 

 of great management concern for either 

 species. However, given that the current 

 exploitation rates are considered mod- 

 erate (10-30%), and that Seamount 

 aggregations of yellowfin and bigeye 

 tuna are highly vulnerable to low-cost 

 gear types, it is recommended that 

 further increases in fishing effort for 

 these species be monitored at Cross 

 Seamount. 



Manuscript accepted 28 October 2002. 



Manuscript received 31 December 2002 

 at NMFS Scientific Publications Office. 



Fish. Bull. 101:215-228 (2003). 



Dynamics of bigeye (Thunnus obesus) and 

 yellowfin (71 albacares) tuna in Hawaii's 

 pelagic fisheries: analysis of tagging data with a bulk 

 transfer model incorporating size-specific attrition 



M. Shiham Adam 



John Sibert 



David Itano 



Pelagic Fisheries Research Program 



Joint Institute ol Marine and Atmospheric Research 



University of Hawaii at Manoa 



1000 Pope Road, Marine Sciences BIdg. #313 



Honolulu, Hawaii 96822 



E mail address (for M S Adam): msadam®hawaii.edu 



Kim Holland 



Hawaii Institute of Marine Biology 

 University of Hawaii 

 Kaneohe, Hawaii 96744 



Around the Hawaiian Islands, a vari- 

 ety of small and medium-scale fisher- 

 ies target bigeye (Thunnus obsesus) and 

 yellowfin tuna IT. albacares) associated 

 with offshore seamounts, weather mon- 

 itoring buoys, an inshore network offish 

 aggregating devices (FADs), and natu- 

 ral aggregation sites (Itano and Hol- 

 land, 2000). These fisheries, conducted 

 from longline, troll, and handline (and 

 to a lesser extent pole-and-line) vessels 

 provide an important source of revenue 

 for the state of Hawaii (Boggs and Ito, 

 1993; Ito and MachadoM. The small- 

 gear fleet (essentially trolling and the 

 handline vessels) supports recreational 

 and subsistence fisheries for both resi- 

 dents and the tourist industry (Pooley, 

 1993: Hamilton and Huffman, 1997). 



An important sector of the small-scale 

 commercial fisheries is the offshore 

 handline fishery, which targets mixed 

 species aggregations found in associa- 

 tion with offshore NOAA weather-moni- 

 toring buoys and seamounts (Itano and 

 Holland, 2000). Most of the catch and 

 effort in this fishery, which currently 

 lands roughly 500 t per year, concen- 

 trates on the Cross Seamount and takes 

 mostly juvenile and subadult yellowfin 

 and bigeye tunas. Concerns have been 

 raised as to whether the seamount 

 fishery intercepts too many juveniles 



that might otherwise recruit to inshore 

 fisheries or to the offshore longline 

 fishery (Holland et al., 1999). There is 

 also concern among the handline fish- 

 ermen exploiting the seamount that 

 further increases in fishing effort could 

 overexploit offshore tuna resources 

 and reduce the economic viability of 

 their fishery. Moreover, yellowfin and 

 bigeye tuna around Hawaii are part of 

 the wider Pacific Ocean stock that are 

 being exploited by the various coastal 

 and high seas fisheries (Hampton and 

 Fournier, 2001; Hampton and Fournier^) 

 and therefore the overall health of 

 the Pacific-wide stock is important for 

 the viability of the local fisheries. In 

 these concerns, the Hawaiian Cross 

 Seamount fishery exemplifies resource 



Ito,R.Y.,andW. Machado. 1999. Annual 

 report of the Hawaii-based longline fishery 

 for 1998. Honolulu Laboratory Admin 

 Report. H-99-06, 62 p. National Marine 

 Fisheries Service, NOAA, SWFSC, 2570 

 Dole St., Honolulu. HI-96822-2396. 

 Hampton, J., and D. Fournier 2001. A 

 preliminary stock assessment model for 

 bigeye tuna in the Pacific Ocean. Working 

 Paper submitted to the Fourteenth Meet- 

 ing of the Standing Committee on Tuna 

 and Billfish, 9-16 August 2001. Secre- 

 tariat of Pacific Community, HP D5, 98848 

 Noumea Cedex, New Caledonia. 



