MAJOR PREDATOR PREY INTERACTIONS IN FISHES 



TABLE 2. — Comparison of .school formation and breakup mPranesus insularum with twilight phenomena recorded near Lilipuna Pier 



and HIMB lagoon, Kaneohe Bay, Hawaii. 1 

 (Mean school formation = -29.4 min (before sunrise), mean school breakup = +19.1 min (after sunrise).] 



'One-way ANOVA comparison of times of starting to school/schooling and starting to break up/complete breakup (P - 0.004) 



Midday (1000- 1 500 H, Local Time) 



In the pier area accurate counts of jumps made 

 by the silversides during the time between 1000 

 and 1500 h local time were usually difficult to 

 make due to waves caused by wind and nearby 

 vessel activity. 



Figure 1 presents the data collected during 

 representative midday periods near the pier when 

 interference was minimal. Generally, the silver- 

 sides formed large elongated schools (hundreds to 

 thousands of individuals) under or near the pier. 

 The schools were largely inactive except when 

 predators or potential predators such as barra- 

 cuda, lizardfish, jacks, and needlefish, approached 

 or attacked. When the tide level was low, the 

 schools condensed and moved into deeper water 

 near or under the end of the pier. 



In the lagoon area at HIMB, the behavior and 

 distribution of silversides was much the same 

 during midday as it was near the pier (Figure 1). 

 Small schools of silversides were strung out along 

 the sides of the channel. Large schools of hundreds 

 to thousands of fish were relatively inactive and 

 concentrated over deeper water in the center of 

 the lagoon. Barracuda and jacks were the most 

 frequent predators, but lizardfish and leather- 

 jackets were occasionally active in the lagoon. 



Evening Twi light 



As sunset approached, predator-prey activity 

 increased in frequency in the pier area (Figure 1). 



Peak activity occurred between sunset and the 

 end of the period of civil twilight and then declined 

 rapidly to stop just after the end of the nautical 

 twilight period. The silversides moved off the reef 

 along, but in the direction opposite to, the path 

 taken during the morning twilight movement 

 onto and across the reef. Attacks and jumping 

 occurred near the pier, then out over the reef, 

 moved eastward along and parallel to shore, 

 finally northward to the edge of the reef at the 

 easternmost end of the observation area. As 

 darkness increased, attacks and jumping grad- 

 ually diminished in frequency and intensity 

 (fewer individuals in smaller and fewer schools 

 jumped). 



In the lagoon area midday jumping activity in 

 shallow and deep water continued until just after 

 sunset, then stopped abruptly (Figure 1). The low 

 number of jumps in deep water in the late 

 afternoon and evening in the lagoon contrasts 

 sharply with the frequency of jumps in the earl}' 

 morning (Figure 1). This difference may be 

 related to the low levels of incident light striking 

 the surface of the lagoon in the afternoon and 

 evening due to the vegetation and the mountains 

 and clouds to the northwest obscuring the sun. In 

 the morning the lack of high vegetation and 

 mountains nearby to the northeast resulted in 

 light striking the lagoon's surface so that the 

 silverside were presumably visible to their pred- 

 ators. 



Light meter readings of 0.035-0.402 foot candle 

 (Table 1) were made during the time (20-24 min 



421 



