HOBSON and CHESS: MOVEMENTS OF PRANESUS PING VIS 



ranked eighth, is consistent with its relative 

 numbers in the plankton on these two dates 

 (Table 2). As with other prey of the silverside, 

 the presence of C. discaudata in the night collec- 

 tions, and its absence from the day collections 

 (Table 2), probably can be attributed to a diel 

 vertical migration. 



Vndimilii i'//lgi/ris 



In the samples from 4 to 5 m deep, U. vulgaris 

 was equally abundant day and night; however, 

 it was numerous in the surface collections only 

 at night (Table 2). Thus, although this copepod 

 swims at the surface after dark, it seems to be at 

 least a few meters deeper during the day. This 

 conclusion is in agreement with Johnson (1949). 

 who found U. vulgaris abundant at or above 2 m 

 only at night and concluded that it descends to 

 deeper water in daylight. 



Based on the plankton hauls, U. vulgaris was 

 present in similar numbers on both nights — 

 with perhaps more at the surface on 23 March. 

 Consistent with this, it was ranked fifth in 

 the gut contents on the 23d, and sixth on the 

 24th. 



Other Prey 



The silverside preyed on many other forms at 

 the time of this study, but in fewer numbers 

 than the above (Table 1). Probably various of 

 these lesser prey, though of secondary impor- 

 tance here, assume major status under appropri- 

 ate conditions. In general, these other prey 

 have many of the characteristics of the major 

 prey: most are crustaceans or larval forms of 

 other groups, and generally they seem to rise 

 into the surface waters at night, then descend 

 to deeper water during the day (Table 2). 



Prey Size 



Undoubtedly size is important in determin- 

 ing whether or not a given organism is suitable 

 prey for the silverside. Based on the gut-content 

 analysis, most prey are between 0.5 and 3 mm 

 in their greatest dimension. Nevertheless, other 

 factors are also important. For example, the 

 size range of the abundant larvaceans in the 



plankton samples was precisely the size range 

 of most prey — 0.5 to 3 mm — and yet only a 

 single larvacean was found among the gut con- 

 tents. Obviously some characteristic of the larva- 

 ceans, apart from size, excluded them as prey 

 for the silverside. 



BEHAVIOR OF SMALL 

 SILVERSIDE 



Limited observations indicate that individuals 

 smaller than about 30 mm behave differently 

 than the larger fish, discussed above. During 

 the day the smaller fish do not .school with the 

 larger individuals along the beach; instead, 

 they occur in small schools 1 m or so in dia- 

 meter that swim in the upper part of the water 

 column 20 m or more away from shore. The fish 

 in these schools appear to be actively feeding, as 

 individuals are continually breaking away from 

 the school, snapping at objects in the water that 

 were too small to be seen by us, and then 

 immediately dashing back to the school — all in 

 one rapid, continuous movement. None of these 

 small active individuals were collected during 

 the day for gut-content analysis; however in 

 the collections offshore at night, in which the 

 64 larger individuals were collected for the gut- 

 content analysis (Table 1), 5 individuals less 

 than 30 mm long also were taken (3 on 23 

 March; 2 on 24 March). Significantly, although 

 the guts of all 64 larger individuals contained 

 food, the guts of all 5 small individuals were 

 empty. 



These limited data suggest that the small 

 silverside may feed during the day but not at 

 night. 



CONCLUSIONS 



1. During the day in the Majuro Atoll lagoon 

 silverside larger than 35 mm long assemble in 

 relatively inactive schools along the beaches. 

 The.se fish do not feed at this time. 



2. Shortly after sunset these schools migrate 

 away from shore, following the same route day 

 after day. 



3. Once out into the lagoon the schools dis- 

 perse, and the fish spread out just under the 

 water's surface. They feed throughout the night, 



785 



