RALSTON ET AL.: BOTTOM FISH RESOURCE AT JOHNSTON ATOLL 



ing individual contingency table cells showed that 

 the greatest contribution to the total chi-square was 

 for P. filamentosus (58% of total). Specifically, under 

 the hypothesis of independence, 16.5 were expected 

 downcurrent but only 5 were caught, while 26.5 were 

 expected upcurrent where 38 were landed. The ap- 

 parent surplus of P. filamentosus along the eastern 

 exposure, where trade winds prevail and oceanic cur- 

 rents first impact the atoll (Barkley 1972), may relate 

 to this fish's habit of feeding on large deepwater 

 plankton, especially salps (genus Pyrosoma). Bray 

 (1981) has shown that small resident planktivores 

 will travel to the upcurrent edge of a reef to access 

 pelagic plankton. The distribution of P. filamentosus 

 at Johnston Atoll may represent a similar situation 

 on a much larger scale. 



Bottom Fish Catch Rate 



One-way analysis of variance (ANOVA) of CPUE 

 data was used to examine whether geographical dif- 

 ferences exist in bottom fish abundance, i.e, the two 

 treatment classes were upcurrent and downcurrent 

 regions (see above). The ANOVA was insignificant 

 (F = 1.62, df = 1, 21, P = 0.21), although the mean 

 catch rate along the eastern exposure (5.6 bottom 

 fish/line-h) was 60% greater than downcurrent (3.5 

 bottom fish/line-h). This result suggests the lack of 

 significance may have been due to small sample size 



The CPUE data were analyzed by time of day to 

 determine if catchability fluctuates through the day. 

 The results in Figure 6 show that fishing was 

 distinctly better during the morning than afternoon. 

 In this figure individual values of drift CPUE (n = 

 23) have been plotted against the midpoint of the 

 drift time interval. The solid line represents aggre- 

 gate catch rates, calculated by pooling both catch and 

 effort statistics from all areas into 1-h intervals and 

 then forming CPUE ratios. Different symbols repre- 

 sent each of six separate fishing locations (Fig. 1). 

 Note that catch rates were highest when fishing 

 began each day and consistently declined to a low 

 during the midafternoon. The data further indicate 

 that catch rates may increase again with the onset 

 of the evening crepuscular period, although the data 

 are meager. This pattern was evident both within 

 and among the six sites fished and, when averaged 

 out, resulted in morning catch rates 2.07 times 

 greater than afternoon rates. 



Catchability 



Having the Makalii and Townsend Cromwell at 

 Johnston Atoll at similar times prompts comparison 



15 -i 



10 



Q. 

 (J 



5- 



O- 1 



I 1 1 1 1 1 1 1 1 



0800 1000 1200 1400 1600 



Time of Day 



Figure 6.— The effect of time of day on the catch rate of bottom 

 fish at Johnston Atoll. Catch rates calculated for each drift of the 

 vessel and presented for each of six different fishing stations (see 

 Figure 1). 



of the assessment techniques. We assume that in the 

 1-mo interim between visits no changes occurred in 

 overall levels of abundance, because Johnston Atoll 

 is a National Wildlife Refuge where no fishing is per- 

 mitted and the fishes are typically long lived (Ralston 

 and Miyamoto 1983; Ralston see footnote 8). Any 

 differences in assessment are then likely due to dif- 

 ferences in method. 



To compare surface estimates of bottom fish abun- 

 dance with those derived from submersible surveys, 

 we matched fishing stations (numbers) with submer- 

 sible dives (letters) which occurred nearby (Fig. 1). 

 Specific pairings were F-l, E-2, B-3, H-4, 1-5, and D-6. 

 For each dive the overall abundance of bottom fish 

 was estimated by forming the ratio of total fish 

 counted to total number of quadrat counts, and then 

 converting to density measured in bottom fish/ha. 

 The CPUE statistics were used to estimate abun- 

 dance for each fishing station, after correcting for 

 fluctuating catchability (Fig. 6). The result is pre- 

 sented in Figure 7. There is a positive correlation 

 between CPUE and bottom fish density (r = 0.54), 

 although it is insignificant. 



One means of estimating catchability, q, is to deter- 

 mine the slope of the regression of CPUE on stock 

 density. We estimated the functional regression 

 (Ricker 1973) of the data presented in Figure 7 (solid 

 line) and determined that q = 0.0215 ha/line-h. A 

 second estimate of q is obtained by forming the ratio 

 of the average catch rate of bottom fish at the atoll 



151 



