RALSTON ET AL.: BOTTOM FISH RESOURCE AT JOHNSTON ATOLL 



downward. Although not easily seen in the figure, 

 a small but prominant ledge 5-10 m high encircled 

 the atoll between 130 and 140 m. Somewhat deeper, 

 between 180 and 275 m, the bottom was uniform in 

 slope and its surface relatively smooth and devoid 

 of features. Slope angles approached the vertical at 

 most sites in the 300-350 m depth range, with over- 

 hanging caves formed by subaerial dissolution. 6 At 

 the deepest points visited (360 m) the bottom became 

 less precipitous, and in some areas a sediment-laden 

 terrace had formed along the base of the deep 

 dropoff. 



Based on estimates of slope angle, existing charts, 

 and a hydrographic survey by the Townsend Crom- 

 well, habitat areas for the three depth zones were 

 determined. The positions of the 10 and 100-fathom 

 isobaths were already known, but they were refined 

 and the locations of the 50-, 150-, and 200-fathom 

 isobaths estimated. Figure 1 is a simplified repre- 

 sentation of a much larger chart which was digital- 

 ly analyzed to determine the horizontal (i.e., level) 

 areas bounded by isobaths (Table 1). The results show 

 that emergent lands (Johnston, Akau, Hikina, and 

 Sand Islands) account for only 1% (305 ha) of the 

 level planar area of the atoll. The largest area 

 (60%) lies between sea level and 10 fathoms. The 



6 Keating, B. H. Geologic history and evolution of Johnston 

 Island: Submersible dive results. Manuscr. in prep. University of 

 Hawaii, Honolulu, HI 96822. 



total horizontal extent of the atoll is about 25,000 

 ha. 



These results can be misleading, however, because 

 a vertical slope provides no horizontal habitat area, 

 and yet both reef fish diversity and standing crop 

 are known to be positively correlated with topo- 

 graphic relief (Luckhurst and Luckhurst 1978; Glad- 

 felter et al. 1980; Carpenter et al. 1981). At John- 

 ston Atoll the structural complexity of the sub- 

 stratum frequently increased with slope angle. A 

 better estimate of total habitat is the area of bot- 

 tom irrespective of slope angle, estimated by dividing 

 the horizontal planar area of a depth stratum by the 

 cosine of the slope angle within it. This adjustment 

 almost doubles the estimate of total habitat area in 

 the 150-200 fathom zone, simply due to the precipi- 

 tous dropoff found there. A composite 5,165 ha of 

 habitat occurs between 50 and 200 fathoms. 



General Observations 



While this study focused primarily on the deep- 

 water ichthyofauna of Johnston Atoll, many obser- 

 vations were made on the oceanographic, geologic, 

 and biotic characteristics of the study area. These 

 are briefly recounted here. 



Currents running in directions parallel to the slope 

 were frequently encountered. They were generally 

 weak and did not exceed 0.3 kn (0.15 m/s). They 

 sometimes exhibited reversals with depth. During 



100- 



150- 



i 200- 



a 

 s 

 a 



250- 



300- 



350- 



— i 50 



100 



E 

 o 



x: 



O 

 N 



150 



200 



Figure 3— Composite reconstruction of the deep slope at Johnston Atoll. Horizontal and vertical scales equal. 

 Average slope angles (0) were measured for each of three 50-fathom depth strata. 



145 



