RALSTON ET AL.: BOTTOM FISH RESOURCE AT JOHNSTON ATOLL 



depth ranges. For example, all species with depth 

 ranges exceeding 200 m are large (i.e, Caranx lugu- 

 bris, Epinephelus quernus, and Seriola dumerili). 

 Moreover, among extensively observed species, a 

 significant Spearman correlation exists between 

 ranked average weight and depth range (r s = 0.52, 

 df = 25, P < 0.01). This finding should be viewed 

 with caution because of potential biases in depth 

 distributions (see above). 



The last column in Table 2 gives sighting scores 

 for all species. Those assigned a value of 1 indicate 

 species dominating the deep slope fish community 

 in terms of species sightings. Note that some species 

 were seen infrequently, but when encountered they 

 were observed in large numbers (eg, Elagatis bipin- 

 nulata, Fig. 4). Similarly, Pristipomoides filamen- 

 tosus was not seen on every dive and was thus as- 

 signed an abundance score of 3. In spite of this, when 

 seen, it was abundant and it was the most frequent- 

 ly caught while fishing (see next section). Sighting 

 scores therefore do not indicate relative species' con- 

 tributions to total standing crop biomass of the deep 

 slope fish fauna. 



Quadrat Sampling 



A total of 974 quadrat sample counts were made 

 during the 10 submersible dives. No attempt was 

 made to estimate abundance separately for each 

 species. Rather, the total number of bottom fish was 

 recorded, regardless of species composition. Al- 

 though severely reducing the detail of the data base, 

 this did have the desirable effect of averaging biases 

 due to attraction or repulsion of fishes to and from 

 the Makalii. It was evident, for example, that some 

 species were attracted to the submersible and follow- 

 ed it about (e.g., Seriola dumerili and Caranx lugu- 

 bris), whereas others were repelled and actively 

 avoided the submersible's lights (eg., Pristipomoides 

 filamentosus and Etelis coruscans). Still others did 

 not seem to be greatly influenced (eg., Cookeolus 

 boops, Epinephelus quernus, Pristipomoides zonatus, 

 and Pontinus macrocephalus). By pooling species 

 quadrat counts, the abundance of some species was 

 overestimated, some underestimated, and some 

 estimated without bias. Due to averaging, we believe 

 that pooled counts provide the best available 



Figure 4— Johnston Atoll deep slope fishes. A. Caranx lugubris 

 with wire coral; B. Epinephelus quernus peering out of cave; C. 

 Seriola dumerili (foreground) and Caranx lugubris (background); 

 D. school of Elagatis bipinnulata with Carangoides orthogrammus 

 (above); E. Heniochus diphreutes with black coral; and F. aggrega- 

 tion of Myripristis chryseres and Neoniphon aurolineatus. 



estimates of total bottom fish density along the deep 

 slope of Johnston Atoll. 



Some 367 bottom fish were counted in quadrat 

 samples, resulting in a mean encounter rate of 0.38 

 fish/quadrat. The data were fitted to the Poisson 

 distribution to ascertain the dispersion pattern. A 

 chi-square goodness of fit test yielded x 2 = 325.32, 

 df = 3, P « 0.005, demonstrating nonrandom dis- 

 persion. The variance to mean ratio calculated from 

 the frequency distribution of bottom fish/quadrat 

 observations was 4.64 and was significantly greater 

 than 1 (P « 0.005), indicating strong contagion. 

 One of the principal explanations for this result 

 is shoaling by Pristipomoides filamentosus and Ete- 

 lis coruscans. Both are large species, which formed 

 aggregations of up to 100 individuals well off the bot- 

 tom (20 m) in the vicinity of underwater headlands 

 and promontories. These monospecific groups ap- 

 peared to feed in open water on plankton, consis- 

 tent with previous dietary studies of P. filamentosus 

 (Kami 1973; Ralston 8 ). When either was observed, 

 there was an increased likelihood of encountering 

 conspecifics. As a consequence 10 or more P. fila- 

 mentosus were seen in one quadrat on 7 occasions. 

 Another factor contributing to clumping was non- 

 random distribution with depth (Fig. 5). This figure 

 presents the relationship between mean number of 

 bottom fish per count and depth (vertical bars = 

 standard errors). Note the two abundance peaks, the 

 first at about 170 m and the second at 250 m. The 

 former was due primarily to large numbers of 

 Caranx lugubris and P. filamentosus. The location 

 of the second peak was just below the second 

 thermocline and was largely the result of local in- 

 creases in numbers of Epinephelus quernus and P. 

 zonatus. 



The mean numbers of bottom fish per quadrat, 

 stratified into 50-fathom depth intervals, are also 

 shown in Figure 5 (i.e, 0.57, 0.47, and 0.06 fish/count). 

 These data were converted to densities (1 quadrat 

 = 0.003 ha) such that from 50 to 100 fathoms an 

 average of 190 bottom fish are estimated to occur 

 per hectare of habitat. Similarly, in the two deeper 

 strata, estimated densities of 156 and 20 bottom 

 fish/ha occur. 



Given estimates of bottom fish density and depth- 

 specific estimates of total available habitat (Table 1), 

 estimates of the total standing crop of bottom fishes 

 at Johnston Atoll indicate that about 339,000 fish 

 occurred in the 50-100 fathom zone, 221,000 between 



"Ralston, S. Unpubl. data. Southwest Fisheries Center Hono- 

 lulu Laboratory, National Marine Fisheries Service, NOAA, Hono- 

 lulu, HI 96812. 



149 



