Margulies et al Spawning and early development of captive Thunnus albacares 



257 



where Y = egg stage duration (h), 

 D - mean egg diameter, and 

 T = mean incubation temperature (°C). 



The diameter of individual fertilized eggs ranged 

 from 0.85 to 1.13 mm and averaged 0.97 mm (Table 1). 

 Each egg contained a single oil globule that averaged 

 0.22 mm in diameter. The dry weight of individual 

 eggs ranged from 34 to 60 ,ug and averaged 43 ug 

 (Table 1). Mean daily egg diameter was inversely re- 

 lated to water temperature during the period when the 

 original broodstock (stocked in 1996) were spawning 

 (Fig. 5A; df=892, P<0.001), although there was consid- 

 erable scatter about the regression (r-=0.23). Mean egg 

 diameter was significantly correlated (r=0.45, df=553, 

 P<0.001) with the mean weight of spawning females 

 (all females >20 kg) in the original broodstock group 

 (Fig. 5B). 



Larval size at hatching and hatching success 



At hatching, the larvae averaged 2.5 mm NL (range 

 2.0-2.9 mm NL; SD = 0.16) (Table 1). Larvae at hatching 

 had unpigmented eyes, no alimentary tract nor mouth, 

 and exhibited a large, elliptical yolk mass containing a 

 single posterior oil globule. The dry weight of individual 

 larvae at hatching ranged from 25 to 41 pg, averaging 

 30 ^g (SD = 4) (Table 1). Mean larval length at hatching 

 was positively correlated (r=0.46, df=498, P<0.001) with 



mean egg diameter (Fig. 6A), and negatively correlated 

 (r=-0.27, df=774, P<0.05) with mean incubation tem- 

 perature (Fig. 6B). 



The percentage hatching of eggs spawned in cap- 

 tivity ranged from 9.5<7c to 99.0% and averaged 83% 

 (SD=14.7%). In general, the hatching success was high, 

 with over 84% of the hatchings occurring at rates >70%. 

 The percentage hatching showed no relationship with 

 either water temperature or larval length. 



Egg and early larval development 



Yellowfin tuna egg development was rapid. The develop- 

 mental duration from fertilization to hatching followed a 

 curvilinear pattern (Fig. 7), and the egg-stage duration 

 was temperature dependent (Fig. 3B). At a modal incu- 

 bation temperature of 27.0°C, egg-stage duration was 

 21.65 h (Fig. 7). At 27.0°C, cell cleavage occurred <1 h 

 after fertilization, early embryo gastrulation occurred 

 about 6 h after fertilization, the embryo first formed 

 after 8 h of development, and the embryo was in the 

 tail-free stage approximately 17 h after fertilization. The 

 specific gravity of fertilized eggs changed with develop- 

 ment. Upon fertilization, eggs were positively buoyant 

 and would rise to the surface neuston layer of the brood- 

 stock tank. Eggs remained positively buoyant into the 

 tail-free stage, but 2 to 4 h (dependent on temperature) 

 before hatching the specific gravity changed and the eggs 

 became negatively buoyant until they hatched. The nega- 



