NOTES Ratty et al. Reproductive biology of Thunnus alalunga 



215 



age of sexually mature males in the size category above 

 90 cm FL which may be related to the method of cap- 

 ture and/or the type of habitat where collected. All fish 

 larger than 90 cm, except one, were caught during sub- 

 surface longline fishing operations within about 75 

 miles of the coast of the New Zealand north island, 

 whereas nearly all other fish were caught by surface 

 trolling in midocean waters. Oceanographic observa- 

 tions made concurrently with albacore troll-fishing op- 

 erations indicate that albacore in the midocean region 

 of the South Pacific are associated with Subtropical 

 Convergence Waters (Laurs 1986, Laurs et al. 1987). 



Female albacore have been reported to attain sex- 

 ual maturity and first spawn at about 90 cm FL (Ueya- 

 nagi 1955, Otsu and Uchida 1959); however, size at 

 maturity for males has not been well established. Otsu 

 and Hansen (1961) found that some male albacore are 

 probably mature when they attain a length of about 

 90 cm FL. Based on the general appearance of testes 

 and oozing of milt, Ueyanagi (1957) postulated the 

 smallest mature male to be 97 cm FL in a sample of 

 albacore collected in the western North Pacific. Brock 

 (1943) reported that "Milt could be squeezed from the 

 testes of some of the more mature males, but no 

 females were found in which eggs could be discerned 

 by the unaided eye. ..." Brock (1943) examined fish 

 caught off the Oregon coast that ranged in fork length 

 between 53 and 90 cm, but did not specify the size of 

 fish that contained milt. 



Partlo (1955), in the only published histological study 

 that includes albacore testes as well as ovaries, con- 

 cluded that both males and females in his age groups 

 V and VI were in a condition approaching spawning. 

 He calculated that the mean fork length for males and 

 females in age-group V is approximately 70 cm, and 

 for age-group VI is about 85 cm for males and 79 cm 

 for females. He also reported observing small numbers 

 of fully mature sperm restricted to tubules of the 

 posterior portion of testes in age-group IV, which he 

 reported as having a mean fork length of 62 cm. Partlo 

 (1955) examined a total of 44 testes and ovaries, but 

 unfortunately did not specify the sex or number of in- 

 dividuals in each age group. 



Godsil and Byers (1944) reported that the fat body 

 was present in albacore collected throughout the North 

 Pacific, except in three specimens from the area around 

 the Hawaiian Islands, with its greatest development 

 always on the right side of the fish. However, they 

 made no mention of asymmetry in gonad size. 



There appears to be a functional relationship between 

 meiotic activity and the amount of fat in the mesentery. 

 We presume the fat body provides an energy source 

 for spermatogenesis, although we have no information 

 from the present study on the timing of sperm release 

 from the albacore testis. Godsil and Byers (1944) also 



speculated that the fat body acts as an energy reserve 

 for female albacore that is expended during the spawn- 

 ing season. They based their supposition on the absence 

 of the fat body in three recently spawned albacore from 

 the Hawaiian Islands. Schaefer (1987) noted that 

 spawning in the black skipjack, Eiifhymrus UtieatHs, 

 is probably regulated by energy available in fat storage. 

 When the fat stores fall below a minimum level, ovarian 

 atresia probably occurs over a short period of time. 



Conclusions 



We provide quantitative evidence that a substantial 

 portion of male albacore caught in midocean areas of 

 the South Pacific Subtropical Convergence Zone about 

 lat. 35°-40°S, may be sex-ually mature when they attain 

 a fork length of 71-80 cm. Also, the proportion of ma- 

 ture fish increases with increasing length. A lower pro- 

 portion of males caught by subsurface longline fishing 

 in coastal waters off New Zealand showed development 

 in sexual maturity, although not as accelerated as males 

 caught by surface trolling in the midocean region. Fe- 

 male fish 55-95 cm FL caught in both regions showed 

 few signs of sexual maturation. South Pacific albacore 

 are believed to spawn generally in the region of the 

 Southern Tropical Convergence waters about lat. 

 20°-10°S (Knox 1970). This region is about 1000-1500 

 miles north of the Subtropical Convergence Zone where 

 the albacore investigated in this study were caught. 

 The adaptive significance of sexual maturity in male 

 albacore at times, locations, and ages when females ai-e 

 not in spawning condition is not clear. Sampling of both 

 males and females over the entire spawning cycle and 

 in locations where albacore are believed to spawn will 

 be required to understand the reproductive biology of 

 the South Pacific albacore population. 



Acknowledgments 



The authors wish to express their appreciation to Dar- 

 lene Pickett and Christine Hopkins for their assistance 

 in making histological preparations, to E. Roger Mar- 

 chand for preparing photomicrographs, to Robert S. 

 Garrett for producing scanning electron micrographs, 

 and to Lana Nimmo for supplying technical help in 

 image analyzer operations. In addition, we extend spe- 

 cial thanks to Mark Hess for skillful computer applica- 

 tions used in processing and analyzing results and to 

 Kurt M. Schaefer for constructive comments on a draft 

 of the manuscript. We also acknowledge Talbot Mur- 

 ray for handling the logistics of collecting gonad sam- 

 ples on the RV Kaharoa and Japanese longline fishing 

 vessels, and the scientific party, crew and officers of 

 the RV Townsend Cromwell. 



