400 



Fishery Bulletin 88(2), 1990 



the surface mixed layer met the top of the thermocline 

 at depths varying between 75 and 100 m. During the 

 daytime, the fish moved in and out of the top of the 

 thermocline, with occasional excursions towards the 

 surface; during the night, movements were more com- 

 pletely confined within the mixed layer with more time 

 spent very close to the surface. 



Fish 8804 (also estimated to weigh 60 kg) was caught 

 and tagged by an HIBT team at 1310, 11 August 1988, 

 3 nmi off Keahole Point. Fight time and release condi- 

 tion were similar to fish 8803. After an initial, brief 

 movement towards shore, this fish also proceeded 

 directly west for about 8 hours before curving south- 

 ward after sunset on the first night (Fig. 2). This direc- 

 tion was maintained all day during the second day until 

 after sunset when it turned more shoreward. However, 

 a southerly course was resumed after sunrise of the 

 third day. The track was terminated after 42 hours. 



The daytime vertical movements of this fish were 

 focused around the interface between the mixed layer 

 and the thermocline. From that depth (around 90 m) 

 intermittent excursions were made to the surface. 

 Nighttime distribution was shifted markedly towards 

 the surface (Fig. 3B). 



Fish 8807 was caught by the Kaahele'ale at 0900 on 

 27 August 1988, 3 nmi off Keahole Point. The fish, 

 which was fought for 35 minutes, was caught using an 

 artificial lure with a single hook that lodged in the base 

 of the marlin's bill. The fish, weighing in excess of 160 

 kg, was completely immobile and floating belly-up at 

 the side of the boat when the transmitter was attached 

 near the midline in line with anterior insertion of the 

 anal fin. 



When the leader was cut, the fish sank slowly upside 

 down under the boat but, after ~30 seconds, righted 

 itself and began to swim slowly downwards. During the 

 next hour, the fish traveled 3.25 nmi and, as with the 

 previous two Keahole fish, headed steadily west, in this 

 case for 7 hours. After sunset, the fish slowed and 

 assumed a southeasterly course which it retained for 

 the remainder of the track. After sunset on the second 

 night, the fish returned close to the island and briefly 

 visited fish aggregating device (FAD) TT off Milolii 

 (Fig. 2). The fish was lost in rapidly deteriorating sea 

 conditions off South Point, Hawaii, after 42 hours of 

 tracking. At this time, the fish appeared to be con- 

 tinuing in a southerly direction. 



Immediately upon release, this fish descended into 

 the uppermost 2 or 3 degrees of the thermocline, where 

 it remained for 6 hours as it swam steadily offshore. 

 Upward excursions towards the surface started 3 hours 

 before sunset, and movements on the second day were 

 mostly in the thermocline-mixed layer interface with 

 frequent upward movements, some of which reached 

 the surface. As with the other Keahole marlin, night- 



time distribution was shifted towards the surface, with 

 considerable amounts of time spent at the surface 

 (Fig. 3C). 



Fish 8903, weighing ~75 kg, was caught and released 

 3 nmi off Keauhou Bay by a team in the 1989 HIBT. 

 The fight time was ~20 minutes and the fish was live- 

 ly when the transmitter was attached. Immediately 

 upon release, the fish headed offshore for 5.5 hours 

 before turning north (Fig. 2). This initial deviation in 

 direction occurred ~11.5 nmi offshore and coincided 

 with the marlin's first movement to the surface from 

 the top of the thermocline, where it had been since the 

 initial release (Fig. 3D). 



Compared with other fish tracked in this study, this 

 fish spent large amounts of time swimming slowly very 

 close to the surface, often with the tips of its dorsal 

 and caudal fins protruding above the surface. Although 

 this "lazy" surface behavior is not uncommonly ob- 

 served by fishermen, this was the only fish to demon- 

 strate this behavior in the current study. The track was 

 terminated after 24 hours, with the fish continuing to 

 swim slowly at the surface in a northwesterly direction. 



Fish 8904, also weighing ~75 kg, was caught and 

 released by an HIBT team at a location ~2 nmi off- 

 shore of Kealakekua Bay, after a 36-minute fight. This 

 fish also ran west (for ~6 hours) to a point 11.5 nmi 

 offshore, at which time contact was lost for ~2 hours. 

 When relocated, the fish had adopted a northwesterly 

 course which it maintained for the remainder of the 

 29-hour track (Fig. 2). Vertical movements were similar 

 to those of other fish, being largely constrained by the 

 top of the thermocline and the surface, and averaging 

 closer to the surface at night than during the day 

 (Fig. 3E). 



Temperature and depth distribution 



Pooling the data from all six fish indicates that 82% 

 of the daytime and 97% of nighttime distribution oc- 

 curred in the mixed layer and top 2 degrees of the ther- 

 mocline. In terms of depth, approximately 36% of the 

 daytime and 60% of nighttime was spent between the 

 surface and 30 m, the rest of the time being spent at 

 greater depths (Fig. 4). 



Discussion 



Although the sample size reported here is quite small, 

 these data represent a significant increase in the 

 number of Pacific blue marlin tracked. Also, several 

 aspects of the behavior of these marlin show remark- 

 able consistency. 



For instance, the behavior of all the fish tracked in 

 this study was strongly influenced by the interface 



