FISHERY BULLETIN: VOL. 70, NO. 3 



passage of the ITCZ causes a rapid change from 

 predominantly southeast trades to either north- 

 east trades or light easterly winds/doldrumlike 

 conditions. In the former case, there would be 

 divergence at the equator and formation of a 

 weak convergence some distance south of the 

 equator, while in the latter case there would be 

 equatorial divergence but no convergences. 

 Either way it would seem that there might be 

 significant disruption of the "productivity" 

 bands. However, EASTROPAC data do not 

 show this very clearly, perhaps due to the lag 

 period in the establishment of the higher trophic 

 levels. 



If a surface SECC is present in the eastern 

 Pacific (even seasonally) then eastward move- 

 ment of juvenile skipjack could possibly take 

 place in the SEC near a southern "productivity" 

 band (at the SECC/SEC southern boundary). 

 Subsequently there would be an active migration 

 of juveniles from near the SECC terminus into 

 the southern fishery. However, water tempera- 

 tures alone could prevent any direct recruitment 

 from this area to the southern fishery during 

 part of the southern winter. [In this and sub- 

 sequent migration models speculations are not 

 made on skipjack distribution south of the pos- 

 sible SECC/SEC southern boundary.] 



Tuna have to swim continually to ensure ven- 

 tilation across the gills and to maintain hydro- 

 static equilibrium, and maximum speeds in ex- 

 cess of 10 body lengths per second (bl/sec) have 

 been reported (Blaxter, 1969, and references 

 therein). For the closely related little tuna 

 (Euthynnus af finis) , MsLgnu&on (1970) report- 

 ed the minimum speed for hydrostatic equilibri- 

 um in a 42-cm fish was about 1.4 bl/sec and cal- 

 culated for a 10-cm fish it would be about 3 bl/ 

 sec. After feeding, the average speed of the 

 same captive fish increased to about 2 bl/sec, 

 while Walters (1966) recorded for a 40-cm fish 

 a speed of 5.9 bl/sec on a feeding run on dead fish 

 and a maximum speed of 12.5 bl/sec. Magnuson 

 (1970) also pointed out that the little tuna ap- 

 pears to spend most of its time swimming at 

 speeds near the minimum hydrostatic speed and 

 relatively little near the maximum. For captive 

 skipjack of 38, 39, and 48 cm, John J. Magnuson 

 (personal communication to Maurice Black- 



burn) indicated that the observed mean speed 

 was about 2 bl/sec; for calculations for model 

 fish the minimum speed for a 15-cm juvenile 

 skipjack would be about 3 bl/sec. Actively mi- 

 grating juvenile skipjack may be expected to 

 have a narrow range of theoretical minimum 

 speed, from 2 to 3 bl/sec. 



The tracking of 40-42 cm skipjack in the 

 Hawaiian Islands by Yuen (1970) showed that 

 movement off a bank at night, principally from 

 1800 to 0200 hr, was mainly near the surface 

 and without frequent directional changes, at 

 speeds equivalent to about 1.5-6.0 bl/sec. How- 

 ever, during daylight, apparent speeds fell well 

 below Magnuson's calculated minima which, 

 Yuen concluded, must indicate considerabPe turn- 

 ing from a straight line track, presumably due 

 to food searching and feeding — [skipjack are 

 primarily daylight feeders (Nakamura, 1962)]. 

 In addition, there was a greater variability in 

 depth during the day than the night. On a re- 

 cent cruise (Williams, 1971) small groups of 

 skipjack were observed from the deck and under- 

 water bow chamber swimming just ahead of the 

 RV David Starr Jordan for considerable lengths 

 of time. For the size of fish involved, a sample 

 of four ranged from 60 to 64 cm FL, and the 

 ship's trolling speed of 6I/2 knots, the skipjack 

 were maintaining speeds of about 5.5 bl/sec. 



In view of the above data it is considered rea- 

 sonable to assume a mean swimming speed for 

 incoming young skipjack equivalent to about 

 3 bl/sec "made good" in the direction of the 

 oriented movement (migration) over the 24-hr 

 period, i.e., about 50 miles per day. Hence, the 

 first recruits entering the offshore areas of the 

 southern fishery at the beginning of August may 

 well have passed the meridian of 120°W about 

 6 weeks earlier, i.e., in mid-June shortly after 

 the surface NECC is reestablished east of that 

 meridian. Similarly, the last recruits to this 

 fishery, in entering the offshore areas about the 

 beginning of April, would have passed long 

 120°W about mid-February close to the time of, 

 or shortly after, the interruption of the surface 

 NECC east of that point. Thus, at the time of 

 entry of the principal component of recruits 

 from November to April, peak January and 

 February, the surface NECC is established 



750 



