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



259 



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one spawning female to another. Spawning 

 behavior (swimming in tightening circles 

 with females in the lead position) ensured 

 that eggs and milt would be well mixed. 

 Most spawning groups moved over a range 

 of depths in the broodstock tank, so it is 

 likely in nature that yellowfin tuna in 

 spawning groups move vertically to some 

 degree during release of gametes. 



The aggregations and courtship behav- 

 ior usually occurred unabated for several 

 hours before spawning and represented 

 a significant energetic investment by the 

 broodstock fish. The daily energetic cost 

 of maturing a batch of eggs for a single 

 spawning by yellowfin tuna sampled at 

 sea was estimated at 1.06% of body weight 

 by Schaefer (1996), but that estimate did 

 not include the energetic costs of courtship 

 or spawning behavior. If the behavior of 

 our captive spawning group is representa- 

 tive of yellowfin tuna behavior in nature, 

 then we believe that yellowfin tuna in the 

 wild probably form large spawning aggre- 

 gations and individual fish may invest 

 considerable time (1 to 4 h) and energy 

 (costs unknown) on daily courtship and 

 spawning activities. 



The behavior of the yellowfin tuna 

 broodstock is predictable on a daily basis, 

 which would indicate that a synchroniza- 

 tion mechanism is inherent to the behav- 

 ior. Most of the courtship and spawning 

 behavior of the fish appeared to be driven 

 by female behavior (i.e., females led, males 

 followed). In teleost fishes, the release of 

 eggs and milt during spawning is synchro- 

 nized by the release of pheromones, pre- 

 dominantly by females (Liley et al., 1991). 

 Sex hormones and hormone metabolites 

 are water soluble and indicate the sender's 

 reproductive status. Pheromones studied to 

 date appear to be of two types, steroids and 

 prostaglandins, and they are released by 

 females in concentrated urine trails (Sta- 

 cey, 1984, 1991). Female yellowfin tuna in our spawn- 

 ing group often released discharge trails during the 

 courtship process just before spawning, and at times 

 we misinterpreted these trails as egg emissions. Given 

 the ritualized nature of the courtship and spawning 

 behavior of both sexes in our broodstock, most of the 

 discharge trails during late-courtship may have been 

 pheromone releases by the females, although future 

 confirmation with biochemical or immunoassay analyses 

 would be required. 



Diel patterns of spaw/ning and egg hatching 



The diel timing of spawning by the broodstock yellowfin 

 tuna was finely tuned to water temperature and, com- 



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2.3 2.4 2.5 2.6 2.7 2.8 



Mean larval length at hatch (mm-NL) 



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29 

 2.8 

 2.7 

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 2.5 

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Mean incubation temperature (°C) 



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Figure 6 



(A) The relationship between mean larva! length at hatching and 

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

 length and mean incubation temperature for yellowfin tuna [Thunnus 

 albacares) in captivity. 



bined with the strong inverse relationship between water 

 temperature and egg duration, resulted in a predictable 

 and narrow range in the time of hatching. The majority 

 of spawning occurred after sunset and at higher (gener- 

 ally >27°C) water temperatures. However, when water 

 temperatures decreased, the broodstock fish spawned 

 earlier in the day. This temporal shift resulted in nearly 

 all hatching occurring in the late afternoon or early 

 evening. We believe that this pattern for the diel timing 

 of spawning and hatching is important and has adaptive 

 significance for the early life history of yellowfin tuna. 

 In nature, the maintenance of a conservative time for 

 egg hatching would ensure that newly hatched yellowfin 

 larvae spend the first 12 to 15 h after hatching in dim- 

 ming light or darkness. Because yolksac larvae of marine 



