MA.inK PREDATOR-PREY INTERACTH )\S I \ FISHES 



twilight, interfish distances decreased as polar- 

 ized schools once again formed and moved in 

 coordinated patterns as they did in the field. 



Silverside Jumping Activity Patterns 



Morning Twilight 



In the Lilipuna Pier area prior to nautical 

 twilight, I could hear jumping silversides and the 

 "pop" associated with attacking predators strik- 

 ing the water's surface approximately 20 min 

 after the observation periods had commenced and 

 95 min prior to sunrise (Figure 1). These jumps 

 were made primarily by individual fish in close 

 proximity to the pier in the channel near the edge 

 of the reef. Jumping occurred later by increas- 

 ingly larger numbers of individuals in schools at 

 the easternmost end of the observation area. 

 Jumps occurred initially near the edge of the reef, 

 moved toward, then turned northwest parallel to 

 and along the shore, finally spreading out over the 

 reef and toward the pier. These attacks by 

 predators and jumps of silversides sequentially 

 traced three sides of the perimeter of a rectangle 

 defining the east, south, and west boundaries of 

 the observed area near the pier. Attacks and 

 jumps in shallow water over the reef pre- 

 dominated after the beginning of nautical twi- 

 light, and by sunrise all attacks and jumping 

 occurred within a few meters of the pier. Peak 

 activity in shallow reef and deep channel water 

 was recorded just after the beginning of civil 

 twilight and steadily decreased to midday levels 

 (Figure 1). 



The only predators observed to attack the 

 silversides over the reef in the early morning were 

 lizardfish. Blue jacks and barracuda were ob- 

 served in the channel and occasionally over the 



reef near sunrise and during the late morning. 



In the lagoon area, jumps in the central deeper 

 area of the lagoon were initially recorded 45 to 50 

 min before sunrise (Figure 1). As twilight pro- 

 gressed, jumping was eventually seen in narrow 

 bands of shallow water along the sides of the 

 lagoon, but occurred infrequently. Barracuda and 

 jacks were the principal early morning predators, 

 although lizardfish were also observed attacking 

 the silversides. Since the shallows were relatively 

 small in area, most of the silversides were 

 concentrated over the central deeper water of the 

 lagoon. A period of increased jumping activity did 

 not occur in the lagoon during twilight as it did 

 near the pier. 



Light meter readings of 0.096-0.402 foot candle 

 (Table 1) were made in 1973 during the time ( 18- 

 24 min before sunrise, i.e., the time of civil 

 twilight) when silversides were in the process of 

 forming schools, especially in the lagoon area. 

 Initial schooling became noticeable (individuals 

 moving closer together, becoming more cohesive 

 and polarized when swimming as they did during 

 the day) in 1972 and 1973 as early as 44-23 min 

 before sunrise and was completed as late as 33-18 

 min before sunrise (Table 2). Silversides then 

 remained in schools throughout the day. 



In summary, during the morning, predator 

 attacks and silverside jumping could not be 

 detected until 95 min before sunrise at the pier 

 and 50 min before sunrise in the lagoon. Deep- 

 water attacks were initially noted for individual 

 silversides, but subsequently increased numbers 

 of jumps were recorded in shallower water for 

 increasingly larger schools, especially near the 

 pier. During the time peak jumping occurred (30- 

 10 min before sunrise), silversides were forming 

 cohesive polarized schools (44-18 min before 

 sunrise, mean 29.4 min). 



Breakup 

 of schools 



Formation 

 of schools 



Steven 1959 West Indies 



Shaw 1961 



This report 

 Sept. 1973 



This report 

 Sept. 1973 



Marine Biological 

 Laboratory, Mass. 



Kaneohe Bay, 



Hawaii 

 Kaneohe Bay, 



Hawaii 



Hepsitia 

 stipes 



Menidia 



Pranesus 

 insularum 



Pranesus 

 insularum 



0.06 0.07-0.05 2 Fish in aquariums indoor with windows and door 



closed, no artificial light. Watched until 



nightfall. 

 0.12 0.35-0.03 14 Experimental; gradual reduction of light until 



school began dispersing. Used neutral density 



filters. 

 0.21 0.402-0.035 3 Field, during evening twilight. 



0.18 0.402-0.096 4 Field, during morning twilight. 



'One-way analysis of variance (ANOVA) of all light meter readings (P = 0.57). 



419 



