HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



Enewetak; but there may be a problem with All- 

 dredge and King's sampling technique. Their sam- 

 ples were taken with Plexiglas traps that rested on 

 the bottom and collected zooplankters that rose 

 into the water column at night; however, there 

 were gaps between the rigid lower edges of these 

 traps and irregularities on the sea floor. Conceiva- 

 bly, as Alldredge and King themselves recognized, 

 the samples could have included swimming or- 

 ganisms from the base of the surrounding water 

 column that entered the traps through these gaps. 

 These collections need to be repeated with this 

 possibility for error eliminated. While it would be 

 surprising if the numbers of calanoids they col- 

 lected had actually entered the traps through 

 these gaps, we are concerned that the only 

 calanoid identified in their samples, Acartia spp. 

 (listed as cyclopoids), are of a genus known to 

 include species that are exceedingly numerous in 

 the water column during both day and night (e.g., 

 Emery 1968; Hobson and Chess 1976). We would 

 expect organisms that live in the substrate by day 

 to have morphological features reflecting this 

 habit that distinguish them from holoplanktonic 

 relatives at the generic level or higher. So al- 

 though there may have been nearshore residents 

 among the calanoids whose numbers sharply in- 

 creased after dark at Enewetak, we believe that at 

 least most of them, especially the larger ones, ap- 

 peared following regular movements from deeper 

 water. 



The calanoids that visited the nearshore waters 

 after dark seemed to be part of a nocturnal move 

 shoreward made by many zooplankters, including 

 chaetognaths and larval fishes. Because each of 

 our primary collecting sites probably received noc- 

 turnal visitors from different sources, the two are 

 discussed separately. 



Walt Island 



Perhaps some of the nocturnal plankters that 

 visited the weak-current site were carried from 

 the open sea by the turbulent flow of water that 

 crossed the interisland reef at higher tides, but 

 this would have been a hazardous transit for most 

 zooplankters, and we doubt that significant num- 

 bers came this way. If many had come by this 

 incidental route, at least some would still have 

 been there al daybreak — probably somewhat dis- 

 oriented in these foreign surroundings. But they 

 were always gone by early morning twilight, 

 suggesting they followed a well-established pat- 



tern with consistent and predictable arrivals and 

 departures. 



Probably most of the nocturnal plankters that 

 visited Walt Island came from the deeper waters of 

 the lagoon, moving over the lagoon's shallow 

 periphery as part of a regular nocturnal rise into 

 the surface waters. The general rise of zoo- 

 plankters at night in lagoons of the Marshall 

 Islands has been documented (at Bikini by 

 Johnson 1 949; and at Majuro by Hobson and Chess 

 1973). It has also been noted that by day the mid- 

 lagoon is much richer in zooplankters than is the 

 shallow periphery (Gerber and Marshall 1974), 

 but a shoreward movement among zooplankters at 

 night would reduce this difference between the 

 two regions. Probably it is widespread that zoo- 

 plankters rising from the depths at night spread 

 out over shallow water near shore. At Kona, 

 Hawaii, where great depths lie adjacent to a coast- 

 al shelf (see Hobson 1974), one of us (E. Hobson) 

 often observed myctophids (lanternfishes), and 

 other deep-water forms, in <5 m of water close to 

 shore after dark (unpubl. obs.). 



Swimming to the Walt Island site from the 

 deeper water of the lagoon would usually entail 

 moving against the drift from the reef. Although 

 comparatively weak, this current would neverthe- 

 less obstruct small or weak-swimming forms. The 

 nocturnal shoreward movement of zooplankters 

 at this location, then, would favor the larger, 

 stronger swimming components of the 

 plankton — forms like chaetognaths, larval fishes, 

 and the larger calanoids. Likely for this reason 

 most of the calanoids among the increased num- 

 bers of zooplankters at Walt Island were >2 mm 

 (Table 3), whereas at Bogen Island, where zoo- 

 plankters were carried by currents, most of a much 

 greater number were 1 to 2 mm long (Table 8). 

 Distinction between the two locations is important 

 because it is the larger zooplankters that were 

 important prey of the nocturnal planktivores. Of 

 course, the upcurrent swim from deeper water 

 would take even the most mobile zooplankters 

 some time. Thus, it is significant that larger 

 calanoids were absent in the plankton collections 

 made at Walt Island 1 h after last evening light, 

 but were numerous in the collections made here at 

 midnight and later (Table 3). 



Bogen Island 



We presume that most of the zooplankton col- 

 lected in the flooding tidal currents at Bogen Is- 



149 



