BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



Table 6.-Skipjack in size categories as percent of total, Cruises 

 Jordan 57-Cromwell 51, November-December 1970 and Jordan 

 60, March-April 1971. 



Table 7.-Small skipjack (<45 cm) as percent of total in lati- 

 tudinal zones north of 10°N, Cruises Jordan ol-Cromwell 51, 

 November-December 1970. 



>- 

 o 



z 



UJ 



o 



25 

 20 



15 



10 



5 







25 



20 



15 



10 



5 







JORDAN -57 AND CROMWELL -51 

 NOVEMBER - DECEMBER 1970 

 n = l43 



JTT-^ffHJ" 



Pn-ri 



I I I I I I I ' I 1 I I I 



JORDAN -60 

 MARCH -APRIL 1971 

 n= 70 



a 



I I I I I I I I' I  I 'I ■!  I ■! "I'l "ri "Ti "I "I 'Ti I ■! I I I I I I I ' I 



26 30 34 38 42 46 50 54 58 62 66 70 74 78 82 86 90 



2 cm FL CLASSES 



Figure 9.-Skipjack percent-length-frequency distribution in 

 the study area. Smoothed curves are from 3-figure moving 

 averages. Stated length indicates midpoint of class. 



Commission 1966, 1972). The principal component 

 of the Hawaiian catch usually consists of fish of 

 modal sizes > 60 cm (U.S. Bureau of Commercial 

 Fisheries 1963; Rothschild 1965; Higgins 1966). 

 The principal modes of skipjack caught in the 

 study area, 56 to 60 cm on the two cruises, are 

 intermediate between those in the eastern Pacific 

 coastal fishery and the Hawaiian fishery. Purse 

 seine samples of skipjack obtained in and near the 

 study area in 1970 and 1971, which are mentioned 

 later, had mean sizes from 58 to 61 cm. 



Size of Other Tuna 



Only 34 yellowfin tuna were boarded (39 caught) 

 on the November-December 1970 cruise, and 2 in 

 March-April 1971. None were taken south of lat. 



3°N. Yellowfin ranged from 26 to 112 cm, with 

 mean lengths for different aggregations ranging 

 from 28.5 to 42.3 cm. The majority (68%) were < 45 

 cm. These small yellowfin were mainly (83%) from 

 the same latitudinal zone (10°-14°N) as the small 

 skipjack. On three occasions schools of mixed small 

 tunas-skipjack, yellowfin, and frigate mackerel 

 {Auxis: 30 to 31 cm)-were sampled north of lat. 

 10°N. Small tuna of different species may occur 

 together because of similar environmental and 

 food requirements, and behavior. 



Sex and Maturity of Skipjack 



Sex ratios of skipjack for the two cruises were as 

 follows: 



November-December 1970: Total 143, Sexed 72 



Males 24; females 33; 

 indeterminate 15 



Ratio: Males to females, 1:1.4 



March- April 1971: Total 72, sexed 59 



Males 24; females 35 



Ratio: Males to females, 1:1.5 



Gonad maturity was determined macroscopically 

 in the field, and stages were classified as follows: 



Immature, virgin Roughly 



Immature, resting equivalent 



Maturing to indicated 



Spent stages in 



Spent-recovering Orange (1961) 



1-S 



1 



2 



5-A 



5-B 



The number of gonads in each stage by size of fish 

 are given in Tables 8 and 9. 



In November-December the smallest spent or 

 spent recovering female skipjack was 46.5 cm. In 

 March-April the corresponding size for females 

 was 49.8 cm, and for males 48.3 cm. These data 

 support other evidence that first maturity in 

 female skipjack in the Pacific is reached between 

 40 and 45 cm (Orange 1961; Waldron 1963; 

 Kawasaki 1965). 



A relatively large number of recently spawned 

 fish, i.e., with spent and spent-recovering gonads, 

 was taken on each cruise: 30% of the females and 

 7% of the males in November-December 1970, and 



391 



