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Fishery Bulletin 105(2) 



fishes are vulnerable to predation by 

 planktivorous fishes and invertebrates 

 (Bailey and Houde, 1989), minimizing 

 the amount of time spent in daylight 

 appears to be a mechanism for maxi- 

 mizing survival for yolksac larvae. 



This temporal pattern of hatching 

 would also minimize the exposure 

 of yellowfin tuna yolksac larvae to 

 the ultraviolet effects of sunlight. 

 Although the vertical distribution of 

 yellowfin tuna yolksac larvae in the 

 ocean has not been described, feed- 

 ing-stage larvae of yellowfin tuna are 

 found predominantly in the upper 

 mixed layer (Leis et al., 1991; Boe- 

 hlert and Mundy, 1994). The deleteri- 

 ous effects (mostly genetic damage) of 

 ultraviolet radiation are strongest in 

 the upper mixed layer and may affect 

 yolksac larvae more than other early 

 life stages of marine fishes ( Vetter et 

 al., 1999). Thus, any adaptation that 

 reduces the amount of time yellowfin 

 tuna yolksac larvae spend in daylight 

 increases their chances of survival. 



Most tunas are reported to be noc- 

 turnal spawners, according to histo- 

 logical analyses of the ovaries of adult fishes caught at 

 sea (McPherson, 1991; Nikaido et al., 1991; Schaefer, 

 1998, 2001a). Our experimental results indicate that 

 the view that yellowfin tuna spawn exclusively at night 

 should be reconsidered. The assumption of nocturnal 

 spawning by tunas in past studies has been based on 

 histological analyses of oocyte development and new 

 postovulatory follicles in the ovaries of adult fishes in 

 the wild (collected predominantly at sea surface temper- 

 atures >27°C), but the actual water temperatures en- 

 countered by the fish before and during spawning were 

 not known. Our results indicate that yellowfin tuna 

 respond to variations in water temperature by altering 

 their time of spawning and that the diel changes in 

 spawning time are precise and predictable. The ability 

 to control the timing of spawning by adults may be me- 

 diated by the timing of the release of sex hormones and 

 maturation processes in response to water temperature. 

 It is possible that our results are simply an artifact 

 of captivity because our broodstock fish were confined 

 to a tank and not able to thermoregulate by changing 

 water depth or location as wild fish can. However, we 

 believe that our experimental evidence for the control of 

 the timing of spawning by yellowfin is compelling. The 

 adaptive significance of this spawning pattern is most 

 likely related to the ultimate effects of temperature 

 on development rates of eggs and yolksac larvae (i.e., 

 maintenance of a consistent time of hatching to maxi- 

 mize early larval survival). In the only other similar 

 study on tunas, Miyashita et al. (2000a) reported that 

 cultured bluefin tuna also spawned predominantly be- 

 fore sunset. Daily mean water temperatures in their 



Blastodisc Cleavage Morula Blastula Gastrula Early Tail-bud Tail-free Hatch 



embryo 



Egg stage 



Figure 7 



The development of yellowfin tuna iTIninnus albacares) eggs from fertiliza- 

 tion to hatching at 27°C. Plotted values are mean hours elapsed to reach 

 specific developmental stages. Error envelopes are ±2 SEs. 



4-yr study ranged from 21.6° to 29.2°C, and the mean 

 temperatures during the spawning season ranged from 

 23.6° to 27.0°C. 



Spawning cues 



Spawning substrate, food availability, water tempera- 

 ture, photoperiod, and lunar cycle can strongly influence 

 many functions of the teleost reproductive system (Bye, 

 1984; Stacey, 1984). We consider each of these potential 

 cues for reproduction in yellowfin tuna 



Because yellowfin tuna are pelagic spawners, they 

 have no specific substrate requirements. The yellowfin 

 tuna in our study received controlled daily rations to 

 fuel daily spawning costs (Wexler et al., 2003). Under 

 the nonlimiting food conditions that we provided, no 

 relationship was apparent between spawning occurrence 

 and food abundance. However, this pattern of spawn- 

 ing in captivity does not rule out the possibility that 

 the timing of yellowfin tuna reproduction in tropical 

 or subtropical waters could be influenced by seasonal 

 fluctuations in food abundance, such as those associated 

 with seasonal upwelling. Some species of tropical and 

 subtropical marine fishes exhibit reproductive cycles 

 that coincide with periods of upwelling or increased 

 productivity and food availability (Bye, 1984). Tunas 

 in tropical regions usually spawn year-round, but those 

 occurring in the subtropics exhibit seasonal spawning 

 patterns (Fritzsche, 1978; Schaefer, 1998), whereas 

 bluefin tuna and albacore migrate to warm waters to 

 spawn in distinct areas during restricted periods (Col- 

 lette and Nauen, 1983). Our yellowfin tuna broodstock 



