FEEDING ORIENTED MOVEMENTS OF THE ATHERINID FISH 

 PRANESUS PINGUIS AT MAJURO ATOLL, MARSHALL ISLANDS 



Edmunds. Hobson and James R. Chess' 



ABSTRACT 



The feeding behavior of the atherinid Pranesus pinguis was studied in the lagoon at Majuro 

 Atoll, Marshall Islands, during March 1972. 



During the day, individuals larger than about 35 mm (standard length) assemble in 

 relatively inactive schools along the shore. Shortly after sunset the schools migrate offshore to 

 feeding grounds in the lagoon, following the same route night after night. Once out in the 

 lagoon the schools disperse, and individuals 2 to 4 m apart feed on plankton at the water's 

 surface throughout the night. Their prey include hyperiid amphipods, caridean shrimp larvae, 

 myodocopid ostracods, the tretomphalus stage of foraminiferans, and calanoid copepods. 

 Most of their prey are at the surface at night, but at greater depths in daylight. At first 

 morning light the silverside begin to concentrate in the shoreward part of their feeding 

 ground. Then about 45 min before sunrise, having reached daytime proportions, the school 

 moves inshore over the same route it had taken outward the night before and at about 20 

 min before sunrise arrives at its diurnal schooling site to take up its relatively inactive day- 

 time mode. 



In contrast, limited evidence indicates that individuals smaller than about 30-mm standard 

 length feed by day but not at night. 



The silverside Pranesus pinguih (Lacepede) is 

 widespread in the western Pacific and Indian 

 Oceans (Schultz, 1953) and is a potential bait- 

 fish for tuna fishermen (Hida, 1971). Because 

 biological information of the species does not 

 exist, we studied its feeding activity in the la- 

 goon at Majuro Atoll, in the southeastern 

 Marshall Islands, from 16 to 25 March 1972. 



Majuro Atoll is a ring of shallow coral reefs 

 and low islands encircling a lagoon about 40 km 

 east to west and 10 km north to south (Figure 

 1). Around the perimeter of the lagoon a narrow 

 shelf of sand and isolated coral heads extend 

 offshore under 1 to 6 m of water. At the outer 

 edge of this shelf, several hundred meters from 

 shore, the sea floor falls abruptly to a depth of 

 40 m, which is the approximate water depth over 

 most of the lagoon. 



The study was centered on silverside that 

 schooled during the day (Figure 2) along the 

 shore of Arniel Island (Figure 3). The behavior 

 of fish in this school was judged typical of that 



' National Marine Fisheries Service, Tiburon Fisheries 

 Laboratory, Tiburon, CA 94920. 



seen in other members of the species during a 

 limited survey along the shores of other islands 

 bordering the lagoon. 



Movements and other behavior of this fish 

 were observed directly using a face plate and 

 snorkel at all hours of day and night over the 10 

 days we spent at Majuro. Specimens were 

 collected for analysis of gut contents, and plank- 

 ton samples were taken in the lagoon during 

 both day and night. Sampling methods and pro- 

 cedures are described below where pertinent. 

 All lengths given for the silverside are standard 

 length. Because the horizon frequently was 

 obscured by clouds, times of sunrise and sunset 

 were calculated using the Nautical Almanac, 

 U.S. Naval Observatory. 



ACTIVITY PATTERN 



The Diurnal School 



During each day, individuals about 35 to 65 

 mm long hovered in a quiet school that was 

 strung out along the shore. Here, where it is 1 to 

 2 m deep, the school occupied most of the water 



Manuscript accepted December 1972. 

 FISHERY BULLETIN: VOL. 71, NO. 3, 1973. 



777 



