Abascal et al.: Testicular development in Thunnus thynnus 



415 



esis in fully mature bluefin tuna. The different testicular 

 development of maturing and spawning tuna is reflected 

 by their respective average I G , which was fourfold higher 

 in spawning fish. An equivalent gonad growth was found 

 in the females collected in the same samplings (Medina 

 et al., 2002), indicating a spatiotemporal parallelism in 

 the gonad maturation cycle and a good synchronization 

 of the reproductive peak in the two sexes. The matura- 

 tion schedule differs between the two sexes, however, in 

 that males are capable of generating mature spermatozoa 

 while still on migration, whereas females do not appear 

 to develop fully mature oocytes until they have reached 

 the spawning grounds (Medina et al., 2002). Therefore, 

 even though mature spermatozoa can be found in tes- 

 ticular ducts during prolonged periods throughout the 

 reproductive cycle, it is unlikely that males are actually 

 capable of spawning out of reproductive season. 



The seasonal I G profile of the bluefin tuna appears 

 to be similar to that of the pelagic, highly migratory 

 perciform Rachycentron canadum (Brown-Peterson et 

 al., 2002), and the swamp eel (Synbranchus marmora- 

 tus) (Lo Nostro et al., 2003), in which peak I c; values 

 occur when the reproductive activity is at a maximum. 

 A different situation has been reported in the common 

 snook (Taylor et al., 1998), where the highest I G levels 

 correspond with the mid maturation class and decrease 

 during the latter part of the reproductive season. The 

 biological significance of these different I G profiles in 

 terms of reproductive strategies is yet unknown because 

 a very limited number of species have been examined 

 so far. 



Because spermatozoa are by far the most abundant 

 cells in mature testes, the gonad weight becomes a 

 good indicator of the quantity of sperm produced by a 

 fish (Billard, 1986). Therefore, the significant increase 

 in I G that occurred between samplings off Barbate and 

 the Balearic Islands would indicate that, during migra- 

 tion, bluefin tuna can raise several times the volume of 

 sperm accumulated in the testes. The apparently high 

 spermatogenetic activity observed in bluefin tuna caught 

 on the spawning grounds suggests that bluefin tuna 

 have the ability to regenerate testicular sperm stores. 

 Continuous sperm production could be important be- 

 cause external fertilization requires the release of large 

 amounts of sperm to ensure successful fertilization of 

 eggs, especially when egg size is small. In addition, it 

 should be noted that tunas spawn multiple times (June, 

 1953; Yuen, 1955; Buriag, 1956; Otsu and Uchida, 1959; 

 Baglin, 1982; Stequert and Ramcharrun. 1995) and 

 can spawn almost daily throughout the reproductive 

 season (Hunter et al.. 1986; McPherson, 1991; Schaefer, 

 1996, 1998, 2001; Farley and Davis, 1998; Medina et 

 al., 2002). 



From histological examination of the sperm ducts, 

 and based on the amount of sperm present and the 

 staining of the epithelium, Schaefer (1998) proposed a 

 spawning interval of 1.03 days for spawning male Thun- 

 nus albacares throughout the eastern Pacific Ocean. 

 The spawning rate estimated for reproductively active 

 females with the postovulatory-follicle method was 1.19 



days (Schaefer, 1998), which coincides with the spawn- 

 ing interval estimated for female T. thynnus around the 

 Balearic Islands (Medina et al., 2002). Unfortunately, 

 we could not make a reliable estimation of the male 

 spawning interval in our samples. Two possible reasons 

 may account for this failure. One reason is that many 

 of the samples of gonadal tissue did not include the 

 main sperm duct. On the other hand, no clear evidence 

 of spawning was identified by histological examination 

 of those specimens processed that had sperm ducts. A 

 plausible explanation for this fact is that recent sperm 

 release can be detected only within 12 hours after the 

 spawning event (Schaefer, 1996); hence for male spawn- 

 ing to be detected the fish would have to be sampled 

 in a narrow range of times following spawning, which 

 Schaefer (1996) established between 00.01 and 12.00 

 hours after spawning for Thunnus albacares. It would 

 be worth conducting further research on bluefin tuna at 

 their spawning grounds, by attempting to cover a broad 

 range of sampling times in order to ensure collection of 

 specimens shortly after gamete release. In this way, use- 

 ful information would be obtained on such reproductive 

 parameters as spawning schedules, fecundity, and the 

 energy cost of spawning, which are essential for ecologi- 

 cal assessments of the reproductive potential. 



It is noteworthy that male tuna, as small as 20 kg 

 in weight (-100 cm L F ), were caught on the spawning 

 grounds in our study. They had gonad indices over 5% 

 and histological features indicative of full maturity. 

 These observations indicate that the eastern stock of 

 Atlantic northern bluefin tuna can reach maturity at 

 age 3 years and thus support conclusions of previous 

 studies (Rodriguez-Roda, 1967; Hattour and Macias, 

 2002; Susca et al., 2001a, 2001b; Medina et al., 2002); 

 western bluefin tuna, on the other hand, mature at an 

 older age, which has been estimated at 6 years (Baglin, 

 1982). 



Prior to sexual maturation, marine fish generally 

 accumulate large lipid deposits, primarily triacylgly- 

 cerols, which are subsequently mobilized to support 

 gonad development and spawning migration (Bell, 1998). 

 The major lipid storage sites are the mesenteric tissue, 

 muscle, liver, and subdermal fat layers (Ackman, 1980). 

 In bluefin tuna the liver does not appear to play an 

 important role in lipid storage but is mainly involved 

 in processing fatty acids mobilized from other bodily 

 sources (Mourente et al„ 2002). This metabolic pattern 

 is consistent with our observations of weight modifica- 

 tions for liver and fat body from maturation through the 

 spawning period. Although I L increases only slightly 

 with sexual maturation, I F undergoes a marked decrease 

 at the time of maximum gonad development. Thus, the 

 regression analysis of the relationship between I G and I F 

 shows a significant negative correlation, which reveals a 

 depletion of mesenteric fat stores as the testes grow. The 

 occurrence of a similar situation in females (Medina et 

 al., 2002; Mourente et al., 2002) and in male and female 

 Thunnus alalunga (Ratty et al., 1990) has led to the con- 

 clusion that fat-body lipid reserves provide an important 

 energy source for gametogenesis in tunas. 



