NOTE Graves et al : An evakiation of satellite tags for estimating postrelease survival of Makaiia nigncam 



137 



marlin (Fig. 3). Temperature readings demonstrated that 

 tagged individuals spent the majority of their time at tem- 

 peratures above 26°C (Fig. 4). The maximum temperature 

 range recorded for any of the eight individuals was 9°C 

 (22-3rC, tag no. 24033). Block et al. ( 1992), using acoustic 

 tracking, determined that the six blue marlin which they 

 tracked spent half of their time in the upper 10 m of the 

 water column in water temperatures 2.5-27°C, and Hol- 

 land et al. (1990) reported that blue marlin in waters off 

 Hawaii remained at temperatures of 26° or greater. 



Differences in thermal histories were evident among the 

 individuals in our study. Blue marlin no. 24029 (Fig. 3G) 

 spent the vast majority of time at temperatures equal to 

 that of the surface waters (30-31°C). In contrast, individu- 

 al no. 24527 (Fig. 3H) spent much less time in the warmer 

 surface waters and repeatedly moved up and down in wa- 

 ters between 23° and 31°C. Several individuals appeared 

 to remain at or very near the surface for extended peri- 

 ods, evident in Figure 3 as a continuous string of tempera- 

 ture readings at or slightly above 30°C. An analysis of the 

 data examining diurnal-nocturnal periods with tempera- 

 ture (inferred depth) indicated a high level of variability 

 between individuals and no clear pattern was apparent 

 (Fig. 3). In contrast, Holland et al. ( 1990) determined that 

 blue marlin spent a higher proportion of their time (-50%) 

 in the upper 10 m at night than during the day (-25% ). 



It was possible to infer swimming depths of blue marlin 

 by comparing water temperature values with the temper- 

 ature-depth profiles at station "S" provided by the Bermu- 

 da Biological Station for Research.- All blue marlin en- 



Although this station is situated 24 km to the southeast of the 

 island, similar temperature-depth profiles would be expected 

 for the general region (Johnson, R. 2000. Personal commun. 

 Bermuda Biological Station for Research, 17 Biological Lane, 

 Ferry Reach, St. George's GEOl Bermuda). This allowed us to use 

 the station S profiles to infer swimming depth, realizing that, 

 depending on when an animal was tagged and where it moved, 

 there would be some differences for which we could not account. 



tered cooler waters at various times during the five-day 

 period, with excursions to depths as great as 40 meters. 

 Temperature records were consistent with the tagged blue 

 marlin actively undertaking vertical movements in the 

 upper 40 meters of the water column. However, six of the 

 eight fish spent >75% of their time in the upper 10 m of 

 the water column for the five-day duration of the study. If 

 the data from all eight fish are pooled, this yields a mean 

 value of 79.9% (SD 15.8%) of the time spent in this zone. 

 This is a higher percentage of time spent in the upper 10 

 m than that obsei-ved by Block et al. ( 1992), who reported 

 that fish spent about half of the time in this zone. How- 

 ever, this comparison should be viewed with some caution 

 because the Block et al. (1992) data were based on con- 

 tinuous tracking, whereas each data point in our analysis 

 was the average of two hourly measurements. 



All post-pop-up inclinometer values were 254 or 255, 

 where 255 represented the maximum (vertical) inclinom- 

 eter value expected for an upright, floating tag. Pre-pop- 

 up inclinometer values ranged from 203 to 251, with three 

 individuals at 233 and four between 247 and 251. These 

 values indicate tags were inclined at an angle below 30 

 degrees above horizontal for more than 40% of the 1830 

 sampling times for each individual, and are consistent 

 with sufficient forward propulsion to depress the positive- 

 ly buoyant tag more than 60 degrees from vertical. There 

 was no correlation between pre-pop-up inclinometer values 

 and net displacement. The fish with the largest net dis- 

 placement (no. 24059) had the second highest inclinometer 

 value. This was not unexpected because the difference be- 

 tween the lowest and highest pre-pop-up inclinometer val- 

 ues represents a minor difference in the time the tag was 

 depressed below 30 degrees above horizontal. Also, the re- 

 lationship between total movement and net displacement 

 could be quite different for different individuals. 



Three different lines of evidence provided by the pop- 

 up satellite tags (net movement, water temperature, and 

 tag inclination) each suggested that at least eight of the 



