ually. They swam actively out of the gloom, 

 then veered away and circled for a short time 

 before disappearing. 



Continuing the descent, at approximately 

 180 m we became aware of the presence of a 

 large concentration of fish at the periphery of 

 the light field. The descent was stopped and we 

 maneuvered in an attempt to obtain visual and 

 photographic data. Each time the propulsion 

 motors were activated, the fish would hurriedly 

 retreat from view. Their movements were as 

 a disciplined school. Several times we ap- 

 proached the school with the lights out, a tech- 

 nique which enabled the submersible Alvin to 

 penetrate schools of myctophids in the Atlantic 

 (Backus et al., 1968). Although we were never 

 able to penetrate the school, the darkened sub- 

 mersible apparently attracted a few individuals 

 who remained near the windows when the lights 

 were switched on. Stunned by the sudden illu- 

 mination, they remained transfixed long enough 

 for good visual identification. The northern an- 

 chovy, with its projecting snout and thin body, 

 is sufficiently distinct from other local clupeoids 

 to give us confidence in our identification. Un- 

 fortunately, the photographs taken were blurred. 



Three more blue sharks were observed at the 

 same depth as the anchovy school. The deepest 

 sighting was at 275 m. Because these were ob- 

 served individually, the sightings could all have 

 been of the same individual. Possibly, one of 

 the same sharks sighted at 100 m may have fol- 

 lowed Deepstar during the descent. We also ob- 

 served a few squid, presumed to be Loligo opal- 

 escens, at the depth of the school. The spatial 

 proximity of the sharks, squid, and anchovy sug- 

 gests a predator-prey relationship, although no 

 predation was observed. 



The lower limit of the anchovy school was at 

 310 m and after dropping below it we descended 

 to 590 m. There we released the descent weight 

 and rose rapidly to the surface. The school was 

 observed between 300 and 200 m during the 

 ascent. The dive ended at 1407. 



There is little doubt of the relationship be- 

 tween the anchovies and the large targets on 

 the recording. It is difficult, however, to be sure 

 that there is a correlation between the sharks 

 and the discrete targets. Whatever is respon- 



sible for these traces has a high acoustic target 

 strength at 12 kHz. This can be seen by com- 

 parison on the figure with the trace made by 

 Deepstar, a 2-m-diameter air-filled sphere. 



Literature Cited 



Backus, R. H., J. E. Craddock, R. L. Haedrich, D. L. 

 Shores, J. M. Teal, A. S. Wing, G. W. Mead, and 

 W. D. Clarke, 



1968. Ceratoscopelus maderensis: peculiar sound- 

 scattering layer identified with this myctophid 

 fish. Science (Wash., D.C.) 160:991-993. 

 Strasburg, D. W. 



1958. Distribution, abundance, and habits of pe- 

 lagic sharks in the central Pacific Ocean. U.S. 

 Fish Wildl. Serv., Fish. Bull. 58:335-361. 



Isaac E. Davies 



Marine Bio-Science Division 



Naval Undersea Research and Development Center 



San Diego, CA 92132. 



Robert P. Bradley 



Arctic Marine 



3870 Sharon Drive 



W. Vancouver, B.C., Canada. 



THE USE OF CONCENTRATION INDICES 

 IN FISHERIES '- ' 



The temporal-spatial distribution of organisms 

 has generated considerable interest among bi- 

 ologists. This interest has generated many 

 studies which deal primarily with the distribu- 

 tion of a single organism in space. In this note 

 we consider the relationship between two organ- 

 isms in space or time. We are interested, in par- 

 ticular, in an index of the relation between a 

 predator (a fishing fleet) and a prey (the pop- 

 ulation to be harvested). We begin by writing 



^ Quantitative Science Paper No. 12. This series is 

 prepared under the general sponsorship of the Quanti- 

 tative Ecology and Natural Resource Management 

 Program supported by Ford Foundation Grant Number 

 68-183. 



" Portions of this study were undertaken while B. J. 

 Rothschild was affiliated with the Center for Quantita- 

 tive Science in Forestry, Fisheries, and Wildlife, Uni- 

 versity of Washington, Seattle. 



511 



