NOTES Holland et al : Capture and release behavior of Pacific blue marlin off Hawaii 401 



Figure 4 



Aggregate depth distribution of six Pacific blue marlin off Hawaii 

 during da,v1ime (A) and nighttime (B). 



between the surface mixed layer and the top few 

 degrees of the thermodine, even though the depth of 

 this interface varied between tracks. Unlike yellowfin 

 tuna Thurinus albMares, which are also associated with 

 this feature (Carey and Olson 1982, Yonemori 1982, 

 Holland et al. In press), the marlin only rarely dove 

 below the top of the thermodine. Thus, a major por- 

 tion of the distribution of these marlin was bracketed 

 by the surface and the bottom of the mixed layer. This 

 behavior— not making deep dives, and spending most 

 time in the mixed layer and upper thermodine— is 

 similar to that of striped marlin tracked off California 

 (Holts and Bedford In press), but differs greatly from 

 that of swordfish which make frequent deep dives into 

 cold water (Carey and Robison 1981). Compared with 

 the striped marlin tracked off California, most of the 

 blue marlin in Hawaii spent a relatively small amount 

 of time very close to the surface. This may be due to 

 the shallower depth of the thermodine off California 

 (15-25 m) compared with Hawaii (35-90 m). With the 

 exception of fish 8903, which spent an atypical amount 



of daytime very close to the surface, the marlin tracked 

 in the present study moved closer to the surface at 

 night. This differs from the striped marlin data, but 

 is consistent with the behavior reported for skipjack 

 tuna Katsuwonus pelamis (Yuen 1970), swordfish 

 (Carey and Robison 1981), yellowfin tuna (Yonemori 

 1982, Holland et al. In press), and bigeye tuna T. obesus 

 (Holland et al. In press). 



Upon release, all six marlin dove into the upper layers 

 of the thermodine and remained there for several 

 hours. Onset of consistent upward excursions from the 

 thermodine appeared to represent the end of the post- 

 capture recovery period, usually between 4 to 6 hours. 

 The recovery period was initiated by comparatively 

 high-speed swimming, with four of the six fish travel- 

 ling farther in the first hour following release than at 

 any other time during the tracks. Because marlin are 

 obligate ram-ventilators, these fish may be swimming 

 fast enough to repay the anaerobic metabolic debt in- 

 curred during the fight, but not so fast as to acquire 

 new debt. The fast speeds of the first hour are even 

 more remarkable considering that at least two of the 

 fish appeared to be completely exhausted when tagged 

 and released. Holts and Bedford (In press) also report 

 deeper than normal depths and heightened activity 

 levels immediately upon the release of striped marlin. 



With the exception of slow-moving fish 8903, the 

 range of swimming speeds of individual fish in our 

 study (1.2-2.18 kn) are similar to the blue marlin (1.2- 

 1.9 kn) of Yuen et al. (1974). This is somewhat faster 

 than the striped marlin speeds (0.75-1.45 kn) observed 

 by Holts and Bedford (In press). In the present study, 

 most of the fish swam slightly slower at night than dur- 

 ing the day. 



One of the most remarkable features of the current 

 study is the consistent direction of movement displayed 

 by the marlin in the first several hours of each track. 

 Even though the three Keahole fish could have moved 

 anywhere within a 240 "-sector without running into 

 land, all three swam along parallel westerly courses 

 that took them directly offshore. Also, initial deviation 

 from these parallel tracks occurred in the same general 

 area ~12 nmi offshore. Similarly, both fish caught off 

 Keauhou swam offshore for '^-■6 hours along almost 

 identical paths, and both made their first major direc- 

 tional changes between 11 and 12 nmi offshore. This 

 point in these two tracks was separated by only ~0.5 

 nmi, even though the release sites were over 4 nmi 

 apart. The previously longest track of a Pacific blue 

 marlin (22.5 hours; Yuen et al. 1974) was also initiated 

 off Keauhou, and also commenced with a 7 nmi-move- 

 ment offshore, as did the Barbers Point marlin tracked 

 in the current study. 



These results suggest that direct movement offshore 

 may be a common response to the trauma associated 



