INDUCED SPAWNING OF A TUNA, 

 EUTHYNNUS AFFINIS 



Investigations into the biology of young tuna have 

 been hampered by the difficulty of capturing and 

 maintaining live larvae or early juveniles from the 

 wild. The production of young in captivity would 

 provide an obvious solution; however, the inherent 

 difficulties of maintaining and working with such 

 powerful, fast-swimming, pelagic fishes under 

 conditions of close confinement have discouraged 

 attempts to artificially stimulate their spawning. 

 The following describes what we believe to be the 

 first induced spawning of any tuna, accomplished 

 with specimens of Euthynnus affinis, one of the 

 smaller sized of the true tunas (taxonomic rela- 

 tions of tunas were recently reviewed by Collette 

 (1978)). These were held captive in tanks at the 

 Kewalo Research Facility of the Southwest 

 Fisheries Center, Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA. 



The specimens used in these spawning trials 

 were three females in captivity for 2-4 wk and 

 three males in captivity for about 4 mo. All were 

 captured by hook and line and transported to the 

 laboratory in the baitwells of either a commercial 

 fishing vessel or the NOAA Ship Townsend 

 Cromwell. They were maintained outdoors in cir- 

 cular, 7.3 m in diameter by 1.1 m deep tanks with 

 water flow of about 3.0-3.5 1/s and temperatures 

 ranging from 23.3° to 26.2° C. They were fed 

 thawed surf smelt, Hypomesus pretiosus, and 

 squid Loligo opalescens at a rate of about 15-20% 

 of their body weight per day. 



Sex and gonadal maturation of each specimen 



were determined by biopsy, with samples obtained 

 by catheterization through the urogenital aper- 

 ture (Shehadeh et al. 1973). While the fish were 

 physically restrained for the biopsies, each was 

 marked with an identifying numeral or letter with 

 silver nitrate (Thomas 1975). Males in advanced 

 stages of maturation could be recognized by the 

 presence of milt in the catheter, while females in 

 any stage of maturation could be identified from 

 the biopsied ovarian tissue. The diameters of 25 

 ova of the largest size class present in the 

 catheterized sample were measured with an ocu- 

 lar micrometer and the average diameter calcu- 

 lated as an index of ovarian maturation. 



Gonadal maturation of captive specimens was 

 monitored by monthly biopsies, starting in March 

 1979 with 37 specimens maintained since the pre- 

 ceding January and February. After March, newly 

 captured specimens were also biopsied on the day 

 of delivery to the laboratory. The most advanced 

 ovaries in our captive fish between March and 

 August 1979 were in three females delivered to the 

 laboratory on 6 June. Their largest ova averaged 

 0.54, 0.48, and 0.44 mm on arrival and 0.55, 0.53, 

 and 0.50 mm, respectively, when biopsied again 2 

 wk later on 19 June. The first two (females A and 

 B, Table 1) were used in the first spawning trial, 

 which started that same day, on 19 June. The third 

 female (female C, Table 1) was biopsied again after 

 an additional 2 wk, and its largest ova had in- 

 creased to an average diameter of 0.56 mm. It was 

 used in the second spawning trial, which started 

 that same day, on 2 July. Males from groups cap- 

 tured earlier in January and February were also 

 biopsied on the starting dates of the spawning 



Table l. — Spawning responses oiEuthynnus affinis to treatment with salmon pituitaries (SP), human chorionic gonadotropin (HCG), 

 and pregnant mare serum (PMS). Ova sizes represent mean diameters of 25 spawned ova after preservation in 3-5'^ Formalin, 'or the 

 largest size class in fresh biopsied samples. 



Trial 1 : 



Female A 53 



Female B 0.55 



Male A 

 MaleB 

 Trial 2: 

 FemaJe C 0.56 



MaleC 



0.03 



0.05 



0.03 



SP 5mg 

 HCG 100 lU 



SP 10mg 

 HCG 500 lU 



HCG 100 lU 

 HCG 500 lU 



SP lOmg 

 HCG 500 lU 



HCG 1 ,000 lU 



054 



0.57 



0.70 



0.04 



0.04 



0.04 



SP 5 mg 

 HCG 100 lU 

 PMS 1,000 lU 

 SP 10 mg 

 HCG 500 lU 

 PMS 2,000 lU 

 HCG 100 lU 

 HCG 500 lU 



SP 10 mg 

 HCG 1 ,000 lU 

 PMS 2,000 lU 

 HCG 1,000 lU 

 PMS 1 ,000 lU 



0.55 0.04 Did not spawn. 



0.77 0.02 Spawned ( >35, 000 released spontaneously; 



0.98 0.02 <10,000 stripped and fertilized). 



No hydration of milt. 

 Hydration of milt. 



0.78 0.03 Spawned (>185,000 released spontaneously; 



1,02 0.03 >40,000 stripped and fertilized). 



Hydration of milt. 



'Reference to trade names does not imply endorsement by the National Marine Fisheries Service, NOAA. 



FISHERY BULLETIN; VOL. 79, NO. 1, 1981. 



185 



