out the study area (100-365 m), the length of the 

 atoll's 183 m (100 fathom) isobath (64 km) provides 

 a convenient measure of total deep slope habitat 

 (Ralston and Polovina 1982). The average point-to- 

 point distance covered by the submersible during one 

 4-h dive was 2.27 km (s = 0.56 km). An aggregate 

 22.7 km were thus surveyed during this study, com- 

 prising 35% of the deep slope habitat at the atoll. 



Temperature 



Ambient temperature and depth were recorded 

 often during dives, from which temperature-depth 

 profiles were later constructed. The results are sum- 

 marized in Figure 2. The solid line represents me- 

 dian temperatures at depth, with the shaded area 

 encompassing the range of temperatures observed 

 among all 10 dives. Surface water temperature was 

 typically 27°C and the mixed layer 100 m deep. A 

 second weak thermocline was found around 240 m. 

 Although its depth varied somewhat (220-245 m), it 

 was present around the entire atoll, i.e., both wind- 

 ward and leeward exposures, and was observed as 

 a shimmering layer below the submersible as it 

 descended. This effect is believed due to refraction 

 of light passing through variable density water, a 

 result of the thermocline in association with a de- 

 crease in salinity. 4 Ambient water temperature usual- 

 ly had dropped to 8.5 °C at a depth of 350 m. 



Slope Angle 



The relationship between the bottom's slope and 

 depth was also measured. These data were sum- 

 marized after each dive and bottom contours plot- 

 ted. Overall, there was little variation in slope angle 

 around the atoll, i.e., the general pattern was one of 

 uniformity at all sites visited. Figure 3 presents pool- 

 ed results for all slope angle-depth determinations. 

 In the figure, horizontal and vertical scales are equal 

 and the composite contour of the bottom (100-365 

 m) at Johnston Atoll is shown in profile. The slope 

 was stratified into three 50-fathom depth zones for 

 later analysis. 5 The slope angle between 50 and 100 

 fathoms averages Q 1 = 25° (Table 1). Similarly, 2 

 = 47° and 3 = 59°. There is a definite trend at 

 Johnston Atoll for the slope to steepen with in- 



4 E. Chave, Hawaii Undersea Research Laboratory, University of 

 Hawaii, Honolulu, HI 96822, pers. commun. June 1984. 



Stratification of depth into zones was performed using units of 

 fathoms (1 fathom = 1.83 m) because nautical charts, hydrogra- 

 phic surveys, and fathometers are so measured. For the sake of 

 brevity and clarity, isobaths and depth strata will henceforth be 

 given only in this unit of measure 



FISHERY BULLETIN: VOL. 84, NO. 1 



Temperature - °C 



10 15 20 25 30 



50 



100 



150 



E 



i 



Q. 

 



Q 



200 



250 



300 



350 



Figure 2.— The pooled relationship (w = 10) between temperature 

 and water depth at Johnston Atoll. Solid line = median values; 

 shaded area = range of values. 



Table 1.— Total habitat areas stratified by depth zones at Johnston 



Atoll. 



creasing depth, at least between 100 and 365 m. 

 In the shallowest regions surveyed (<125 m) the 

 bottom was a monotonous sandy plain in the shore- 

 ward direction, but at 125 m it began to slope steeply 



144 



