Schaefer: Spawning time, frequency, and batch fecundity of Thunnus albacares 



batch fecundity estimates and natural logarithms of 

 FL for each fish was calculated. The result of the 

 regression analysis is given in the antilog form as a 

 power curve. Analysis of covariance was applied to 

 the log-transformed batch fecundity and length data 

 from this study and the data of Joseph ( 1963 ) ( whose 

 data were truncated so that the length ranges of the 

 two data sets coincided) to compare the two sets of 

 data. 



In order to estimate the cost of spawning a single 

 batch of eggs, 12 preserved oocytes (10% neutral- 

 buffered formalin) in the migratory-nucleus stage 

 from each of two fish, were individually measured, 

 rinsed in distilled water, placed on a nucleopore fil- 

 ter for about 10 seconds, and then weighed to the 

 nearest 0.001 mg. The average weight of an oocyte 

 in the migratory-nucleus stage multiplied by the 

 mean relative fecundity provided an estimate of the 

 cost of spawning a single batch of eggs in percentage 

 of body weight (Hunter et al., 1986). 



The costs of a single spawning for both females 

 and males were also estimated by subtracting the 

 percentage of calculated body weight of the gonads 

 just after spawning from the percentage of body 

 weight of the gonads just before spawning occurred. 



Results 



Classification of ovaries and spawning 

 incidence 



2000 h had hydrated oocytes; 2) ovaries collected 

 between 2230 and 0330 h had new postovulatory fol- 

 licles ( Tables 1 and 2 ). The earliest spawning appears 

 to have occurred around 2230 h, as evident in an 

 1145-mm FL female; this fish had new postovulatory 

 follicles with no evidence of cellular degeneration and 

 had large numbers of ovulated eggs within the lu- 

 mina of both ovaries. 



The mean diameter of the most developed group 

 of oocytes within ovaries, plotted by time of day (Fig. 

 3), provided further evidence of the spawning time 

 for yellowfin tuna. From 0001 to 1200 h, the mean 

 oocyte diameter increased from about 0.55 mm to 0.64 

 mm. From 1201 to 2000 h, the oocytes began the 

 migratory-nucleus stage, during which the mean 

 diameter increased to about 0.80 mm by 1601 to 

 2000 h. This advanced batch of oocytes became hy- 

 drated, was then ovulated, and was spawned some- 

 time after 2000 h. The mean diameter of the remain- 

 ing most-developed modal group of oocytes was about 

 0.50 mm. 



Postovulatory follicles with no signs of degenera- 

 tion were classified as to 6 hours old (Fig. 4, A and 

 B). These postovulatory follicles had an involuted 

 shape with numerous folds and a relatively open fol- 

 licular cavity. The granulosa cell layer lining the lu- 

 men (epithelium) showed regularly aligned cuboidal 

 cells with prominent basal nuclei. The thecal con- 

 nective tissue layer was distinct, in only minimal 

 contact with the granulosa, and appeared to be of 

 about the same thickness as the latter. 



The estimate of spawning time and rates of resorp- 

 tion of postovulatory follicles for yellowfin tuna was 

 based on the classification of ovaries taken from 58 

 fish sampled at different times of the day (Table 1). 

 All females had reached sexual maturity. Sexually 

 mature females were those whose ovaries contained 

 advanced yolked oocytes (oocyte classes 3, 4, or 5) in 

 the most advanced modal group, or had partially 

 yolked oocytes ( oocyte class 2 ) along with a or f5 atre- 

 sia, or both (atretic classes 1, 2, 3, or 4). Of the 58 

 females sampled, 52 were classified as reproductively 

 active. Reproductively active females are those whose 

 ovaries contain advanced yolked oocytes and may or 

 may not contain postovulatory follicles and those in 

 whom a atresia is either absent or less than 50% 

 (atretic states or 1). 



Analyses of the ovaries sampled in 1986 and 1987 

 indicated that yellowfin tuna spawning occurred be- 

 tween approximately 2230 and 0330 h. This estimate 

 was based on the following observations: 1) ovaries 

 sampled during the afternoon (1300-1800 h) had 

 migratory-nucleus stage oocytes in the most ad- 

 vanced mode and those sampled between 1800 and 



0001- 0401- 0801- 1201- 1601- 2001- 

 0400 0800 1200 1600 2000 2400 



Time ol day (h) 



Figure 3 



Mean diameters (mm) of the most advanced group of oo- 

 cytes of yellowfin tuna, Thunnus albacares, by time of day. 

 Means are represented by solid circles, ranges by vertical 

 lines, and the numbers of females are noted. 



