407 



Abstract— Testis histological structure 

 was studied in bluefin tuna {.Thunnus 

 thynnus I from the eastern Atlantic and 

 Mediterranean during the reproductive 

 season (from late April to early Junei. 

 Testicular maturation was investi- 

 gated by comparing samples from 

 bluefin tuna caught on their eastward 

 reproductive migration off Barbate 

 i Strait of Gibraltar area) with samples 

 of bluefin tuna fished in spawning 

 grounds around the Balearic Islands. 

 Histological evaluations of cross sec- 

 tions showed that the testis consists of 

 two structurally different regions, an 

 outer proliferative region where germ 

 cells develop synchronously in cysts, 

 and a central region made up of a well- 

 developed system of ducts that convey 

 the spermatozoa produced in the prolif- 

 erative region to the main sperm duct. 

 Ultrastructural features of the differ- 

 ent stages of the male germ cell line are 

 very similar to those described in other 

 teleost species. The bluefin tuna testis 

 is of the unrestricted spermatogonial 

 testicular type, where primary sper- 

 matogonia are present all along the 

 germinative portion of the lobules. All 

 stages of spermatogenesis were pres- 

 ent in the gonad tissue of migrant and 

 spawning bluefin tuna, although sper- 

 matids were more abundant in spawn- 

 ing fish. The testis size was found to 

 increase by a factor of four ( on average ) 

 during migration to the Mediterranean 

 spawning grounds, whereas the fat 

 bodies (mesenteric lipid stores associ- 

 ated with the gonads) became reduced 

 to half their weight, and the liver mass 

 did not change significantly with sexual 

 maturation. Linear regression analy- 

 sis of the pooled data of migrant and 

 spawning bluefin tuna revealed a sig- 

 nificant negative correlation between 

 the gonad index (I G ) and the fat tissue 

 index (I F ), and a weaker positive cor- 

 relation between the gonad index (I G ) 

 and the liver index (I L ). Our analyses 

 indicate that the liver does not play a 

 significant role in the storage of lipids 

 and that mesenteric lipid reserves con- 

 stitute an important energy source for 

 gametogenesis in bluefin tuna. 



Testicular development In migrant and spawning 

 bluefin tuna {Thunnus thynnus (L.)) from 

 the eastern Atlantic and Mediterranean 



Francisco J. Abascal 



Cesar Megina 



Antonio Medina 



Departamento de Biologia 



Facultad de Ciencias del Mar y Ambientales 



Universidad de Cadiz 



Av Republica Saharaui 



11510 Puerto Real 



Cadiz, Spain 



E-mail address (for A Medina, contact author): antonio.medina@uca.es 



Manuscript submitted 27 April 2003 

 to Scientific Editor's Office. 



Manuscript approved for publication 

 25 March 2004 by the Scientific Editor. 

 Fish. Bull. 102:407-417 (2004). 



The Atlantic northern bluefin tuna 

 (Thunnus thynnus thynnus (L.)), is 

 one of the most commercially valu- 

 able wild animals in the world. In the 

 last two decades this species has been 

 subject to intense over-fishing, which 

 has caused a decline in both the east- 

 ern and western populations because 

 of lowered recruitment (Mather et 

 al., 1995; Sissenwine et al., 1998). 

 The bluefin tunas (7! thynnus and T. 

 maccoyii) are unique among tuna spe- 

 cies in that they live mainly in cold 

 waters and move into warmer waters 

 to spawn (Olson, 1980; Lee, 1998; 

 Schaefer, 2001); therefore the migra- 

 tory pattern of these species depends 

 substantially on reproduction. The 

 eastern stock of Atlantic bluefin tuna 

 spawns from June through August in 

 the Mediterranean Sea. where natural 

 conditions are apparently optimal for 

 the survival of offspring. From late 

 April to mid June, bluefin tuna breed- 

 ing stocks migrate from the North 

 Atlantic to spawning grounds in the 

 Mediterranean (Mather et al., 1995; 

 Ravier and Fromentin, 2001). A good 

 understanding of the reproductive 

 parameters (especially sexual matu- 

 ration, fecundity, and spawning) of 

 tunas is of paramount importance for 

 population dynamics studies and the 

 management of fisheries that target 

 tunas. Nevertheless, "a very limited 

 amount of scientifically useful infor- 



mation is available on the reproduc- 

 tive biology for most tunas" ( Schaefer, 

 2001). Recent work has increased our 

 knowledge on the reproductive biol- 

 ogy of female Thunnus thynnus in 

 the eastern Atlantic and the Medi- 

 terranean (Susca et al„ 2000, 2001a, 

 2001b; Hattour and Macias, 2002; 

 Medina et al., 2002; Mourente et al., 

 2002), but many questions remain 

 still to be answered regarding male 

 reproductive activity in this and other 

 tuna species. 



Histological examination of gonads 

 is a useful tool for assessing the ma- 

 turity state offish. However, very few 

 light-microscopy studies have been 

 published on bluefin tuna and no ul- 

 trastructural studies of reproductive 

 organs are yet available. The male 

 reproductive cycle of T thynnus has 

 been characterized histologically by 

 Santamaria et al. (2003), and Ratty 

 et al. ( 1990 ) and Schaefer ( 1996, 1998 ) 

 have reported valuable histological 

 descriptions on male and female go- 

 nads of the Pacific albacore (Thun- 

 nus alalunga) and the yellowfin tuna 

 (Thunnus albacares), respectively. In 

 this article we report biometric and 

 histological data on male T. thynnus 

 caught during their reproductive mi- 

 gration and spawning period in order 

 to provide further information on the 

 biological aspects of reproduction for 

 this species. 



