cooling to warming occurs early in the January- 

 February-March period; when this shift is late 

 in the period catches will be below average. 



The quality of the predictive index bears an 

 important relation to the biology of the skipjack. 

 For example, if the time of warming index ac- 

 counts for a high degree of variability in catch 

 (in the Hawaiian fishery, catch is closely corre- 

 lated with catch-per-unit-of-effort, R. Uchida, 

 personal communication), then variations in 

 catch are probably related to features of the en- 

 vironment that operate not more than several 

 months prior to the summer peak fishing period. 

 If this is the case it is unlikely that year class 

 strength, for example, would affect the catch 

 since year class strength of the fishable stock 

 is probably fixed at least several months prior 

 to the period of peak fishing. As the degree of 

 variability in catch, accounted for by its regres- 

 sion on time of warming, is decreased, the pos- 

 sible operation several months prior to the sum- 

 mer peak fishing period of mechanisms unrelated 

 to the time of warming become increasingly 

 apparent. 



It seemed that a statistical evaluation of the 

 relation between sea-surface warming time and 

 skipjack catch would be appropriate. For this 

 analysis sea-surface temperatures taken about 

 every other week off Koko Head, Hawaii, were 

 used. Least squares parabolas were fitted to 

 the temperature curves for the period between 

 December 1 and May 31 for the years for which 

 data were available (1956-63). The time of shift 

 from cooling to warming was obtained by taking 

 the first derivative of each parabola, setting the 

 derivative equal to zero, and solving to obtain 

 the minimum temperature. The point on the 

 abscissa where temperature is at a minimum is, 

 of course, the time of shift from cooling to warm- 

 ing. 



The correlation coefficient between time of 

 warming and total catch was 0.664, which is non- 

 significant at the 5-percent level with six degrees 

 of freedom. This neither confirms nor denies 

 the existence of this relation, but if a relation 

 does exist the available degrees of freedom are 

 not sufficient to demonstrate significance. Let 

 us assume, however, that two more degrees of 

 freedom are acquired and that the relation is 

 adjudged significant with a correlation coefficient 

 of 0.664. We would then say that 44 percent of 

 the variation in catch is accounted for by time of 

 warming; the other 56 percent is unexplained and 

 could be due to events which are uncorrelated 

 with time of warming such as inter alia year 

 class strength and size of fish. 



Subpopulations 



Evidence for the existence of subpopulations of 

 skipjack in the Pacific has been advanced by 

 Sprague and HoUoway (1962) and Sprague, Hollo- 

 way and Nakashima (1963). This evidence is 

 based on the existence of immunogenetically sep- 

 arable groups of skipjack. The location of these 

 subpopulations is charted by Sprague (1963). 

 His figure shows two subpopulations (I and II) in 

 the Hawaiian region, one (III) in the equatorial 

 region, three in the South Pacific (IV, V, VI), 

 and one (VII) in Micronesia. A cursory examina- 

 tion of subpopulation data shows no grossly ap- 

 parent relation between subpopulations I and II 

 and size of fish and time of year for the Hawaiian 

 samples. Attention is called to Hennemuth's 

 (1959) paper which shows morphometric differ- 

 ences in skipjack among areas within and between 

 the eastern and central Pacific. 



HYPOTHESES 



In this section the material on spawning, size 

 distribution, movements, and gonad indices is 

 synthesized into a set of hypotheses concerning 

 the origin of the skipjack exploited in the eastern 

 Pacific, their mode of entrance into the eastern 

 Pacific, the variables associated with their de- 

 parture from the eastern Pacific, and the origin 

 of the fish exploited in Hawaii. I am not the first 

 to present some of the ideas on which these hy- 

 potheses are based, viz., "...it appears that some 

 of the west coast population /of skipjack/ range 

 far to the westward" (Schaefer, 1963: p. 50) and 

 "these /tag/ recoveries are strong support for 

 the hypothesis, which we have frequently men- 

 tioned, that the skipjack of the eastern Pacific 

 may undertake long offshore-inshore migra- 

 tions" (Schaefer, 1963: p. 57). These allusions 

 are similar in spirit to some of the hypotheses 

 entertained in this paper, but it will be seen that 

 they are somewhat different in concept. 



The phenomena which the hypotheses of this 

 paper attempt to account for are undoubtedly 

 complex, and for this reason I suspect that in 

 some cases the explanations offered are over- 

 simplifications. I hope that the simplified pic- 

 ture presented here will generate further ques- 

 tions on the biology of the skipjack tuna. 



Skipjack Exploited in Eastern Pacific 



It appears as though skipjack tuna that are 

 taken in the eastern Pacific Ocean are generated 

 from spawnings in the central Pacific Ocean. 

 This concept is based on the distribution of skip- 

 jack larvae, which indicates apparently negligible 

 spawning (relative to the numbers of skipjack 



11 



