HOBSON and CHESS: MOVEMENTS OF PRANESUS PINGUIS 



Figure 3. — The lagoon shore of Arniel Island, viewed from the point at the western end of the bay in which 



this study was centered. 



column. Perhaps the outstanding characteristic 

 of the school at this time was the inactivity of its 

 members. 



The situation began to change late in the 

 afternoon, when fish in the school gradually 

 drew together at a point in the center of the 

 bay. By sunset the school was a compact unit, 

 concentrated just beneath the surface of the 

 water. 



As evening approached, the fish became in- 

 creasingly restless, and often the school was 

 briefly in motion, only to return to its original 

 position. The fish were highly responsive to 

 disturbances at this time. Repeatedly a segment 

 of the school exploded from a point within, so 

 that a void suddenly appeared. Sometimes this 

 maneuver could be traced to a predator— 

 usually a carangid or a belonid — but more often 

 no basis for the action was noted. 



The Offshore Migration 



Shortly after sunset the silverside migrated 



away from their daytime schooling site. We 

 recorded this phenomenon on all three occasions 

 when observations were made at this time of 

 day. 



The school could not be seen effectively from 

 out of water at this time of day, owing to the 

 fading light and wind-waves on the water's 

 surface. Consequently, we watched from, under- 

 water: once (21 March) from the center of the 

 school, when the fish around us began to migrate 

 14 min after sunset; once (22 March) at the 

 eastern edge of the school, when the fish in view 

 began moving out at 9 min after sunset; and 

 once (24 March) at the western edge, when the 

 observed fish got underway at 4 min after sun- 

 set. Because we made observations from three 

 widely separated parts of the school, these times 

 are not directly comparable. Nevertheless, on 

 each occasion the last of the silverside had left 

 the schooling site about 10 min after the first 

 individuals moved out. 



When the migration was recognized to be 

 underway, one of us remained at the schooling 



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