AN ECOLOGICAL SURVEY AND COMPARISON OF 



BOTTOM FISH RESOURCE ASSESSMENTS 



(SUBMERSIBLE VERSUS HANDLINE FISHING) AT JOHNSTON ATOLL 



Stephen Ralston, 1 Reginald M. Gooding, 1 and Gerald M. Ludwig 2 



ABSTRACT 



The deep slope (100-365 m) environment at Johnson Atoll in the central Pacific was surveyed with a submer- 

 sible and the standing crop of commercially important bottom fishes (i.e, lutjanids, serranids, and carangids) 

 estimated by visual quadrat censusing. Results are compared with an assessment made by hook-and-line 

 fishing. 



Overall, 69 species of fish were recorded from the submersible and 10 from fishing. Well over half 

 of the sightings from the submersible were new locality records. Bottom fish abundance estimates (fish/hec- 

 tare and fish/line-hour) varied by site but agreed broadly with one another. Tbgether they are used to 

 estimate catchability (0.0215 hectare/line-hour), which is shown to vary through the day. 



Bottom fish were contagiously dispersed along both vertical and horizontal dimensions, with increased 

 numbers of the snapper Pristipomoides filamentosus in upcurrent localities. On a finer scale this species 

 and Etelis coruscans were aggregated near underwater promontories and headlands, but at different 

 depths. 



Numerous observations concerning the deep slope environment of this central Pacific Ocean atoll 

 are included. 



Perhaps the most widespread precept in fisheries 

 today is the supposition that catch rate is propor- 

 tional to stock abundance (Gulland 1974; Ricker 

 1975; Pitcher and Hart 1982). Even so, there are 

 numerous studies which demonstrate exceptions to 

 this assumption (see for example MacCall 1976; Ban- 

 nerot and Austin 1983). A departure from linearity 

 in the relationship of these two variables reflects 

 varying catchability. This variation may be due to 

 schooling behavior, gear saturation, or any number 

 of other factors which affect catch per unit effort 

 (CPUE) in addition to stock abundance (Rothschild 

 1977). It is often difficult, if not impossible, to 

 evaluate the validity of the linearity assumption in 

 most practical situations. A multiple approach to 

 stock assessment has often been suggested as a 

 means of circumventing this problem, including the 

 use of hydroacoustics (Barans and Holliday 1983; 

 Thorne 1983), underwater television-diver surveys 

 (Powles and Barans 1980), and submersibles (Uz- 

 mann et al. 1977) to corroborate CPUE data. Con- 

 sistency in results among a set of independent 

 assessment techniques is necessary for validation 

 and verification of data. 



'Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, Honolulu, HI 96812. 



2 U.S. Fish and Wildlife Service, Honolulu, HI 96850; present ad- 

 dress: Florida Fishery Research Station, U.S. Fish and Wildlife Ser- 

 vice, P.O. Box 1669, Homestead, FL 33030. 



Submersibles in particular have also proven useful 

 in studying the distribution of fishes in various deep- 

 water habitats (Brock and Chamberlain 1968; Stras- 

 burg et al. 1968; Colin 1974; Shipp and Hopkins 

 1978), in identifying nursery grounds of commercial- 

 ly important rockfish species (Carlson and Straty 

 1981), and in assessing the effectiveness of baited 

 longline gear (High 1980; Grimes et al. 1982). In 

 many situations submersibles provide an ideal means 

 of independent assessment (Uzmann et al. 1977) if 

 questions concerning bias in visual surveys can be 

 adequately addressed (Colton and Alevizon 1981; 

 Sale and Douglas 1981; Brock 1982). 



The purpose of this study was to examine the 

 distribution and abundance of tropical deep slope 

 bottom fishes (i.e, lutjanids, serranids, and carangids) 

 at Johnston Atoll in the central Pacific Ocean with 

 a research submersible and to compare the results 

 with an assessment made by fishing. This compari- 

 son provides not only a basis for testing the validity 

 of a CPUE statistic, but also for estimating the 

 catchability coefficient. Both are important issues 

 because of the widespread use of hook-and-line catch 

 and effort statistics in resource assessments of bot- 

 tom fish stocks worldwide (Moffitt 1980; Ralston 

 1980; Ivo and Hanson 1982; Ralston and Polovina 

 1982; Munro 1983; Forster 1984). Of special interest 

 was determining the relationship between CPUE 

 and visual estimates of bottom fish standing stock. 



Manuscript accepted April 1985. 



UlCirfDV DITI T ITTTTM. VOI O A M(l 1 1QOC 



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