FISHERY BULLETIN: VOL. 84, NO. 4 



ity and population density of a deepwater caridean 

 shrimp, Heterocarpus laevigatas. This circumglobal 

 species is found in depths of 400-950 m in subtropical 

 and tropical latitudes (King 1984). Experimental 

 trapping surveys have shown it to be abundant at 

 widespread localities in the central and western 

 Pacific (King 1983), and a developing commercial 

 fishery for this species has emerged in the Hawaiian 

 Islands (Gooding 1984). Interest by Pacific island na- 

 tions in promoting the harvest of this shrimp is great 

 (King 1981), providing the impetus for an assess- 

 ment of the Heterocarpus resource in the Mariana 

 Archipelago. Additional results of this research 

 program are reported elsewhere (Moffitt and 

 Polovina 2 ). 



MATERIALS AND METHODS 



Intensive fishing for Heterocarpus laevigatus was 

 conducted in an area 3.5 km off the north end of 

 Alamagan Island in the western Pacific (lat. 

 17°39'N, long. 145°50'E). Alamagan is part of the 

 Commonwealth of the Northern Mariana Islands, 

 lying 450 km north of Guam (Fig. 1). It is small, 

 uninhabited, and of recent volcanic origin. While the 

 ocean bottom slopes steeply away from the island 

 at an angle of 25° to the east, south, and west, a 

 broad shelf, approximately 6.5 km 2 in area and 

 lying 600-800 m deep, extends well off the north end 

 of the island. This shelf was selected as a study site 

 because 1) good catches of H. laevigatus were pre- 

 viously obtained in the area, 2) 700 m is an ideal 

 target depth for trapping this species (Moffitt and 

 Polovina fn. 2), 3) the relatively uniform bottom 

 topography would facilitate setting and retrieving 

 fishing gear, and 4) the area had no known history 

 of prior exploitation. 



Fishing was conducted over a 16-d period, 9-24 

 January 1984, from the NOAA ship Townsend 

 Cromwell. Shrimp traps of standard Honolulu Lab- 

 oratory design were set daily in four strings of five 

 traps each. All traps were half round in shape (91 

 x 66 x 46 cm), with a frame constructed of 1.27 

 cm reinforcing steel, covered with 1.27 x 2.54 cm 

 mesh hardware cloth (illustrated in figure 3 of 

 Gooding 1984). Individual traps within a set were 

 spaced 40 m apart and were baited with three 

 chopped Pacific mackerel, Scomber japonicus. All 

 traps were set between 1100 and 1300 h in 600-800 



2 Moffitt, R. B., and J. J. Polovina. In prep. Distribution and 

 yield of the deepwater shrimp resource in the Marianas. South- 

 west Fisheries Center Honolulu Laboratory, National Marine 

 Fisheries Service, NOAA, 2570 Dole Street, Honolulu, HI 

 96822-2396. 



m and were retrieved the following day between 

 0800 and 1100 h. In addition, a large (150 x 150 

 x 150 cm) pyramidal commercial shrimp trap was 

 sometimes deployed alone. 



When fishing gear was recovered, the traps were 

 individually emptied and the contents sorted, 

 counted, and weighed to the nearest 0.01 kg by 

 species lot. On three occasions a random length- 

 frequency sample of trap-caught H. laevigatus was 

 saved for later study. All shrimp in these samples 

 were measured to the nearest 0.1 mm CL (carapace 

 length) with dial calipers. 



To accurately delimit the bottom topography of 

 the study area, an unregistered reconnaissance 

 hydrographic survey was conducted over the site on 

 9 February 1984, with the Townsend Cromwell. 

 Depth soundings from a Raytheon 3 fathometer were 

 recorded every 3 min over an 8.5-h period as the 

 vessel ran a predefined cruise track which covered 

 the entire study area. The position of the vessel was 

 recorded to the nearest 0.01 min at each sounding. 



The Townsend Cromwell returned to the study site 

 again from 12 to 16 May 1984, to assess the recovery 

 of the H. laevigatus population in the study area and 

 to determine the effect of different baiting practices 

 on CPUE. Four sets of six traps each were set over- 

 night on each of four occasions. Half these traps con- 

 tained three chopped Pacific mackerel whereas the 

 other half (i.e., every other one) contained two whole 

 Pacific mackerel. The catch was sorted and treated 

 as discussed previously. 



RESULTS 



Hydrographic Survey 



A total of 164 depth soundings were obtained over 

 the study site. The data were contoured using the 

 GCONTOUR procedure of SAS/GRAPH (SAS 1981) 

 and the resulting chart is presented in Figure 2. 

 Solid lines represent isobath contours spaced at 200 

 m depth intervals. Note that the shrimp study site 

 is a saddle point; concave upwards along the north- 

 south axis and concave downwards from east-west. 

 The hydrographic survey revealed a small but steep 

 pinnacle and a deep canyon immediately adjacent 

 to the study area. 



In the figure the locations of each string of five 

 standard traps are shown as open circles (n = 60) 

 whereas single sets of the large pyramid trap are 

 given as closed circles (n = 8). Fishing effort was 



3 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



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