379 



Abstract — Thirty-three skipjack tuna 

 {Katsuwonus pelamis} (53-73 cm fork 

 length) were caught and released with 

 implanted archival tags in the eastern 

 equatorial Pacific Ocean during April 

 2004. Six skipjack tuna were recap- 

 tured, and 9.3 to 10.1 days of depth 

 and temperature data were down- 

 loaded from five recovered tags. The 

 vertical habitat-use distributions indi- 

 cated that skipjack tuna not associ- 

 ated with floating objects spent 98. 69^ 

 of their time above the thermocline 

 (depth=44 m) during the night, but 

 spent 37.7% of their time below the 

 thermocline during the day. When 

 not associated with floating objects, 

 skipjack tuna displayed repetitive 

 bounce-diving behavior to depths 

 between 50 and 300 m during the day. 

 The deepest dive recorded was 596 

 m, where the ambient temperature 

 was 7.7°C. One dive was particularly 

 remarkable because the fish contin- 

 uously swam for 2 hours below the 

 thermocline to a maximum depth of 

 330 m. During that dive, the ambient 

 temperature reached a low of 10.5°C, 

 and the peritoneal cavity temperature 

 reached a low of 15.9°C. The vertical 

 movements and habitat use of skipjack 

 tuna, revealed in this study, provide a 

 much greater understanding of their 

 ecological niche and catchability by 

 purse-seine fisheries. 



Vertical movement patterns of skipjack tuna 

 (Katsuwonus pelamis) in the eastern equatorial 

 Pacific Ocean, as revealed with archival tags 



Kurt M. Schaefer (contact author) 

 Daniel W. Fuller 



Email address for K. M. Schaefer: kschaeferffl iattc.org 



Inter-American Tropical Tuna Commission 

 8604 La Jolla Shores Drive 

 La Jolla, California 92037-1508 



Manuscript submitted 10 October 2006 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



12 January 2007 by the Scientific Editor. 



Fish. Bull. 105:379-389 (2007) 



Limited information on the vertical 

 movements of skipjack tuna (Kat- 

 suwonus pelamis) is available from 

 analyses of catch data for tuna long- 

 line gear (Yabe et al., 1963), ultra- 

 sonic telemetry (Dizon et al., 1978; 

 Schaefer and Fuller, 2005; Matsumoto 

 et al.'), and archival tags (Ogura^). 

 These studies have indicated that 

 skipjack tuna inhabit predominantly 

 the mixed layer but make occasional 

 brief dives below the thermocline. 



Skipjack tuna are distributed 

 throughout the world's tropical and 

 subtropical oceans (Collette and 

 Nauen, 1983; Matsumoto et al., 1984). 

 In the eastern Pacific Ocean (EPO), 

 catches of this species have occurred 

 from about 34°N off southern Califor- 

 nia to about 27°S off northern Chile. 

 The species is limited to surface tem- 

 peratures of about 17° to 30°C (Wild 

 and Hampton, 1994). Skipjack tuna 

 are one of three principal targets 

 of a large-scale purse-seine fishery 

 in the EPO and are caught mostly 

 in association with floating objects 

 between about 10°N and 15°S. The 

 purse-seine catch of skipjack tuna 

 in the EPO during 1995-2004 av- 

 eraged 197 thousand metric tons (t) 

 (range: 129 to 295 thousand t), which 

 is 160% greater than the average of 

 76 thousand t (range: 51 to 95 thou- 

 sand t) during the previous 10-year 

 period (Anonymous, 2005). 



The anatomical and physiological 

 adaptations of skipjack tuna charac- 

 terize them as highly efficient oppor- 

 tunists for exploiting their oceanic 

 vertical habitat. Their body shape, 

 fin configurations, and musculature 



are close to optimum for relatively 

 fast, sustained, and burst swimming 

 (Magnuson, 1978; Altringham and 

 Shadwick, 2001). They lack a swim 

 bladder (Godsil and Byers, 1944), are 

 capable of physiological thermoregu- 

 lation (Dizon and Brill, 1979), and 

 have relatively rapid gastric evacua- 

 tion rates (Magnuson, 1969). 



Skipjack tuna possess a well-de- 

 veloped large central rete, in addi- 

 tion to epaxial and hypaxial ves- 

 sels and retia (Stevens et al., 1974; 

 Graham and Dickson, 2001). The 

 counter-current retia provide a ther- 

 moconserving mechanism enabling 

 metabolic heat to be retained within 

 the muscles and thus elevate body 

 temperatures above that of ambient 

 water temperatures (Graham, 1975; 

 Stevens and Neill, 1978). The ana- 

 tomical specializations enhance the 

 thermal inertia of this species and 



1 Matsumoto, T., H. Okamoto, and M. 

 Toyonaga. 2006. Behavioral study 

 of small bigeye, yellowfin and skipjack 

 tunas associated with drifting FADs 

 using ultrasonic coded transmitter in 

 the central Pacific Ocean. Information 

 Paper 7, 25 p. Second regular session 

 of the scientific committee. Western and 

 Central Pacific Fisheries Commission, 

 P.O. Box 2356, Kolonia, Pohnpei FM 

 96941. 



- Ogura, M. 2003. Swimming behav- 

 ior of skipjack, Katsuwonus pelamis, 

 observed by the data storage tag at the 

 northwestern Pacific, off northern Japan, 

 in summer of 2001 and 2002. Working 

 Paper SKJ-7, 10 p. Sixteenth meeting 

 of the standing committee on tuna and 

 billfish, Sec. Pac. Comm., Noumea, New 

 Caledonia, B.P. D5 Noumea Cedex, New 

 Caledonia. 



