was later reduced to a random sample of 2.5 of the 

 most developed group. Following Snedecor (1946, 

 p. 457), I analyzed the data on ova size distribu- 

 tions; my analysis showed that 25 was an adequate 

 number to approximate the mean size. 



DEVELOPMENTAL STAGES OF OVA 



Four rather distinct developmental stages were 

 recognized and designated as ''early developing," 

 "developing," "advanced," and "ripe". The 

 early developing category includes ova which, 

 at their most primitive, appear as simple trans- 

 parent cells present in all ovaries. The larger 

 ova in this stage contained a relatively large 

 nucleus. The mean diameter of the early develop- 

 ing ova ranged from 0.16 to 0.33 mm. The ova 

 assigned to the developing stage were completely 

 opaque because of the deposition of yolk granules. 

 Their mean diameter was 0.37 to 0.66 mm. 

 The advanced stage comprised ova that were 

 still relatively opaque in appearance and con- 

 tained a cluster of small oil droplets to ova that 

 were semitransparent and had a well-developed, 

 bright yellow oil globule. The mean diameter of 

 these ova was 0.49 to 0.74 mm. The collection 

 had no ripe ovaries; however, a sample of ova from 

 a running ripe skipjack caught on January 26, 

 1957, was available for examination. This skip- 

 jack was caught by pole and line at 9°33' S., 

 139°55' W., about 10 miles southeast of the island 

 of Hua Pou in the Marquesas. Its ova were used 

 in an unsuccessful attempt at artificial fertilization 

 and were preserved and brought to the laboratory. 

 They are almost perfectly spherical and trans- 

 parent, with a distinct straw-colored oil globule. 

 Fifty were measured. Their diameters were 0.85 

 to 1.12 mm., with a mean of 0.96 mm. The oil 

 globule was about 0.14 mm. in diameter. 



The sizes of these ripe ova are similar to those 

 described by other investigators. Brock (1954) 

 found ripe ova from a skipjack in Hawaii to 

 average 1.125 mm. in diameter. Yabe (1954) 

 measured some ova from a ripe skipjack caught in 

 the southern waters of Japan and found that 

 they were 0.S0 to 1.17 nun. in diameter, averaging 

 1.00 mm. 



DEGREE OF MATURITY 



Skipjack were classified according to the stage 

 of development of the most developed group of 

 ova: the nomenclature used was the same as 



for the stages of ova development. Throughout 

 this report, unless otherwise stated, the designa- 

 tions early developing, developing, advanced, 

 and ripe will be used interchangeably to describe 

 a skipjack, its ovaries, and the most developed 

 ova contained therein. 



The early developing category included ovaries 

 as small as 2 g., from a sexually immature fish, 

 and those (from adult fish) which were relatively 

 large but which contained only early developing 

 and a few residual ova. This category also 

 included some ovaries containing ova thai ap- 

 peared to have attained the developing stage but 

 had begun to degenerate. These ova were 

 grayish, relatively soft, and easily broken. 



TREATMENT OF DATA 



Skipjack from the Marquesas and the Tuamotu 

 areas were considered together in all aspects of 

 this study, because I assumed that they belonged 

 to the same population or at least were similar 

 physiologically insofar as spawning was concerned. 

 It would have been interesting to determine 

 whether skipjack from the two areas did indeed 

 have similar spawning habits; however, the 

 samples from the Tuamotus were too few to 

 treat that area separately. Of the 402 pairs of 

 ovaries collected, 372 came from around the 

 Marquesas and 30 from the Tuamotus. 



SIZE AT FIRST SPAWNING 



The approximate body length at first spawning 

 was determined by arranging the lengths and the 

 stage of development of all the skipjack used in 

 this study into a frequency distribution (fig. 2). 

 The smallest skipjack measured 39 cm. This 

 fish had thin, ribbonlike ovaries which weighed 

 2 g. and contained only primitive ova. Presum- 

 ably it had never spawned. The ovaries of a few 

 fish, 43.7 to 50.7 cm. long, were classified as early 

 developing and also showed no positive evidence 

 of past spawning. A greater number in that size 

 range, however, had either developing or advanced 

 ovaries. The smallest fish that had advanced or 

 developing ovaries was 43 cm. long. The few fish 

 more than 55 cm. long that were classified as early 

 developing either showed evidence of past spawn- 

 ing or at least of having attained a stage of develop- 

 ment past early developing, for their ovaries con- 

 tained residual or degenerating ova. 



SKIPJACK SPAWNING IN MARQUESAS AND TUAMOTU AREAS 

 774-711 O— 66 13 



481 



