254 



Fishery Bulletin 105(2) 



Seawater characteristics and spawning 



Courtship and spawning behavior 



Spawning in the main broodstock tank occurred at salin- 

 ities of 26 to 36 ppt, but during most months salinities 

 ranged from 30 to 34 ppt. Dissolved oxygen ranged from 

 65% to 107% of saturation but usually exceeded 80% of 

 saturation. The seawater pH ranged from 7.6 to 8.3, and 

 ammonia was always below detection levels. During peri- 

 ods of cessation of spawning, salinities were generally 

 stable between 30 and 34 ppt, and dissolved oxygen, pH, 

 and ammonia were also within normal ranges. 



Spawning 

 No spawning 



Apr-97 



Apr-9e 



July-97 



July- 



Ocl-98 Apr-99 Ocl-99 Apr-OO 



18 Jan-99 July-99 Jan-OO 



24 



22 



B 



Oct-96 Apr-97 Ocl-97 



Jan-97 July-97 Jan-i 



Apr-98 Ocl-98 Apr-99 Oct-99 Apf-00 



8 July-98 Jan-99 July-99 Jan-OO 



Date 



Figure 2 



Mean daily water temperature in (A) the main broodstock tank, and 

 (B) the incubation tanks from October 1996 through March 2000. 

 In (A), plotted symbols are individual dates on which there was 

 either spawning (indicated by o) or no spawning (indicated by xl. In 

 (B), plotted symbols are individual dates on which egg incubation 

 occurred. 



The daily courtship and spawning behavior of the 

 broodstock fish followed a consistent pattern. Courtship 

 behavior usually began in the afternoon, and spawning 

 occurred most often in the late afternoon or evening. The 

 courtship behavior was usually initiated with a loose 

 aggregation by most of the fish in the central, bottom 

 area of the tank, although at times some pairing behav- 

 ior was observed before formation of the aggregation. 

 Smaller groups offish (two to five individuals per group) 

 would break off from the main aggregation and exhibit 

 courtship behavior that included paired 

 swimming, chasing, and bursts of speed 

 throughout the tank. The paired swimming 

 and chasing was often in the pattern of 

 loops throughout the water column. A court- 

 ship group consisted of a single fish, pre- 

 sumed to be a female, followed closely by one 

 to three fish, assumed to be males. During 

 the courtship process males would often 

 flash vertical bars (also termed feeding bars 

 by tuna biologists) along the sides of their 

 bodies. Fish presumed to be females would 

 often release concentrated discharge trails 

 from their vents during the late stages of 

 courtship. Courtship behaviors would con- 

 tinue unabated for 1 to 4 h prior to actual 

 spawning. 



Two to eight spawning groups would 

 eventually break off from the courtship ag- 

 gregation and spawning would take place 

 nearly simultaneously in the tank. When 

 the fish spawned, they would typically swim 

 in an ever-tightening circle — one female in 

 front and one to five males following closely 

 behind in single file. During actual spawn- 

 ing we always observed the trailing indi- 

 viduals releasing milt. As the females in 

 each group began to release their eggs, they 

 would tighten their swimming circle and 

 the males would do the same while they 

 released milt. This action resulted in the 

 entire spawning group swimming in a very 

 tight circle, which appeared to facilitate the 

 mixing of eggs and milt. During spawning, 

 the swimming speeds of the fish usually 

 decreased, compared to the speeds during 

 courtship, but at times swimming would re- 

 main quite rapid while eggs and milt were 

 released. The fish within spawning groups 

 moved horizontally as they spawned, but 

 the fish often added some vortex-like move- 

 ment upward through the water column 

 as spawning occurred. Within each group, 

 spawning was usually completed within 30 

 to 45 sec. The entire spawning event was 

 often finished within 60 to 90 sec, although 

 at times spawning events would continue 

 for up to 5 to 10 min. 



