Margulies et al,: Spawning and early development of captive Thunnus albacares 



255 



Spawning periodicity and size 

 and age at first-spawning 



The yellowfin tuna broodstock spawned 963 

 times, at almost daily frequencies, over the 

 study period, although not all the fish were 

 sexually mature during the entire period. 

 Genetic analysis of female broodstock and 

 their eggs and larvae (Niwa et al., 2003) 

 corroborated our observations and video- 

 tape analysis of courtship and spawning 

 behaviors and spawning frequencies. The 

 mitochondrial DNA analysis confirmed that 

 multiple females were contributing to indi- 

 vidual spawns over protracted time peri- 

 ods (weeks to months). From August 1999 

 through August 2000, there were ten identi- 

 fied females in the tank. We identified up to 

 six female genotypes in egg or larval sam- 

 ples on individual spawning dates, and we 

 observed the same genotypes for at least five 

 females on successive sampling dates over 1 

 to 3 months. The spawning frequencies esti- 

 mated by this analysis were undoubtedly 

 conservative because the sampling was not 

 conducted every day, even though spawning 

 was occurring daily. 



When spawning first occurred (8 Oc- 

 tober 1996), we estimated that the aver- 

 age age of the broodstock females was 1.6 

 years (range 1.3-2.0 years, ?!=24). For the 

 1999-2000 period, we estimated that the 

 weight of a female at first spawning was 12 

 to 28 kg and its size was 75 to 112 cm FL. 

 These measurements correspond to an av- 

 erage age of 2.0 years (range 1.6-2.8 years, 

 n=7), where the majority of females would 

 be slightly younger than 2.0 years. 



Timing of spawning and hatching 



Q. 



0.82 



€ ".^r- =. 



The time of day that spawning occurred 

 ranged from 1330 to 2130 h, and was most 

 frequent between 1600 and 2100 h (Fig. 3A). 

 The time of day of spawning was strongly 

 and positively correlated (r=0.82, df=937, P<0.001) with 

 mean daily water temperature. For example, at mean 

 daily water temperatures of 25°C or below, spawning 

 occurred in the afternoon between 1330 h and 1745 h. At 

 temperatures near 27°C, spawning took place near 1800 

 h, and at water temperatures of 27.5°C or above, spawn- 

 ing occurred at night between 1845 h and 2130 h. 



Egg-stage duration was inversely related to mean 

 incubation temperature (Fig. 3B). Mean egg-stage du- 

 ration ranged from a maximum of 28 h at 24°C to a 

 minimum of 18 h at 29°C or above. The relationship 

 was best described by a highly significant linear regres- 

 sion (r2=0.47, df=827, P<0.001). 



Water temperature imposed opposite effects on the 

 time of day of spawning (direct relationship) and egg- 



Mean temperature (°C) 



Figure 3 



The relationship between mean daily water temperature and (A) 

 time of day of spawning, (B) egg-stage duration, and lC» time of 

 day of hatching for yellowfin tuna iThunnus albacares) in captivity. 

 Plotted values are daily estimates. 



Stage duration (inverse relationship), and the net effect 

 of these relationships was a narrow range for the time 

 of day at hatching (Fig. 3C). Over the 3.5-yr study 

 period, the eggs hatched between 1300 h and 2145 h. 

 However, over 95% of the hatch times occurred in a nar- 

 row time frame between 1500 h and 1900 h, regardless 

 of water temperature. 



Egg production and daily ration 



For the original broodstock group, there was no defini- 

 tive relationship between standardized egg production 

 and mean size of spawning females, nor was there a 

 significant overall correlation between standardized 

 egg production and daily ration over the entire study 



