FISHERY BULLETIN: VOL. 75, NO. 2 



after sunset, i.e., during civil twilight) silverside 

 schools were breaking up, the individuals spread- 

 ing out just under the surface of the water. In 1972 

 and 1973 schools began to break up (increased 

 interfish distances became noticeable) between 14 

 and 16 min after sunset and were spread out by 18- 

 26 min after sunset (Table 2). 



In summary, with the approach of dusk, 

 predator attacks and silverside jumping increased 

 in frequency and intensity to peak during the 

 period of civil twilight, shortly after sunset, near 

 the pier. In the lagoon there was no peak activity; 

 the last attacks and jumps were recorded imme- 

 diately after sunset. Peak jumping near the pier 

 was recorded 5-15 min after sunset, just before the 

 time the silverside schools were observed to break 

 up becoming less polarized and cohesive (14-26 

 min after sunset, mean 19.1). In the lagoon, 

 however, attacks stopped before the prey schools 

 spread out; this may have been due to the shadows 

 and increased darkness caused by heavy vegeta- 

 tion along the northwest side of the lagoon. 



Silverside Behavior: Conclusions 



The temporal pattern of predatory attacks and 

 silverside jumping relative to sunrise was the 

 mirror image of that relative to sunset, at least for 

 the Lilipuna Pier area (Figures 1, 2). For each of 

 the four environmental situations studied, Figure 

 2 simplifies and graphically presents (at 50-min 

 intervals) the mean frequency of silverside jumps 

 illustrated in Figure 1. Midday (1000-1500 h) 

 jumps were combined and were not divided into 

 50-min intervals. Statistical comparisons (analy- 

 sis of variance, P=£0.05) of the jumping data for 

 sunrise ( -50 to +50 min), midday, and sunset 

 (-50 to +50 min) for each of the four situations 

 indicated that, at least for the shallow-water reef 

 area near Lilipuna Pier, the frequencies of jumps 

 at sunrise and sunset were similar and differed 

 from the number during midday. 



The mean time of school formation occurred just 

 prior to the beginning of civil twilight in the 

 morning, and the mean time of the breakup of 

 schools occurred just before the end of civil 

 twilight in the evening. Peak predator activity 

 occurred just after schools formed (mean time) in 

 the morning and just prior to their breakup (mean 

 time) in the evening. The data presented indicate 

 that related events (e.g., school formation versus 

 breakup) occurred in the study sites significantly 



E 



S 200 



3 

 O 



\ 



W Pier -Shallow 



. 



INTERVALS OF OBSERVATION (minutes relative 10 sunrise or sunset) 



FIGURE 2. — Mean frequency of Pranesus insularum jumps for 

 nine 50-min intervals (except midday). Based on data also pre- 

 sented in Figure 1. 



earlier (about 5-15 min) in the evening, relative to 

 sunset compared with the morning events, rela- 

 tive to sunrise (Table 2). This discrepancy may be 

 due to the shadow effect of the clouds and 

 mountains near Kaneohe Bay, which produce 

 evening twilight conditions 5-15 min earlier than 

 predicted, as discussed above. The relatively low 

 frequency of deepwater attacks near the pier in 

 the evening indicated that by the time silversides 

 had moved off the reef and/or spread out, it may 

 have been too dark for predators to see individual 

 silversides. In the morning, the lack of mountains 

 and vegetation and increasing light levels re- 

 sulted in sufficient light being available for 

 predators to see their prey. 



Observations of free-living and particularly 

 captive silversides, as well as my observations of 

 other schooling prey species (striped mullet, 

 Mugil cephalus, and Hawaiian anchovy, Stole- 

 phorus purpureus) in Hawaii, indicate that 

 predation is of prime importance in shaping the 

 behavioral patterns of prey species. When held 

 captive in the absence of predators for days or 

 weeks, individual prey in schools increased their 

 interfish distances and appeared to feed more 

 actively than they did in the field. When predators 

 were present, interfish distances within captive 

 schools were similar to interfish distances be- 

 tween individuals in the field. During the day, 

 schooling behavior appears to serve a protective 

 function for individuals, reducing the number of 



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