FISHERY BULLETIN: VOL. 76, NO. 1 



The present paper considers these aspects of the 

 interactions among the plankton and adult plank- 

 tivorous fishes as expressed in the lagoon of a coral 

 atoll. It is based on a study over 21 days at 

 Enewetak, Marshall Islands, during April 1976. 



STUDY AREA 



Enewetak Atoll (lat. 1 1=26 ' N, long. 162°22 ' E) is 

 a ring of shallow coral reefs and low islands en- 

 circling a lagoon about 37 km north to south and 

 56 km east to west. It sits amid the westward 

 flowing North Equatorial Current and was buf- 

 feted throughout our visit (as during most of the 

 year) by trade winds from the east. So with surface 

 waters generally moving to the west, it was not 

 surprising that tidal currents in passes between 

 the open ocean and the lagoon on the windward 

 side of the atoll were strong on the flood, but weak 

 on the ebb. Furthermore, water over the windward 

 interisland reefs, driven by the incessant trade 

 winds and seas breaking over the outer reef, 

 flowed in just one direction — into the lagoon. Pre- 

 sumably the situation was reversed on the lee- 

 ward side of the atoll, as described for Bikini and 

 Rongelap, two other Marshallese atolls (von Arx 

 1948). 



From most islands, and interisland reefs, a nar- 

 row shelf of sand and isolated patch reefs extend 

 several hundred meters into the lagoon. At the 

 outer edge of this shelf, where the water in most 

 places is about 20 m deep, the sea floor drops 

 sharply to about 50 m, which is the approximate 

 water depth over much of the lagoon. Our study 

 centered on the lagoon's nearshore shelf along the 

 eastern (windward) side of the atoll, where the 

 waters are sheltered from the trade winds and 

 prevailing seas. Initially, we made observations 

 from Aoman Island in the north, to Enewetak Is- 

 land in the south — a distance of about 32 km. 

 Underwater visibility ranged from about 5 to over 

 30 m, and so at all times was suitable for observing 

 activity. From these observations we gained a 

 general impression of how the planktivorous 

 fishes were distributed, as well as something of 

 their activities. 



It was soon apparent that the distribution of the 

 planktivorous fishes was strongly influenced by 

 nearshore current patterns. This knowledge per- 

 mitted us to select fruitful locations for more in- 

 tensive work, including sampling the plankton 

 and gut contents of planktivorous fishes. Because 

 time was short, we limited intensive study to two 



134 



sites that represented opposing extremes in pre- 

 vailing current velocities, weak and strong — a 

 variance that proved to identify certain major 

 influences on fish-plankton interactions. 



Currents were weak or nonexistent at our site in 

 7 m of water among coral heads on level sand 

 about 100 m from Walt Island, close in the lee of 

 the interisland reef (Figure 1, site A). These weak 

 currents were most evident when water covered 

 the reef, and always flowed from the reef We made 

 observations here at all hours of day and night 

 during both spring and neap tides, and our collec- 

 tions sampled the full range of currents encoun- 

 tered, from no perceptible water movement to a 

 velocity of 9 cm/s. 



'^Bogen Is. 



,^, 



Japton 



i Is 





'"\^^5:> 



LAGOON 



I62°20' 



EAST CHANNEL 



Parry Is. 



PACIFIC 

 OCEAN 



Figure l.— The study area, Enewetak Atoll, Marshall Islands. 



Strong tidal currents fed by water entering the 

 lagoon through East Channel periodically swept 

 through our site in 13 m of water among coral 

 heads on gently sloping sand about 600 m wind- 

 ward of Bogen Island (Figure l,siteB). During our 

 sampling here, currents ranged from 15 to 90, x = 

 51, cm/s, always on flood tide. Observations (but 

 no sampling) were also made at this station at 

 slack water and during ebb tide when there was 

 little perceptible current. Although there was 

 scant evidence of an ebb current at the collection 



