FISHERY BULLETIN: VOL. 76, NO. 1 



Collecting Where Currents Were Strong 



All collections at Bogen Island were made at the 

 height of flood tide, when currents often were too 

 strong to swim with the net, so here we worked 

 from a boat anchored fore and aft above the study 

 site. The net was secured to a line that passed from 

 the boat, through a block anchored on the reef 

 below, and returned to the boat. It was positioned 

 at the three collecting depths — bottom, mid- 

 depths, and surface — by pulling the line one way 

 or the other through the block (Figure 3). The 

 collections were extended to 15 min (compared 

 with 5 min at the other station) to reduce error 

 introduced by organisms taken during the few 

 seconds it took to raise and lower the net. In pre- 

 senting these data, however, we make the values 

 equivalent to a 5-min collection. These collections 

 depended on the current (which was measured 

 with every collection) to carry plankton into the 

 net, and the weakest current sampled, 15 cm/s, 

 was judged close to the minimum necessary. Two 

 series of collections were made during the day — at 

 midday and in midafternoon — and two series were 

 made at night — 1 h after last evening light and at 

 midnight. 



There are problems in comparing data collected 

 by these different methods at the two stations, but 

 we had the advantage of sampling precisely 

 defined positions — a critical requirement when 

 relating the plankton to food habits of specific 

 fishes. 



The volume of water filtered by this stationary 

 net varied with the different current velocities. 



which strongly influenced the numbers of 

 plankters taken. Nevertheless, these numbers ac- 

 curately reflect the relative numbers of plankters 

 available to fishes feeding in these currents. On 

 the other hand, differences in volumes of water 

 sampled must be considered when comparing es- 

 timates of the plankton in the water column from 

 one time or place to another. Therefore, plankton 

 volumes from the strong-current site are pre- 

 sented two ways: volumes actually sampled and 

 volumes adjusted for current differences. In ad- 

 justing for current, the volumes in all collections 

 were made equivalent to those taken in a net mov- 

 ing at the same relative speed that we pushed the 

 net at the Walt Island site — 28 cm/s. These ad- 

 justments also permit rough comparisons with 

 data from California (Hobson and Chess 1976), 

 where plankton were collected in the same way 

 and by the same swimmers. 



Fishes 



A total of 154 fish specimens of 16 species were 

 speared immediately after the plankton collec- 

 tions. Species names are those used by Schultz et 

 al. (1953, 1960), except where more recent taxo- 

 nomic study has indicated change. 



The specimens were preserved in 10% Forma- 

 lin^ immediately after collection. Later, food items 

 in the gut were identified and their positions in the 

 gut noted. The following data were recorded for 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Figure 3.— Collecting plankton at 

 Bogen Island, Enewetak Atoll, site of 

 strong currents. 



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