WILLIAMS: MODELS OF MIGRATION OF YOUNG SKIPJACK 



mature cod was under thyroid control, a distinct 

 cycle in thyroid follicular cell height being ob- 

 served although less than in mature fish. Wood- 

 head (1959a) and Woodhead and Woodhead 

 (1965) suggested production of such an active 

 migration could be due to a general increase in 

 swimming activity and in reaction to water cur- 

 rents (particularly a tendency to swim against 

 them) caused by the increase in thyroid hor- 

 mones. The thyroid in tuna, unlike most teleosts 

 (including the cod), is a discrete bilobed struc- 

 ture (Honma, 1956 — bluefin tuna; Williams, un- 

 published — skipjack and yellowfin tuna). If, as 

 the juvenile skipjack approaches the size asso- 

 ciated with first migration, the thyroid becomes 

 more active and one effect of this is to initiate a 

 similar behavioral response as in the cod (in- 

 creased locomotor activity) , then there would be 

 a subsequent demand for more food if the growth 

 of the animal is to be continued. At such time 

 it is suggested that the increased food require- 

 ments — amount, type, size — cannot be supplied 

 in the central Pacific near-surface waters, or that 

 the young skipjack come increasingly into com- 

 petition for the available food with other species 

 and also their own adults, (See also discussion 

 of evolution of migration patterns in Rothschild 

 and Yong, 1970. ) Additionally, the endocrine in- 

 duced internal stimuli may also act to lower 

 thresholds for recognition of changes in environ- 

 mental conditions to the magnitude of those 

 found in the equatorial central Pacific. 



Sequences of events, such as mentioned above, 

 could possibly trigger the active migration of ju- 

 venile skipjack out of the central Pacific — the 

 main component being to the feeding grounds 

 in the neritic eastern Pacific. It is proposed that 

 the migrating skipjack juveniles are principally 

 located in the equatorial areas of the west-flow- 

 ing NEC and SEC. With near-surface temper- 

 atures optimal, except at the equator west of the 

 Galapagos Islands at times of intense upwelling, 

 the fish are located within or close to the equa- 

 torial and northern "productivity" bands de- 

 scribed previously. In view of the apparent 

 intermittent nature of the northern band, it is 

 possible that the incidence of small, but frequent 

 fronts may also act as concentrating mechanisms 

 for food organisms normally observed at a low 



density in this area, lat 5°-10°N (Murphy and 

 Shomura, 1972). Skipjack may also be asso- 

 ciated with the possible "productivity" band 

 around lat 10°-15°S. 



The mechanisms which maintain the overall 

 eastward orientation of the skipjack in an off- 

 shore oceanic area (in the absence of reference 

 points) are unknown, although as Hoar (1953) 

 commented "fish possess an elaborate and deli- 

 cate array of highly specialized peripheral sense 

 organs and appendages" and these could all be 

 involved in some type of navigating ability in 

 tuna. In this respect the probable role of the 

 pineal apparatus of tunas as a photoreceptor 

 (Rivas, 1953) should not be overlooked, Royce, 

 Smith, and Hartt (1968) in a discussion of pos- 

 sible guidance mechanisms in oceanic migration 

 models of Pacific salmon concluded that they may 

 depend on electromagnetic cues from ocean cur- 

 rents and that responses to all migratory cues 

 are inherited. Recent work by Yuen (1970) on 

 tracking of small skipjack (with ultrasonic tags 

 and continuous-transmission frequency-modu- 

 lated sonar) moving on and off banks in the Ha- 

 waiian Islands lead him to imply that skipjack 

 can navigate and have a sense of time. 



It is hypothesized that incoming juvenile skip- 

 jack move continuously eastwards past the me- 

 ridian of long 130 °W, orientating largely to the 

 zonal "productivity" bands (northern and equa- 

 torial) until entering the offshore areas of the 

 southern fishery from August to April. How- 

 ever, the interruption or cessation of the surface 

 NECC east of long 120°W in the period January- 

 February to April-May might be expected to dis- 

 rupt the orientation and movement of incoming 

 skipjack juveniles, mainly through significant 

 changes in the position or continuity of the "pro- 

 ductivity" bands and food-concentrating mecha- 

 nisms. It is at this period of the year that re- 

 cruitment to the northern fishery is postulated. 



Certainly the cessation of the NECC would 

 cause the deepening of the mixed layer in the 

 vicinity of the previous northern boundary of 

 the NECC, and hence the elimination of possible 

 ridging and subsequent increased biological pro- 

 ductivity, though it is very difficult to estimate 

 the lag period. In the case of the equatorial band 

 (east of long 120 °W) the annual southward 



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