MAJOR: PRKDATOR-PREY INTERACTIONS IN FISHES 



that there may be a biological (circadian) rhythm 

 related to school formation and breakup and the 

 availability of specific food resources. Thus, the 

 breakup of schools may reflect a preemptory 

 predilection of individual silversides to spread out 

 and feed rather than remain within the safety of 

 compact polarized schools. Concurrently, pred- 

 ators are rapidly losing their ability to distinguish 

 individual silversides in the fading light, but their 

 presence remains a threat. 



During the morning the process is reversed as 

 light levels increase with predators becoming 

 increasingly active and presumably more success- 

 ful at capturing silversides. It is during relatively 

 short daily time spans within the periods of twi- 

 light that the silversides become particularly 

 vulnerable to certain predators. It is at these 

 times that the silversides are passing to or from a 

 period of feeding to a period of relative quiescence. 

 In some areas, exposure to predators may be 

 increased because the transition involves the 

 movement from one location to another. The 

 timing of such movements and the behavioral 

 changes that occur within schools appear to be 

 related to the threat of predation, the availability 

 of food and the visual sensitivity and thresholds of 

 both the silversides and their predators. 



ACKNOWLEDGMENTS 



I thank Edmund S. Hobson, Kenneth S. Norris, 

 John S. Pearse, Mary E. Silver, and an anonymous 

 reviewer for editorial advice. M. Gadsden of 

 Aberdeen University provided information con- 

 cerning twilight phenomena. My wife, Elaine A. 

 Major, typed and helped edit various drafts of the 

 manuscript. The figures were drafted by D. 

 Heinsohn of the University of California at Santa 

 Cruz, and the Audio Visual staff of Simon Fraser 

 University in Canada. I am particularly indebted 

 to the Edwin F. Pauley Fund for providing 

 financial assistance. 



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