Margulles et al. Spawning and early development of captive Thunnus albacares 



261 



did not exhibit any patterns of spawning occurrence 

 that were related to food abundance in captivity (but 

 see the following subsection on "Egg production and 

 daily rations"). 



Our focus on the processes controlling maturation 

 and spawning of captive yellowfin tuna has centered on 

 the influences of water temperature, photoperiod, and 

 lunar cycle. Water temperature appeared to provide the 

 main exogenous control over the occurrence and timing 

 of spawning of captive yellowfin tuna. The fish spawned 

 over a range of daily mean temperatures from 23.3° to 

 29.7°C. Spawning occurred at daily mean temperatures 

 <24°C on only two out of 963 dates (single spawning 

 events at 23.3° and 23.9°C), and spawning became in- 

 termittent or ceased within 24 hours after those two 

 dates. This thermal range for spawning of yellowfin 

 tuna is similar to that reported from collections at sea 

 of reproductively active adults (Schaefer, 1998) and 

 early-stage larvae (Nishikawa et al., 1985; Boehlert 

 and Mundy, 1994; Lauth and Olson, 1996). In general, 

 tunas spawn at water temperatures 224°C (CoUette 

 and Nauen, 1983), although larvae of bullet or frigate 

 tunas (or larvae of both) (Auxis spp.) and skipjack tuna 

 have been collected at sea at temperatures near 22°C 

 (Richards and Simmons, 1971; Boehlert and Mundy, 

 1994). In their study of spawning of captive bluefin tuna 

 in Japan, Miyashita et al. (2000a) reported spawning 

 temperatures from 21.6° to 29.2°C. 



This study is the first to investigate the relation- 

 ship on a daily basis between water temperature and 

 spawning by yellowfin tuna over a protracted period. 

 Our results indicate that the sensory ability of yellow- 

 fin tuna to detect ambient water temperature and the 

 associated feedback mechanisms involved in neuroen- 

 docrine control of ovulation and spawning behavior are 

 rapid and characterized by subdaily response times. 

 Spawning usually ceased within one day after water 

 temperatures decreased by only 0.1 to 0.2°C, and usu- 

 ally recommenced in less than one day after similar 

 minute changes in water temperature. The broodstock 

 altered the time of day of spawning predictably in rela- 

 tion to water temperature, but this behavioral change 

 in the time of day of spawning occurred only after sev- 

 eral days of exposure to changes in water temperature. 

 Maturation and spawning in female fishes, in particular 

 the initiation of vitellogenesis, oogenesis, and ovula- 

 tion, is under hormonal control (Goetz, 1983). Sensory 

 inputs to neuroendocrine control of reproduction include 

 information on external water temperature transmitted 

 via temperature-sensitive afferent nerve fibers (Van der 

 Kraak et al., 1998). The spawning periodicity of the 

 broodstock and their ability to adjust the time of day 

 of spawning in response to water temperature indicate 

 that yellowfin tuna have the ability to rapidly integrate 

 sensory information on water temperature and, often 

 within hours, adjust the hormonal control of final matu- 

 ration processes and spawning. 



Our analysis of spawning by the yellowfin tuna brood- 

 stock did not indicate a strong influence of lunar cycles 

 on the timing of spawning or egg production within 



monthly periods. We originally anticipated some lunar 

 periodicity to spawning because many tropical marine 

 species exhibit lunar spawning rhythms that usually 

 peak around the new moon or full moon (spring tides) 

 (Johannes, 1978; Bye, 1984). Peak spawning on new or 

 full moons in the tropics may be an adaptation to maxi- 

 mize offshore transport of eggs and larvae by spring 

 tides away from increased predation pressure in coastal 

 habitats (Johannes, 1978). Our captive yellowfin tuna, 

 however, were exposed only to lunar cycles in captivity, 

 but not to tidal influences. The mean daily egg produc- 

 tion from individual spawnings in our study was high- 

 est during phases of greater lunar illumination. This 

 trend in egg production could be viewed as evidence 

 that yellowfin tuna increase their spawning efforts 

 just prior to full moons, but the trend from our data is 

 not definitive. Although adult yellowfin tuna in nature 

 sometimes aggregate around islands and seamounts 

 (Boehlert and Mundy, 1994), spawning by yellowfin tu- 

 na is widespread throughout the tropical and subtropi- 

 cal oceans (Nishikawa et al., 1985). Because yellowfin 

 tuna often spawn in pelagic open-ocean habitats, they 

 may not require the same level of synchronization with 

 lunar cycles to aid in egg and larval dispersal as that 

 required by coastal tropical species. 



Photoperiod is an important environmental cue for 

 spawning in temperate fish species (Lam, 1983). Howev- 

 er, in the tropics, photoperiod hardly varies throughout 

 the year and is usually not a major factor in the control 

 of maturation and spawning of tropical fishes (Lam, 

 1983). Photoperiod changed very little during our study, 

 and the yellowfin tuna broodstock showed no detectable 

 responses to slight changes in day length. 



Egg production and daily rations 



Yellowfin tuna in this study exhibited the ability to 

 boost egg production in response to periodic increases 

 in daily food rations. Egg production peaked over peri- 

 ods of 1 to 3 weeks (average 12 days) after food rations 

 were increased. These increases in daily egg production 

 sometimes exceeded 200%, providing strong evidence 

 that yellowfin can convert peaks in exogenous energy 

 consumption into higher egg production in a matter of 

 days or weeks. The adaptive significance of this repro- 

 ductive pattern is obvious. The ability to increase egg 

 production in response to greater food abundance in 

 oceanic habitats would provide yellowfin tuna with the 

 opportunity to exploit patchy food resources and periodic 

 increased production that can occur in the vicinity of 

 islands, seamounts, and in coastal or equatorial upwell- 

 ing zones. Results from several field studies, where the 

 abundance of tuna larvae was examined, support our 

 laboratory results. Boehlert and Mundy (1994) suggested 

 that increased abundance of Thunnus spp. larvae on the 

 leeward side of Oahu, Hawaii, may be related to greater 

 forage for adult tunas in that region. Lauth and Olson 

 (1996) reported peak abundance of Euthynnus lineatus 

 and Auxis spp. larvae during periods of peak seasonal 

 upwelling and secondary production in the Panama 



