general. Salinity, likewise, showed no relationship with 

 catches of larval Auxis, but Klawe et al. (1970) reasoned 

 that this lack of relationship may be due to the narrow 

 range of salinity encountered during the period that the 

 study was made. Salinity, therefore, cannot be dis- 

 counted as a factor affecting distribution. In fact, Richards 

 and Simmons (1971) determined that larval Auxi's were 

 found in a relatively narrow salinity range from 33.27.. to 

 35.47~ compared to that of other tunas, with the excep- 

 tion of little tunny, Euthynnus alletteratus. 



Concerning the effects of light intensity, Higgins 

 (1970) observed that juvenile Auxis were more 

 vulnerable to the midwater trawl at night than during 

 daylight hours. Furthermore, they were more readily 

 caught in shallower than in deeper tows, although the 

 data suggested that they may occur as deep as 100 m. 

 Strasburg (1960) reported that larval Auxis occur at the 

 surface very infrequently during daylight hours but are 

 often found there in large numbers at night. Noting that 

 this pattern of an increase in catch at night could result 

 from either net avoidance or vertical migration, Stras- 

 burg reasoned that if net avoidance only were involved, 

 one would expect the catch to be essentially constant at 

 night. The results of his study proved otherwise; there- 

 fore, he concluded that vertical migration appears to be 

 the major factor causing the increase in surface catch at 

 night. Contrarily, in a later study by Klawe et al. (1970), 

 it was observed that larval Auxis show no significant 

 diel movement in the water column. From a statistical 

 te^t they conducted for time of day as well as for inter- 

 action between time of day and type of tow, Klawe et al. 

 (1970) found no interaction between type of tow and 

 time of day; therefore, they concluded that no vertical 

 movement had occurred. They reasoned that any ability 

 larval Auxis may have had to avoid the sampling gear 

 was independent of time of day. 



2.4 Hybridization 



2.41 Hybrids; frequency of hybridication; species 

 with which hybridization occurs; methods of 

 hybridization 



There has been considerable progress in studies on egg 

 and larval development of tuna and tunalike fishes in 

 recent years. Most impressive are the experiments by 

 Japanese scientists on artificial fertilization of eggs and 

 the subsequent rearing of the larvae. Among the tuna 

 species that have been used in these experiments were 

 yellowfin, skipjack, frigate, and bullet. In 1970, the 

 Japan Fisheries Agency coordinated a 3-vr cooperative 

 program to artificially fertilize and rear tunas (Ueyanagi 

 et al. 1973). The experiments were conducted through 

 the cooperative efforts of Tokai University, Kinki 

 University, Shizuoka Prefectural Fisheries Experimen- 

 tal Station, Mie Prefectural Owase Fisheries Experi- 

 mental Station, Nagasaki Prefectural Fisheries Experi- 

 mental Station, and the Far Seas Fisheries Research 

 Laboratory of the Japan Fisheries Agency. 



Experiments conducted at Kinki University Fish- 



eries Experimental Station (1974) involved cross- 

 fertilization of A. thazard and A. rochei. The ripe eggs 

 taken from A. thazard caught by set net at Kashino, 

 Oshima, on 17 July 1973 at 0530 were fertilized and 

 transported to the experimental station at Shirahama. 

 At 1350 on the same day, there were about 35,120 viable 

 eggs which averaged 0.86 mm in diameter and about 

 7,(XX) dead eggs. Twenty-five thousand viable eggs were 

 placed in a 3-ton (3,000 liter) tank and 10,000 in a 1-ton 

 (1,(XX) liter) tank. Water temperature in the tanks was 

 about 26°C. Larvae started to emerge the next day, 

 approximately 30 h after fertilization and all hatching 

 was completed during the day. The larvae were kept in 

 seawater with Chlorella which had a density of 

 400,000/cc. Aeration was also provided. Rotifers were 

 added to the tank 2 days after hatching and marine 

 plankton was added the next day. All the larvae in the 3- 

 ton tank died after 6 days. In the 1-ton tank, although 

 mortality was heavy, there were some survivors. At 14 

 days, the water was circulated, and at 18 days after 

 hatching, artemia and eggs of frozen yellowtail, Seriola 

 quinqueradiata, were used to supplement the marine 

 plankton. Mortality continued and the last survivor, 

 which attained a length of 4.8 cm and a weight of 0.907 g 

 died after 31 days. 



3 BIONOMICS AND LIFE HISTORY 

 3.1 Reproduction 



3.11 Sexuality 



Like all other scombrids, Auxis are heterosexual. 

 There are no externally visible characters that aid in 

 distinguishing males from females. Internally, the 

 paired and elongated gonads are nearly symmetrical 

 and are suspended by mesenteries extending almost the 

 entire length along the roof of the abdominal cavity. At 

 the posterior end of the abdominal cavity, the gonads 

 extend along both sides of the anal fin. This posterior ex- 

 tension, according to Kishinouye (1923), is due to the 

 narrowness of the abdominal cavity. 



3.12 Maturity 



Studies of gonads of troll-caught fish from waters of 

 Kaneohe Bay, Oahu, in the Hawaiian Islands indicate 

 that frigate tuna are nearly mature at a size of about 35 

 cm (Tester and Nakamura 1957), but in Japanese 

 waters, they have been reported to reach maturity at 

 about 29 cm. Yasui (1975) investigated the use of liver 

 weight as a possible index of sexual maturity. He plotted 

 liver weight against body length of frigate tuna caught 

 off the Izu Islands and off Mera, Shizuoka Prefecture, 

 and observed a point of discontinuity at a body length of 

 29 cm. His data also showed that 97% of the fish <29 cm 

 were caught after September whereas 95% of those 

 larger than that were caught before August. The repro- 

 ductive index, according to Yasui, was highest in July. 



15 



