SPAWNING OF YELLOWTTN TUNA 



59 



stages. The group of data shown in the upper 

 part of figure 6 represents fish with ovaries in the 

 ripe and maturing stages of development, D 

 through J. Because stage F (shown by dots) was 

 the only maturing stage which was well repre- 

 sented in the collection, it was used to describe the 

 relation between ovary weight and fish weight at 

 the approach of spawning. Over the ranges of 

 fish size and ovary size, the regression of ovary 

 weight on fish weight for ovaries in stage F * was 

 found to be best described by the linear equation 



y=43.48HX- 1327.8573. 



The regression coefficient had a standard error 

 of 6.060. The lines drawn parallel to the regres- 

 sion line indicate plus and minus twice the stand- 

 ard deviation from regression. 



It is evident from figure 6 that the slopes and 

 levels of the two regression lines, one representing 

 fish with ovaries in the immature, or resting, stage 

 (A) and the other representing fish in a maturing 

 stage (F), are significantly different. The dif- 

 ference in slope is due to the fact that in stage F 

 the larger fish tend to have relatively larger ova- 

 ries. The difference in level reflects the increase in 

 size of the ovaries between stage A and stage F. 



The values representing fish with ovaries in 

 other maturing stages (D, E, G, H, and I) and the 

 ripe stage (J), and those in which there was no 

 evidence of previous spawning, are indicated in 

 figure 6 by an x. Unfortunately, there are too few 

 samples in any one of these stages to establish 

 a regression with any degree of confidence. From 

 the fact that most of the values for these stages 

 lie within two standard deviations of the regres- 

 sion line for stage F, it appears that when the 

 ovaries reach stage D they have already greatly 

 increased in weight over stage A and that they 

 increase very little while progressing through 

 stages D, E, F, G, H, and I. The paucity of 

 samples of all of these stages except F also sug- 

 gests that the stages, other than F, probably are 

 passed through very rapidly. The scarcity of 

 some of the younger stages such as D and E might 

 be ascribed to undersampling in May, but irregu- 

 larity of sampling can scarcely explain the con- 

 sistent dominance of stage F over all other stages 



* Only those ovaries In stage F which showed no evidence of 

 previous spawning during the current year, as determined by 

 the absence of remnants of ripe ova In the lumlna, were used in 

 the calculations. 



during June to September. This dominance seems 

 most reasonably explained by stage F being rela- 

 tively long and stable and the other stages being 

 transitory and rapidly passed. For practical pur- 

 poses, then, but two groups need be recognized: 

 (1) Fish with ovaries in stages A, B, and C, or in 

 nonspawning condition ; and (2) fish with ovaries 

 in stages D to J, or in spawning condition. 



For those ovaries in maturing stages in which 

 some spawning had occurred during the season 

 (remnants of mature ova were in the lumina), 

 the points fall somewhere below the upper line but 

 lie above the lower line. These are shown as 

 circles in figure 6. 



With respect to time of year, the regression for 

 ovaries in stage A expresses the relation between 

 ovary weight and fish weight during the off-breed- 

 ing season (November through April), and the 

 regression for ovaries in stage F expresses the 

 relation during the spawning period (mid-May 

 through October) as indicated by these data. 

 (See Spawning Season, p. 61.) 



Thus, by utilizing the scatter diagram of figure 

 6, it is possible, over the size range 36 to 94 kg. 

 (80 to 208 lbs.), to determine whether an indi- 

 vidual yellowfin tuna is in nonspawning or spawn- 

 ing condition by simply locating on figure 6 the 

 coordinates of the weight of its ovaries and of its 

 total weight. The coordinates will probably cross 

 within one set of dashed lines or the other and 

 the fish may be appropriately identified as non- 

 spawning or spawning; some fish will fall at in- 

 termediate coordinate points— they are most prob- 

 ably in spawning condition and have already ex- 

 truded some ova. 



It is of interest that some discrepancy was found 

 between the classification of the ovaries on the 

 basis of their external appearance and their classi- 

 fication on the basis of the arbitrary stages of 

 maturity established by ova-diameter measure- 

 ments. It will be recalled (p. 48) that at the time 

 of collection, the relative maturity of each ovary 

 sample was recorded according to the stages estab- 

 lished by Marr (1948) . Among the 112 samples in 

 the collection, this subjective method of classifica- 

 tion resulted in 13 fish with maturing ovaries being 

 incorrectly identified as spawned out, or roughly, 

 in an error of about 10 percent. The greatest 

 source of error was among those ovaries in stages 

 E and F in which some spawning had taken place 



