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Fishery Bulletin 101(2) 



and longline fishery. Similar to the inshore fisheries, the 

 Seamount fishery targets surface swimming fish favoring 

 small- to medium-size classes. 



Because of the way in which cohorts were aggregated to 

 maximize the number released per cohort, time varying 

 fishing effort could not be used to reparameterize F with a 

 catchability coefficient (e.g. Hampton, 2000 ). Instead, F was 

 estimated as a constant proportion of the numbers avail- 

 able at a given time period. This was considered reasonable 

 because there was no reason to believe that the fishery 

 underwent notable change during the period of the tagging 

 experiment. Under this assumption, F is similar to a catch- 

 ability coefficient. Because both species are targeted with 

 the same suite of gears (Itano and Holland, 2000), the dif- 

 ferences in F would reflect their vulnerability to the gears. 

 Higher overall F (vulnerability) at the Cross Seamount for 

 yellowfin compared to bigeye tuna indicates that yellowfin 

 tuna are more vulnerable there. 



The gross attrition rate for any given spatial component 

 Z, in our model includes size-dependent M and F and size- 

 independent T (emigration rate). At the Cross Seamount, 

 the actual estimates of all three components were generally 

 lower for bigeye than for yellowfin tuna, thereby making 

 the estimated residence times for bigeye tuna roughly 

 twice as high as those for yellowfin tuna (Table 6). Our 

 estimate of residence time for bigeye tuna agree closely 

 with earlier estimates (Holland et al., 1999; Sibert et al., 

 2000). More recently Musyl et al. (2003) found similar 

 results from archival tagging data based on geolocation 

 and vertical movement patterns. They estimated bigeye 

 tuna residence time of 25 ±12 days at the Cross Seamount 

 area — a value consistent with the estimates derived here 

 using conventional tagging data. 



Putting aside M, we do know why yellowfin tuna emigra- 

 tion rate from the Cross Seamount is higher while they ap- 

 pear to be more vulnerable in the fishery than bigeye tuna. 

 Their higher vulnerability could in part be explained by 

 their shallower swimming depths that bring them into more 

 frequent contact with handline and troll gear However, big- 

 eye tuna contribute greatest to the commercial catches by 

 weight from the Seamount (Itano and Holland, 2000). Sibert 

 et al. (2000) suggested that this apparent discrepancy could 

 be due to a much higher biomass of bigeye tuna on the Sea- 

 mount compared to biomass of yellowfin tuna, coupled with 

 longer residence times. 



The apparent longer residence times for bigeye tuna at 

 the Seamount could be due to longer periods of continuous 

 residence or a greater tendency to revisit over time (or to 

 both factors). It is possible that bigeye tuna may gain a 

 trophic advantage by extended association with seamounts 

 (Fonteneau, 1991; Brill and Lutcavage, 2001). Behavior of 

 bigeye tuna associated with Cross Seamount, inferred from 

 archival tag data (Musyl et al., 20031, indicates that their 

 vertical movements are akin to the characteristic open wa- 

 ter behavior That is, they move within the surface mixed 

 layer at night but remain deep during the day except for 

 brief upward excursions (Holland et al., 1990; Dagorn and 

 Josse. 2000). However. Musyl et al. (2003) note the irregu- 

 lar and sometimes more extended day-night transitions of 

 the putative Cross Seamount associated bigeye tuna. This 



modified behavior at Cross Seamount, during day and 

 night, could indicate that bigeye tuna are exploiting a food 

 source that may not be available to or not preferred by sym- 

 patric yellowfin tuna. Unfortunately, similar vertical move- 

 ment observations for yellowfin tuna at Cross Seamount 

 are not currently available. Preliminary investigation on 

 the food habits of bigeye and yellowfin tuna at Cross Sea- 

 mount and offshore weather buoys suggest feeding ecology 

 is very different between the two species even at immature 

 sizes (Grubbsetal.'').They suggest that separation in verti- 

 cal distribution may be maintained during feeding. Bigeye 

 tuna may target the deep-scattering-layer prey while yel- 

 lowfin tuna feed primarily on mixed-layer prey. 



Estimates of horizontal movement patterns of bigeye 

 tuna equipped with archival tags suggest that almost all 

 bigeye tuna released from the Seamount stayed within close 

 proximity of the seamount and around the Main Hawai- 

 ian Island chain (Sibert et al., 2003). The relatively high 

 transfer rates between the Cross Seamount and the NOAA 

 weather buoys and the similar magnitude of transfer rates 

 between Cross Seamount and inshore areas suggests that 

 the apparently lower emigration rate of bigeye tuna is due 

 to returnees contributing to the recapture attrition curve. 

 Given the estimated F at the Seamount for the two species 

 in this study, the number of bigeye tuna residing at Cross 

 Seamount has to be at least an order of magnitude greater 

 than that for yellowfin tuna to match the catch observed in 

 fishery statistics. 



The overall picture emerging from the analysis is similar 

 to the earlier findings of Sibert et al. (2000). At any given 

 time the resident population (or standing stock) of yellow- 

 fin on the Cross Seamount is considerably smaller than the 

 bigeye tuna population. However, during their brief stop- 

 overs on the Cross Seamount, yellowfin tuna are highly 

 vulnerable to the offshore handline-troll fishery that occurs 

 there. They associate with the Cross Seamount but leave 

 quite rapidly, and most of them never return. In contrast, 

 the longer apparent residency, or persistence, of bigeye 

 tuna at the Cross Seamount may be due to longer periods 

 of association and a tendency to return to the Seamount 

 over time. Even though they tend to leave the Seamount 

 (perhaps permanently when they grow to larger sizes), 

 they appear to remain in the Hawaii area, at least for two 

 to three years. Some of them become vulnerable in the 

 inshore area but, if not captured, they are later caught by 

 the longline fishery. This situation is very similar to the ag- 

 gregation of bigeye tuna in the Coral Sea in northwestern 

 Australia (Hampton and Gunn, 1998) where bigeye tuna 

 appear to have a lower attrition rate than yellowfin tuna. 

 Hampton and Gunn ( 1998) argued that although both spe- 

 cies gradually disperse from the Coral Sea area, large num- 

 bers of bigeye tuna remain resident in the area for some 



Grubb.s. R. D., K. Holland, and D. Itano. 2001. Food habits 

 and trophic dynamics of structure-associatod aggregations of 

 yellowfin and bigeye tuna (Thunnus albacarcH, and T. obeaiis) in 

 the Hawaiian Islands. Project description, rationale and prelimi- 

 nary results. Presented at ihf Fourteenth Standing Committee 

 on Billfish and Tunas; Yellowfin Research Group, 9-16 August 

 2001 . Secretariat of Pacific Comnninity. BP D5, 98848 Noumea 

 Cedox, New Caledonia. 



