particularly in the more temperate latitudes. 

 The maximum northward extension of the Equa- 

 torial Zone skipjack occurs during the northern 

 summer, thus also accounting for a large pro- 

 portion of the Hawaiian catch. 



EVIDENCE 



Data that seem pertinent to the problem of the 

 origin of the exploited groups or subpopulations 

 of skipjack have been selected from the litera- 

 ture. The data selected involve the distribution 

 of spawning, length-frequency distribution of 

 Hawaiian and eastern Pacific samples, a com- 

 parison of eastern and central Pacific length- 

 frequency distributions, movements, prediction 

 indices, and subpopulation structure. This sec- 

 tion is, in effect, an appendix to the hypotheses 

 section, but is presented here so that the reader 

 can readily examine the background material 

 before encountering a discussion of the various 

 hypotheses. 



Distribution of Spawning 



Postlarval skipjack and advanced gonad indices 

 have been used as evidence of skipjack spawning. 

 Gonad indices are based on the relative size of 

 the largest ova or the ratio of body weight to 

 gonad weight. These indices are difficult to in- 

 terpret, because running ripe skipjack are prac- 

 tically never taken; therefore, gonad or egg size 

 indices only indicate maturity up to the level at 

 which prespawning fish cease to be available to 

 sampling. The time elapsing between cessation 

 of availability and actual spawning is not known. 



The distribution of the earliest identifiable 

 larvae or postlarvae may be a better index of the 

 temporal-spatial distribution of spawning than 

 are gonad indices. This belief is based on the 

 reasonable assumption that larvae and even post- 

 larval skipjack have a much smaller degree of 

 mobility than the adults and on a less defensible 

 assumption that mortality in skipjack at these 

 stages is approximately uniform with respect to 

 time of year and location. 



Evidence, based on the occurrence of larvae 

 and juveniles, that skipjack spawn during a wide 

 range of months over the enormity of the tropical 

 Pacific should be viewed with caution. It is nec- 

 essary to determine whether the capture of skip- 

 jack larvae or juveniles in incidental numbers 

 in certain areas indicates that the spawning which 

 occurs in those areas is great enough to make a 

 measurable contribution to the exploited stocks. 



Spatial distribution of larvae . --The distribu- 

 tions of skipjack larvae in the eastern and central 

 Pacific are different, because larvae are taken 

 in relatively high numbers in the central Pacific 

 with 1-m. plankton nets, but almost none are 

 caught with similar gear in the eastern Pacific. 

 Klawe (1963) has undertaken the most complete 

 study of the distribution of tuna larvae in the 

 eastern Pacific Ocean. He examined a large 

 number of plankton tows, representing samples 

 taken by several types of gear at various depths. 

 Only a few larval (about 0.02 larvae per tow) 

 and juvenile skipjack were found. Klawe (p. 466) 

 concluded that for skipjack there is "...very 

 limited spawning in coastal and oceanic waters 

 of the area of our study. Young forms /werej 

 collected only on a few occasions, off Central 

 America and as far north as the Gulf of Tehuan- 

 tepec. Young /we r-e_7 also collected off Ecuador." 

 Since only a few skipjack larvae were taken in 

 the eastern Pacific, no temporal patterns were 

 ascertained from the data. 



The relative absence of skipjack larvae in 

 Klawe's data concurs with Matsumoto's (1958: 

 p. 59) figure 31, which shows a drop in skipjack 



o 



larvae per 1,000 m. of water strained in the 

 vicinity of long. 120° W. as compared with areas 

 to the west. 



With regard to latitudinal distribution of skip- 

 jack larvae, Matsumoto's (1958: p. 58) figure 30 

 shows apparently high densities of skipjack lar- 

 vae in the equatorial region, with a reduction in 

 numbers of larvae per 1,000 m. of water strained 

 as one proceeds northward from the Equator. 

 Data in the files of the Bureau of Commercial 

 F isheries Biological Laboratory at Honolulu, the 

 appendices of Matsumoto (1958) and Strasburg 

 (1960), and Matsumoto's (1958) figure 33, for 

 example, suggest that the Hawaiian Islands re- 

 gion, particularly in the summer, is also a locus 

 of high densities of larval skipjack. 



Temporal distribution of larvae. --The tem- 

 poral distribution of tuna larvae in the central 

 Pacific is known from Matsumoto (1958: p. 64), 

 Nakamura and Matsumoto^, and miscellaneous 

 data in the files of the Bureau's Biological Labo- 

 ratory at Honolulu. It appears from these data 

 that skipjack spawn the year-round in the equa- 

 torial region, with peak numbers of larvae per 

 sample from May to September. In the Hawaiian 

 Islands, spawning occurs primarily during the 

 northern summer, whereas in the Marquesas, 



2/ 



Nakamura, Eugene L., and Walter M. Matsumoto. 



Manuscript. Distribution of larval tuna in Mar- 



quesan waters. Bureau of Commercial Fisheries 



Biological Laboratory, Honolulu. 



