FISHERY BULLETIN: VOL. 70, NO. 3 



At a certain size a large component of these 

 juveniles start an active migration eastwards 

 which ends with them on the feeding grounds of 

 the northern and southern fisheries off the 

 American continent. The size at which this 

 active migration commences is not known, but 

 with recruitment to the fisheries at 35-40 cm it is 

 probably at <30 cm. 



When in the eastern Pacific skipjack are feed- 

 ing heavily, and this appears related to both the 

 food requirements of the adolescent fish for nor- 

 mal growth and for the early development of 

 the gonads. The departure of the fish with ma- 

 turing gonads from the feeding grounds is at- 

 tributed to a reproductive drive to return to the 

 central Pacific where final maturation of the go- 

 nads and spawning must take place. 



Richard S. Shomura and Richard A. Barkley 

 (personal communications), with arguments 

 based on central Pacific data, have suggested two 

 spawning groups (northern and southern) are 

 present in the central-east Pacific skipjack sub- 

 population (s) with considerable geographic 

 overlap in equatorial areas. One group spawns 

 during the northern summer, peak in July, and 

 the other in the southern summer, peak in Jan- 

 uary. There is some slight evidence (Orange, 

 1961) that skipjack in the eastern Pacific fish- 

 eries are from two such spawning groups. Fu- 

 jino (in press) has indicated that in the western 

 Pacific subpopulation of skipjack there are two 

 spawning groups (northern and southern) with 

 a large overlap in geographic distribution. 



Even though larval and early juvenile skipjack 

 obviously make diel vertical migrations (Wade, 

 1951; Matsumoto, 1958; Strasburg, 1960; 

 Ueyanagi, 1969, 1970; Higgins, 1970), the lower 

 end of the temperature range is thought to be 

 restricted to about 24 °C, at least for larvae 

 (Ueyanagi, 1969; Richards, 1969; Eric Fors- 

 bergh, personal communication). However, the 

 adolescents arriving on the feeding grounds of 

 the eastern Pacific fishery seem to have attained 

 a physiological condition which permits them to 

 exist at ambient temperatures down to 20 °C, 

 and occasionally 17 °C, which are found in the 

 near-surface waters of these zones (in other 

 areas of the world, such as Tasmania, they may 

 be as low as 15°C, Robins, 1952). Ability of 



skipjack to tolerate low ambient temperature 

 (conserve internal heat) is probably a function 

 of size (sequential physiological events) , and this 

 is also important in relation to the depth capa- 

 bility of the fish in its search for food at various 

 life history stages. It is worth noting that skip- 

 jack have been shown (see below) to maintain 

 body muscle temperatures considerably above 

 ambient water temperatures: 



Ambient Body muscle 

 Skipjack water temper- 

 length temper- ature (°C) 

 (cm) ature (°C) above ambient Source 



Obviously the principal factors to be explained 

 eventually in the the active migration model are 

 those which induce the juvenile skipjack to mi- 

 grate out of the central Pacific. Such a geneti- 

 cally fixed behavioral pattern undoubtedly would 

 be a response to a summation of effects caused 

 by exogenous and endogenous stimuli. Bagger- 

 man (1960) considered that migration, basically 

 a function of locomotion and orientation, only 

 occurs when fish are in the proper physiological 

 condition and subject to external "releasing" 

 factors (stimuli). This physiological condition, 

 in turn, is brought about by endocrinal activity 

 initiated by endogenous rhythms and external 

 "priming" factors. This concept of causation of 

 migration possibly may well apply in the case 

 of tuna. 



Hoar (1959) noted that for fishes with mass 

 cyclical migrations at certain life history stages 

 (such as reproduction, movement from nursery 

 to feeding grounds) changes in endocrine secre- 

 tions (gonadal, thyroidal) appear to play a ma- 

 jor part in the generalized appetive behavior as- 

 sociated with migration. Woodhead (1959a, b) 

 discussed the role of the thyroid in the migra- 

 tions of mature and immature Barents Sea cod 

 described by Trout ( 1957) . It appeared that the 

 "dummy run" contranatant migration of the im- 



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