500 



Fishery Bulletin 88(3), 1990 



ELAPSED TIME (min) 



10 20 25 30 35 





Figure 9 



Fine-scale behavior of bigeye tuna BE8401 shown by 45-minute seg- 

 ment of vertical movements transcribed from data tape, including 

 a rapid dive of 230 m in 1 minute, a rate equivalent to 6.7 body 

 lengths/second. Ambient temperature at the maximum depth of 380 

 m was 9°C. 



Vertical movements 



The tape recordings of the piilsed sig^ial from the trans- 

 mitters resulted in detailed, continuous plots of vertical 

 position. Rapid and small-scale changes in depth were 

 discernible (e.g., Fig. 9), as were longer-term diurnal 

 shifts in vertical distribution. 



Day-night depth distributions In general, yellowfin 

 tuna swam closer to the surface during darkness. For 

 example, a dramatic change in the depth of swimming 

 of fish YF8305 occurred exactly at sunset and lasted 



until first light the following morning, when it again 

 began to swim deeper (Fig. 10). Combining all the off- 

 shore and off-FAD portions of the yellowfin tuna tracks 

 yielded an average daytime depth of 71.3 m (±42.0 SD, 

 A'^ = 333), whereas the average nighttime depth was 

 47.3 m ( + 33.1 SD, iV= 444). These daytime and night- 

 time depth distributions (Fig. 11A,B) are significantly 

 different (ANOVA, p = 0.0001). However, this analysis 

 also indicates significant variability among individual 

 tracks. 



Bigeye tuna swam at significantly greater daytime 

 depths than the yellowfin tuna, and showed even 

 greater shifts between daytime and nighttime distribu- 

 tions. When off-FAD, the predominant bigeye tuna 

 daytime distribution was between 220 and 240 m, 

 whereas the predominant nighttime depth was between 

 70 and 90 m (Fig. 12A,B). 



On-FAD versus Off-FAD depth distributions Swim- 

 ming depths of five yellowfin and one bigeye tuna that 

 were on- and off-FAD during daytime hours support 

 the proposition that FADs tend to bring the fish closer 

 to the surface than they would normally be in other off- 

 shore parts of the ocean. These tracks were analyzed 

 to determine if significant changes in depth distribu- 

 tion occurred when the fish were aggregated around 

 the FADs, independant of diurnal influences. When on- 

 FAD, three of the yellowfin tuna were closer to the sur- 

 face than when off-FAD, one showed no change in 

 depth, and one was deeper. Pooling these data for this 

 subset of yellowfin tuna gave a mean on-FAD depth 

 of 59.3 m (±30.7 SD, iV = 225), which is significantly 

 different from the mean off-FAD depth of 85.2 m 

 (±52.0 SD, N=239). 



DEPTH (m) 



YELLOWFIN 8305 



TIME 



Figure 10 



Plot {24-hour) of vertical movements of yellowfin tuna YF8305. In this and subsequent figures, S = sunset. R = sunrise. Arrows denote 

 the beginning and end of time spent at V FAD. Before and after this period, the fish exhibited the "traveling" behavior of making abrupt 

 shifts between the surface and the top layers of the thermocline. Once at the FAD, the vertical behavior became predominantly sinusoidal. 

 All dives made both on- and off-FAD had a consistent floor around 20°C between 140 and 150 m. The rapidly descending trace at end of 

 plot denotes the tag being shed from the fish. 



