258 



Fishery Bulletin 105(2) 



tive buoyancy in the late egg stage was observed at all 

 salinities at which incubation occurred (26 to 36 ppt). 



The duration of the yolksac larval stage was inversely 

 (but weakly) related to temperature and ranged from 

 56 h at 29.0°C to 65 h at 23.5°C. Eye pigmentation 

 and mouth formation in larvae occurred almost simul- 

 taneously, and at the stage of mouth formation there 

 was usually only a trace of yolk remaining. First-feed- 

 ing larvae averaged 3.3 (SD =0.18) mm NL (range 

 2.7-3.9 mm) and weighed between 14 and 30 f<g (aver- 

 age 22 fig) (Table 1). The mouth width of first-feeding 

 larvae ranged from 225 to 350 .um, averaging 262 f(m 

 (Table Ij. 



0,95 



E 

 E 



tr 090 



°9 



y=1 .254 -0.01 Ox 

 r=-0.48 



24 25 26 27 28 



Mean daily temperature ("C) 



1 05 -, 



1.00 



0.95 



0.90 



B 



y=0.929 + 001x 

 r=0.46 



25 



35 



45 



55 



— I — 

 65 



Mean weight of spawning females (l<g) 



Figure 5 



(A) The relationship between mean egg diameter and mean daily water 

 temperature, and (B) the relationship between mean egg diameter 

 and mean weight of spawning females of yellowfin tuna (Thunnus 

 albacares) in captivity. 



Spawning in the reserve tank 



The fish in the smaller reserve tank began spawning 

 in mid-April 1997. At that time there were four females 

 and four males in the tank. When spawning first com- 

 menced, the females ranged from approximately 7.0 to 

 10.5 kg (average 9 kg) and 70 to 74 cm FL (average 72 

 cm), while the males ranged from 5.5 to 9.0 kg (average 

 7.5 kg) and 62 to 74 cm FL (average 66 cm). We did not 

 determine how many fish were involved in the initial 

 spawning, but it appeared from courtship behavior that 

 only a few of the larger individuals were participating 

 in spawning during the first 1 to 2 months. Spawning 

 continued into October 1997, when we sacrificed the 

 one remaining pair of fish. At the time of 

 death, the female was 16 kg and 94 cm 

 FL and the male was 12 kg and 79 cm 

 FL. We did not monitor the numbers or 

 characteristics of eggs or larvae in the 

 reserve tank. Spawning in the reserve 

 tank occurred over a water temperature 

 range of 24.4° to 29.2°C. 



Discussion 



The spawning by the yellowfin tuna 

 broodstock at the Achotines Laboratory, 

 beginning in 1996, represents the first 

 occurrence worldwide of sustained spawn- 

 ing by yellowfin tuna in landbased facili- 

 ties. Over 3.5 years, the broodstock fish 

 spawned in our large broodstock tank 

 (1362 m-^) at near-daily frequencies over 

 extended time periods. In general, the fish 

 spawned as long as they received adequate 

 daily rations of food and the water tem- 

 perature was 24°C or higher. The reserve 

 group also spawned for a 6-month period 

 in a tank of reduced volume (270 m'^). 



Courtship and spawning behaviors 



No courtship behavior or spawning aggre- 

 gation has ever been observed in tunas 

 in nature. The sustained spawning by 

 the yellowfin tuna broodstock in this 

 study allowed us to observe and analyze 

 reproductive behavior that has not been 

 described previously. The courtship and 

 spawning behavior of the captive yellowfin 

 tuna was ritualized and consistent among 

 three groups of broodstock fish over almost 

 four years. The courtship behavior (chas- 

 ing by males, paired swimming) always 

 occurred after the initial formation of a 

 central aggregation of participating fish. 

 During the actual spawning events, males 

 were not monogamous to single spawn- 

 ing groups and would often move from 



30 



'"O 



