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Fishery Bulletin 105(2) 



Bight, and they suggested that spawning adult tunas 

 may increase batch fecundities in response to greater 

 forage. 



Yellowfin tuna are multiple-spawning fish, and 

 their fecundity is not fixed at the beginning of any 

 spawning period (Schaefer, 2001a). Batch fecundity 

 of yellowfin tuna can vary annually or geographically 

 (Schaefer, 1998), although information on daily pat- 

 terns in batch fecundity of tunas is lacking. Wootton 

 (1979) reported one of the few studies to demonstrate 

 a positive relationship between batch fecundity and 

 daily food ration in nonscombrid species. Our results 

 with captive yellowfin tuna agree with Wootton's data 

 and indicate that over short time periods yellowfin 

 tuna can boost egg production in response to increased 

 food abundance. 



Size and age at first spawning and sex ratios 



Our estimates of the range of sizes at first spawning 

 for our female yellowfin tuna in 1999-2000 (12 to 28 

 kg, 75 to 112 cm FL) (Niwa et al., 2003) are not directly 

 comparable to estimates from some studies of wild yel- 

 lowfin tuna because of slight differences in the repro- 

 ductive conditions measured. We estimated size at first 

 spawning, whereas estimates from studies of wild fish 

 have included estimates of size at maturity and size of 

 reproductively active fish. Size at maturity, although 

 related to size at first-spawning, is a more conservative 

 estimate than size at first-spawning and does not indi- 

 cate true reproductive activity. Schaefer (1998) reported 

 that the minimum length of reproductively-active wild 

 fish observed in the eastern Pacific Ocean was 60 cm 

 FL. Fifty percent of the fish in that study were mature 

 at 92 cm FL, and the portion spawning per day at that 

 size was 0.61. McPherson (1991) estimated that the 

 mean length at maturity of yellowfin tuna in the west- 

 ern Pacific Ocean was 108 cm FL. In general, the sizes 

 of reproductively active fish reported for wild yellowfin 

 tuna are comparable to our estimates of size at first 

 spawning. Also, Schaefer 's (1998) estimate of 92 cm FL 

 as the length at 50% maturity for wild fish is similar to 

 the average length of our broodstock fish in mid-1997, 

 when we estimated that most of our fish appeared to 

 be spawning. 



It appears that the majority of our broodstock began 

 spawning at slightly earlier ages than did the wild yel- 

 lowfin tuna. Schaefer (1998) estimated the age at first 

 maturity of yellowfin tuna in the eastern Pacific Ocean 

 to be approximately 2 years. Our estimates of age at 

 first spawning for the original broodstock fish ranged 

 from 1.3 to 2.0 years, and averaged 1.6 years, and the 

 estimated age at first spawning for the 1999-2000 fe- 

 males ranged from 1.6 to 2.8 years and averaged 2.0 

 years. However, the majority of these females were 

 estimated to be slightly younger than 2 years. The 

 more precocious spawning by most of the captive fish 

 was most likely due to greater food rations and higher 

 growth rates compared to those of wild fish during the 

 first 1-2 years in captivity. Our results indicate that 



fish size, rather than age, is the best predictor of repro- 

 ductive status of yellowfin tuna. 



The sex ratio of the original group of yellowfin tuna 

 in this study was initially 1.2:1.0 (female:male), and 

 females remained slightly more abundant in the spawn- 

 ing group over 3.5 years. A dominance by males in 

 larger length classes has been reported for wild yel- 

 lowfin tuna, and has been attributed to potential differ- 

 ences in natural mortality rates between the sexes (Su- 

 zuki, 1994; Wild, 1994). We saw no evidence of greater 

 mortality in larger females in our broodstock group. 

 However, mortality rates in captivity are expected to 

 be less than in nature because food is not limiting and 

 predators are absent. 



Egg and larval development 



Yellowfin tuna eggs and newly hatched larvae are mor- 

 phologically typical of marine pelagic fishes. Fertilized 

 yellowfin tuna eggs average about 1 mm in diameter, 

 and the larvae hatch at a relatively small size (ca. 2.5 

 mm SL). The weight data that we present in this study 

 are either the first (yolksac larvae, first-feeding larvae) 

 or second (eggs) published estimates for tuna. Yellow- 

 fin eggs weigh about 43 ,((g dry, and weight loss occurs 

 during the embryonic phase, and larvae lose about 33% 

 of the original total weight at hatching and almost 50% 

 of the original weight at first-feeding. The weight loss 

 is due to utilization of yolk and oil during the egg and 

 yolksac stages. 



Scombrid early life history is characterized by high 

 mortality rates, high metabolic rates, and exponential 

 growth (Davis et al., 1991; Margulies, 1993; Wexler et 

 al., 2001). Yellowfin tuna larvae at first feeding are in- 

 termediate in size (3.3 mm SL, 22 jig in this study) com- 

 pared to other scombrid larvae (Tanaka et al., 1996), 

 and they exhibit a large scope for growth as juveniles 

 (Kaji et al., 1999; Wexler et al., 2007). Given the low 

 initial weights of yellowfin eggs and yolksac larvae and 

 the high growth rates of larvae and early-juveniles, the 

 potential for weight gain from the egg stage to the stage 

 at first-recruitment for yellowfin (30 cm FL, 6 months 

 of age, lATTC'M is very high and approaches a gain in 

 weight from xlO" to xlO'. 



Water temperature influences almost every aspect 

 of the egg and yolksac larval stages of yellowfin tuna. 

 Water temperature is inversely related (significantly) 

 to egg size, egg-stage duration, larval size at hatching, 

 and yolksac larval duration. Inverse developmental 

 relationships between water temperature and the sizes 

 and durations of egg and yolksac stages are common 

 in marine fishes (Blaxter, 1969) and have also been 

 reported for cultured bluefin tuna (Miyashita et al., 

 2000a, 2000b). These inverse relationships are most 

 likely the result of slower responses of ontogenetic pro- 



'' lATTC (Inter-American Tropical Tuna Commission). 2000. 

 Annual report of the Inter-American Tropical Tuna Com- 

 mission, 1998, 357 p. 8604 La Jolla Shores Drive, La JoUa, 

 CA 92037. 



