RAPID AND SPONTANEOUS MATURATION, 



OVULATION, AND SPAWNING OF OVA BY 



NEWLY CAPTURED SKIPJACK TUNA, 



KATSUWONUS PELAMIS 



This study was designed to test a hypothesis, for- 

 mulated on the basis of preliminary observa- 

 tions, that skipjack tuna, Katsuwonus pelamis, 

 captured in Hawaiian waters during their breed- 

 ing season and maintained alive would ovulate 

 spontaneously within a few hours after capture. 

 If such did occur, and on a consistent and predict- 

 able basis, this would be of practical value in 

 attempts to spawn these fish in captivity. 



Methods 



These investigations took place at the Kewalo 

 Research Facility of the National Marine Fish- 

 eries Service Honolulu Laboratory. Six deliver- 

 ies of live skipjack tuna were received from two 

 commercial fishing vessels during June and July 

 1980, within the normal spawning season of the 

 species in Hawaiian waters (Brock 1954; Matsu- 

 moto 1966). The fish had been caught by stan- 

 dard pole-and-line methods and transported to 

 the receiving dock of the laboratory in baitwells. 

 Upon delivery they were transferred to circular 

 tanks, 7.3 m diameter by 1.1 m deep, provided 

 with continuous flow of seawater. Time of cap- 

 ture for all groups was between 1500 and 1700; 

 time elapsed between capture and delivery to the 

 laboratory ranged from 3.5 to 8 h, with a mean of 

 5.5 h. Sea temperatures at the capture sites were 

 not measured, but were probably between 25° 

 and 30°C. Water temperatures in the holding 

 tanks were about 25° to 26°C. With all except the 

 first group, a siphon and straining net were used 

 to sample water continuously from the holding 

 tanks to detect the release of their slightly buoy- 

 ant, pelagic ova. For the last four of the six 

 groups, we arranged also to receive specimens 

 fished from the same school but refrigerated on 

 ice immediately after capture. All the specimens 

 were between 40 and 50 cm in fork length (FL) 

 and 1.4 to 2.2 kg; tunas larger than this are diffi- 

 cult to keep alive in the baitwells of these vessels 

 (about 145 by 165 by 130 cm deep on the vessel 

 which delivered five of the six groups). Skipjack 

 tuna of this size are between 1 and 2 yr old and 

 are probably in their first spawning season 

 (Brock 1954; Yoshida 1971). 



We determined gonadal maturation states of 

 specimens at various specified times following 



their capture, either through biopsies on live spe- 

 cimens or through postmortem dissections. 

 Unless a specimen is already running ripe, 

 neither its sex nor gonadal maturity can be de- 

 termined through external appearances. Biop- 

 sies involved extraction of gonadal tissue by 

 catheterization through the urogenital pore of 

 restrained, unanesthetized fish. Ova were teased 

 free from unpreserved, fresh or refrigerated 

 ovarian tissue, immersed in a 0.9% saline solu- 

 tion, and the diameters of 25 from each of the 

 largest and second largest developing groups 

 were measured with an ocular micrometer. Also, 

 since we were interested primarily in the occur- 

 rence and progress of ovulation, we classified 

 females into the following four categories: Un- 

 ovulated — ripe ova not present in ovarian lumen, 

 developing ova enclosed within follicles; ovulat- 

 ing — some ripe ova present in ovarian lumen but 

 not easily stripped from females, follicles contain 

 large, preovulatory ova 0.80 to 1.0 mm in diame- 

 ter; ripe— ovarian lumen filled with large quan- 

 tities of ova which can be easily stripped from 

 females; spent — few residual ova present in ovar- 

 ian lumen, follicles with relatively small ova of 

 <0.5 mm diameter. 



Results 



Responses of each sex remained constant 

 among the six groups. Testes of males sacrificed 

 after 7 to 8.5 h appeared identical to those sacri- 

 ficed and refrigerated on capture. All males had 

 testes that were mature, white, and firm and had 

 thick, viscous milt in the sperm ducts. None 

 yielded milt when moderate stripping pressure 

 was applied. To fertilize ova stripped from fe- 

 males, we had to squeeze milt directly from 

 testes dissected from sacrificed males. 



Observations on all six groups of female skip- 

 jack tuna received from 8 June to 31 July are 

 summarized in Table 1. None of the 16 specimens 

 killed and refrigerated on capture was in an ovu- 

 latory state. The maturing ova in the largest 

 modal group averaged 0.59 to 0.64 mm in 14 of 

 these females and 0.74 mm in another, while the 

 remaining individual had relatively immature 

 ovaries (Table 2). Nine females which died in 

 transit to the laboratory were placed in refriger- 

 ation. Times of death had not been recorded by 

 the fishing crews, but were <5 h after capture in 

 all cases. None of these females had yet ovulated, 

 and the ova in their largest developing modal 

 groups averaged from 0.60 to 0.93 mm in diame- 



FISHERY BULLETIN: VOL. 80. NO. 2. 1982. 



393 



