PREDATOR-PREY INTERACTIONS IN SCHOOLING FISHES 



DURING PERIODS OF TWILIGHT: A STUDY OF THE 



SILVERSIDE PRANESUS INSULARUM IN HAWAII 1 



Peter F. Major 2 



ABSTRACT 



Observations of free living and captive silversides were made in Kaneohe Bay, Hawaii, in October and 

 November 1972 and September 1973. The silversides demonstrated changes in schooling behavior 

 associated with changes in light levels during the periods of twilight. During morning twilight, 

 individual silversides formed schools, which in some areas moved from deep water to shallow water 

 over reefs. All silversides remained in large inactive schools in shallow water or along the edge of 

 channels throughout the day. During evening twilight, schools left the reef and/or broke up, with 

 individual silversides spreading out to feed near the surface. Predation upon the silversides, as 

 evidenced by their jumping behavior, was most intense during the twilight periods as schools formed 

 and broke up. Captive silversides, when not in the presence of predators, tended to increase their 

 interfish distance when in diurnal schools. The formation and breakup of schools of these silversides 

 appear to be very similar to behavioral patterns of related and unrelated species offish in many parts of 

 the world. The formation and break up of silverside schools appear to be related to the threat of 

 predation, the availability of the silverside's food, and the visual sensitivity and thresholds of both the 

 silversides and their predators. 



Daily twilight or crepuscular periods are critical 

 ones with respect to predator-prey interactions 

 between many species of fishes, at least in tropical 

 regions of the world. Hobson 1 1968, 1972), Collette 

 and Talbot (1972), and Domm and Domm (1973) 

 demonstrated the importance of twilight periods 

 on behavioral changes in reef fishes. Hobson 

 ( 1968, 1972, 1974) suggested that such transitions 

 in behavior are shaped by the threat of predation. 

 Predation pressure is also clearly a factor in the 

 evolution of schooling behavior in prey species 

 (Breder 1959, 1967; Hobson 1968; Shaw 1970; 

 Radakov 1973). Most reef fishes hide from their 

 predators amongst the interstices of the coral reef. 

 Many surface and open water prey species lack 

 such hiding places and appear to form schools as a 

 means of cover seeking (Williams 1964, 1966), the 

 school serving as a mobile biological refugium 

 especially during daylight hours. During evening 

 twilight periods many such schools break up with 

 individuals spreading out to feed. During morning 



'Hawaii Institute of Marine Biology Contribution No. 509. 

 From a thesis submitted in partial fulfillment of the require- 

 ments for the degree of Doctor of Philosophy. University of 

 California, Santa Cruz. 



2 Center for Coastal Marine Studies, University of California, 

 Santa Cruz, C A 95064; present address: Department of Biolog- 

 ical Sciences, Simon Fraser Universitv, Burnabv, B.C.. Canada 

 V5A 1S6. 



twilight periods individuals once again form 

 schools (Hobson 1968, 1972, 1973; Hobson and 

 Chess 1973). 



Vision has been shown to be important in the 

 maintenance of schools ( Woodhead 1966; Hunter 

 1968; Shaw 1970; Radakov 1973). In addition, 

 Munz and McFarland (1973) indicated that the 

 behavioral changes of tropical marine fishes 

 during periods of twilight are due to shifts in the 

 visual sensitivity of these fishes with changes in 

 light levels. 



The objectives of this study were to determine if 

 schools of the Hawaiian silverside, the iao, 

 Pranesus insularum, broke up and reformed in 

 response to light levels occuring during twilight, 

 and to determine how the activity of predators of 

 this species of silverside was related to this 

 behavior. 



Study Sites 



Field observations were made at two locations 

 within Kaneohe Bay, along the island of Oahu in 

 the Hawaiian chain. These sites were a 10,000 m 2 

 area of flat reef (water depth ^2 m at high tide ) 

 immediately adjacent to the east side of Lilipuna 

 Pier (Dock), and a 2,500 m 2 area near the central 

 portion of a dredged out (to a depth of 2-3 m) 



Manuscript accepted October 1976. 

 FISHERY BULLETIN: VOL. 75. NO. 2. 1977 



415 



