FISHERY BULLETIN: VOL. 71, NO. 3 



site to follow developments there, while the 

 other accompanied the migration. The major 

 segment of the school did not move directly off- 

 shore; rather, each evening it headed westward 

 along the beach. It continued in this direction 

 until reaching the point of land at the western 

 end of the bay (Figure 4) and then veered off- 

 shore over the deeper water of the lagoon, where 

 contact with it was lost. 



1 KILOMETER 



JtOm 



Figure 4. — Route taken by the silverside when migniting 

 offshore from Arniel Island. The diurnal schooling site of 

 this fish is marked by "X." Soundings are in meters. 

 Adapted from U.S. Hydrographic Office chart 6087. 



Nocturnal Activity 



Later during the night the silverside were 

 found offshore in the lagoon, spread out just 

 below the surface of the water, with 2 to 4 m 

 between each fish. They remained there through- 

 out the night, feeding on plankton (discussed 

 below) ; although some of them ranged as much 

 as 2 km out into the lagoon, most occurred with- 

 in 1 km of the beach. Their distribution at 

 0400 h on 24 March, more than 2 h and 30 min 

 before sunrise, is illustrated in Figure 5. The 

 method used to obtain the distribution of silver- 

 sides in Figure 5 is as follows: Running a skiff 

 along Arniel Island, we established a speed that 

 took us the length of the Island in precisely 4 

 min. Maintaining this speed, we then headed 

 out into the lagoon from the center of the bay, 

 using a light on the opposite side of the atoll to 

 stay on course. Upon passing over the dropoff 



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ISLAND 



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16- 



23 



20 



8 



4 



3 » 















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Figure 5. — Distribution of silverside at the surface along 

 a transect running offshore from Arniel Isldnd at 0400 h 

 on 24 March 1972 (see text for method used). Adapted 

 from U.S. Hydrographic Office chart 6087. 



into deeper water, we used a hand light to count 

 the number of silversides in our path during 

 each minute of a 10-min run. These fish are 

 readily seen using this method owing to the 

 characteristic reflections from their silvery 

 bodies when struck by the light. 



At first morning light, about 1 h and 15 min 

 before sunrise, the silverside began to con- 

 centrate in the shoreward part of their feeding 

 ground. Soon it was apparent that the silver- 

 side were regrouping directly offshore from 

 where they had departed the island the previous 

 evening. 



The fish continued to regroup until about 30 

 min before sunrise, by which time the school 

 had attained daytime proportions and was 

 ready to return to its diurnal location. 



The Inshore Migration 



The lagoon was glassy-calm on each of the 

 three consecutive mornings that we witnessed 

 the inshore migration (23-25 March) so that 

 observations could be made from above the sur- 

 face of the water near the daytime schooling 

 site. By noting agitation on the water's sur- 

 face, the school was seen arriving each morning 



780 



