SPAWNING OF YELLOWFIN TUNA 



49 



ing fish to determine whether the ova matured at 

 the same rate and in the same relative numbers 

 throughout the ovaries. An analysis of variance 

 and a chi-square test (Snedecor 1946) showed no 

 significant differences either in the mean diameters 

 of ova (P>0.05) or in the relative numbers of ova 

 in each size group (P=0.70) from these three 

 regions of an ovary. Furthermore, no significant 

 differences were found in ova size between the two 

 ovaries from the same fish. Similar measure- 

 ments were made for an immature ovary, and no 

 significant differences were found, either among 

 the different parts of the same ovary or between 

 ovaries from the same fish. Thus, it was con- 

 cluded that the ova develop uniformly throughout 

 the ovary and in both ovaries of a fish. All fur- 

 ther ova samples for this study were taken from 

 the middle of the left ovary and are considered 

 representative of both ovaries. 



The samples were taken after the ovaries had 

 hardened in formalin. Ova from a sample were 

 teased out under a binocular microscope to ensure 

 complete separation from the follicles. After the 

 ova were stirred in a watch glass, a small subsam- 

 ple was drawn with a pipette and placed on a glass 

 slide for measurement. Parallel markings with 

 1-millimeter spacing on the slide were an aid in 

 avoiding duplicate measurements of the same 

 ovum. Measurements were made by an ocular 

 micrometer in a compound microscope at a mag- 

 nification of six diameters. Although the ova 

 seldom were perfectly spherical, measurements 

 were made on whatever axis fell parallel to the 

 micrometer graduations. This method of meas- 

 urement has been used by other investigators 

 (Clark 1925, 1931; Schaefer 193G) and, on the 

 basis of comparative test measurements of ova 

 made before and after their preservation, it was 

 found reliable in the present study. 



Maturing ovaries contain not only the translu- 

 cent, immature ova, but also several groups of 

 larger sizes. To ensure adequate numbers of these 

 latter groups without extravagant expenditure of 

 time on the much more numerous immature ova, 

 measurements were made on 300 ova larger than 

 0.16 mm. in diameter for all maturing ovaries. 

 For ovaries in the immature stage, 200 ova were 

 measured. 



DESCRIPTION OF THE OVARIES 



The ovaries of sexually mature yellowfin tuna 

 are paired, elongated organs that extend nearly 

 the length of the abdominal cavity. They are ap- 

 proximately round in cross section and almost 

 symmetrical — the left ovary usually being slightly 

 larger than the right. The ovaries are suspended 

 from the dorsal wall of the body cavity by a fold 

 in the peritoneum. Posteriorly, the ovaries unite, 

 with the lumina joined to a short, wide oviduct 

 that opens as a slit on a papilla behind the anus. 

 Anteriorly, the ovaries receive their blood supply 

 from two of the three branches of the coeliac- 

 mesenteric artery; the No. I branch apparently 

 nourishes the left ovary and the No. Ill branch 

 the right ovary (Godsil and Byers 1944) . As the 

 time of spawning approaches, the superficial blood 

 vessels supplying these organs become enlarged 

 and very conspicuous. 



DEVELOPMENT OF THE OVA 



Within the ovaries of sexually mature fish, sev- 

 eral developmental groups of ova may be distin- 

 guished at one time or another through the year. 

 On the basis of the characteristics of the largest 

 ova present, the ovaries were classified into the 

 following stages of maturity : 



Immature: Immature ovaries contain ova rang- 

 ing from 0.01 to 0.18 mm. in diameter, with a 

 mode of about 0.08 mm. Preserved in formalin, 

 the ova are translucent, spherical bodies, and are 

 invisible to the naked eye. Each ovum contains 

 a rather large, vesicular, eccentric nucleus enclosed 

 in a clearly defined nuclear membrane. Ova of 

 this group are present in the ovaries throughout 

 the year. Fresh ovaries at this stage are pinkish, 

 elongated, and slender. 



Intermediate: Ovaries at this stage contain ova 

 which range from about 0.18 to 0.40 mm. in diam- 

 eter. As the ovum increases from about 0.18 to 

 0.40 mm., the nucleus enlarges and the yolk gran- 

 ules appear. The cytoplasm containing the 

 scattered, spherical yolk granules is enclosed by 

 two membranes. The innermost of these, the vitel- 

 line, or fertilization, membrane, can be clearly dis- 

 tinguished only in the mature ovum. A compara- 

 tively tough, thin, and shell-like outer membrane, 

 the zona radiata, is in direct contact with the sur- 

 rounding ovarian follicle during development of 



