4.3 Natality and recruitment 



4.31 Reproduction rates 



An A. thazard of moderate size (44.2 cm) produces 

 about 280,000 eggs/spawning and about 1.37 million 

 eggs are extruded over the entire spawning season (Rao 

 1964). From Auxis caught in drift nets in Indian waters, 

 Silas (1969) also estimated their fecundity and con- 

 cluded that A. thazard {n = 9) spawn from 197,000 to 

 1,056 million eggs and average 601,000 eggs/spawning; 

 for A. rochei (n = 4), he found fewer eggs per spawning. 

 Silas estimated that females spawn from 31,000 to 

 103,000 eggs and average 52,000 eggs/spawning. 



4.33 Recruitment 



Off Sri Lanka, major recruitment to the exploitable 

 stock of A. thazard occurs annually along the south and 

 southwest coasts between March and August 

 (Sivasubramaniam 1973). There is also evidence that 

 sporadic recruitment occurs during the rest of the year. 

 Off the east coast, recruitment has been observed in 

 July-September, but no clear trend in recruitment has 

 been found in catches along the north coast. For A. 

 rochei, Sivasubramaniam conjectured that recruitment 

 probably takes place at about the same time as that for 

 A. thazard. Therefore, the exploitation of new recruits 

 starts around the middle of the year then shifts to large 

 size classes until the following year's recruitment. 



The time of recruitment influences the annual pro- 

 duction of Auxis in Sri Lanka (Sivasubramaniam 1973). 

 When recruitment occurs during the southwest mon- 

 soon, the schools that enter the fishery are available at a 

 time when the relative abundance of skipjack tuna, yel- 

 lowfin tuna, and kawakawa is low and frigate and bullet 

 tunas are fished until the beginning of the northeast 

 monsoon. But when recruitment occurs late in the year, 

 the fishing effort expended on these two sp>ecies is less 

 intense because of the presence of more desirable species 

 such as skipjack and yellowfin tunas. Thus, fishing em- 

 phasis shifts from trolling to pole and line, longline, and 

 drift netting. November-March marks the peak fishing 

 season for the skipjack tuna pole-and-line fishery in the 

 south and southwest coasts of Sri Lanka. It is at this 

 time that large Auxis (40-50 cm), usually mixed with 

 skipjack and yellowfin tunas, show up frequently in the 

 catch. 



4.4 Mortality and morbidity 



4.42 Factors causing or affecting mortalty 



An instance of natural mass mortality of larval Auxis 

 in Hawaiian waters has been reported by Strasburg 

 (1959). Noting the presence of two distinct conditions of 

 larval Auxis in the preserved plankton samples collect- 

 ed in the vicinity of Lanai (an island in the Hawaiian 

 chain), Strasburg hyjxithesized that these conditions 

 represented fish that were alive and dead just prior to 



capture. In a simple experiment, he placed several fresh, 

 dead specimens in dishes of seawater and allowed them 

 to decompose. Other fresh, dead specimens were pre- 

 served in Formalin. The results showed that fresh pre- 

 served specimens were clean cut and normal in ap- 

 pearance whereas dead, deteriorated specimens were 

 imperfect. Strasburg concluded that large numbers of 

 larval Auxis in the samples were dead before capture, 

 but no cause of death could be isolated. One pKtssible 

 cause of death is the passage of the larvae through an 

 area of marked temperature discontinuity in the sur- 

 face layer. 



5 EXPLOITATION 



5.1 Fishing equipment 



5.11 Gears 



Pole and line is the most commonly used gear to catch 

 Auxis. In Japan, pole-and-line gear designed ex- 

 clusively for fishing A. rochei differs from that used for 

 skipjack tuna fishing. According to Yasui the pwle-and- 

 line gear for bullet tuna consists of a bamboo pole, nylon 

 monofilament, trolling board, and jig, and is attached at 

 the butt end of a tag line to the vessel so that the entire 

 gear is "trolled" at a speed of 2-3 nmi/h while searching 

 for bullet tuna schools (Fig. 49). Two to three fishermen 

 manage the "trolled" poles at the stem. When a fish 



M. Yasui, Shizuoka Prefectural Fisheries Experimental Station, 

 Shizuoka. Japan, pers. commun. December 1975. 



NYLON MONOFILAMENT NO. 10 2 5m 



-) TROLLING BOARD 



Figure 49. — Diagram of the pole-and-iine gear used and the 

 method of fishing in the fisher}' for Auxia spp. in Japanese water 

 (lahida 197!). 



48 



