NOTE Holland et a!.: Residence times of Thunnus albacares and T obesus over a seamount 



395 



fin tuna in the fishery. Similarly, mixed aggi'egations 

 of yellowfin tuna, bigeye tuna, and skipjack tuna have 

 been reported at seamounts in the Atlantic (Fon- 

 teneau, 1991). 



Although the vertical behavior of yellowfin and 

 bigeye tuna seems to merge at Cross Seamount, the 

 current data indicate that the duration of horizontal 

 orientation to the seamount (as measured by resi- 

 dence time ["half-life"] ) is different. 



The underlying advantage of seamounts to tuna 

 biology is not well understood. Although seamounts 

 can cause geographically stable regions of planktonic 

 enrichment, it is not known if this enrichment per- 

 sists long enough to move through the trophic chain 

 to the level of the tuna forage base (Boehlert and 

 Genin, 1987). We do know that Cross Seamount is 

 situated in a very dynamic part of the ocean charac- 

 terized by vortices created on the downcurrent side 

 of the main Hawaiian islands (Flament et al.'). As 

 these eddies spin off from the islands, current direc- 

 tion over the seamount can change frequently. 



If an enriched area of prey does exist, and feeding 

 is the principal underlying reason for tuna aggrega- 

 tions, it is difficult to understand why residence times 

 are different for the two species and quite brief for 

 both. The feeding advantage should impact both spe- 

 cies equally and their residence times at the sea- 

 mount should be similar. A comparison of the stom- 

 ach contents of the two species when caught in sea- 

 mount aggregations would be instructive. It is pos- 

 sible that, rather than acting as feeding stations, 

 seamounts act as orientation points in the larger- 

 scale movement patterns of these fish. Even though 

 they may be too deep for visual detection, seamounts 

 may be recognized by tuna through their ability to 

 detect the effect of seamounts on the earth's mag- 

 netic field (Walker, 1984; Walker et al., 1984; Klimley 

 et al., 1988). The seamounts may act as midocean 

 reference points that may also occasionally harbor 

 increased prey densities, the periodicity and persis- 

 tence of which are driven by events in the surround- 

 ing oceanographic conditions. 



A navigational role might explain why remote sea- 

 mounts aggregate more tuna than seamounts located 

 closer to land masses (Fonteneau, 1991) and why, in 

 our study, the residence times were quite brief for 

 both yellowfin and bigeye tuna species. The differ- 

 ences in the duration of orientation to the seamount 

 might be explained if the navigational importance of 

 seamounts is different in the broader behavioral rep- 

 ertoires of the two species. 



' Flament, P. J., C. Lumpkin, J. Tournadre, P. Kloosterziel, and 

 L.Armi. 1997. Period doubling and vortex pairing in an an- 

 ticyclonic shear flow in the ocean. Manuscript in review. 



Certainly, the current data indicate that Cross Sea- 

 mount hosts transient populations of both tuna spe- 

 cies rather than long-term populations. This brevity 

 of residence at the seamount for both species probably 

 reduces the chances of excessive fishing exploitation. 



Acknowledgments 



We greatly appreciate the expert instruction in tag- 

 ging techniques provided by David Itano, the fish- 

 ing prowess of Tony Frietas, and the role of John 

 Sibert in pointing out the applicability of tag attri- 

 tion curves to these data. This research was funded 

 under Cooperative Agreement No. NA37RJ0199 from 

 NOAA and administered by the Pelagic Fisheries 

 Research Program, Joint Institute for Marine and 

 Atmospheric Research, School of Ocean and Earth 

 Sciences and Technology, University of Hawaii. 



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