FISHERY BULLETIN: VOL. 71, NO. 2 



water trawl samples taken in the Central Pacific 

 Ocean near Hawaii. The majority of the samples 

 were collected during four series which at- 

 tempted to cover the upper 1,000 m day and 

 night at 3-mo intervals over a period of 1 year. 

 The aims of the sampling were to determine the 

 vertical migration patterns of the species present 

 and to examine changes in abundance, size 

 composition, and reproductive state with depth 

 and season. In addition, changes in vertical 

 distribution and avoidance with changing phases 

 of the moon were investigated, and the catching 

 ability of a 10-ft Isaacs-Kidd mid-water trawl 

 was compared with that of a much larger 

 anchovy trawl. The results allow detailed 

 description of many aspects of the life history 

 and ecology of myctophids. It is possible from 

 these data to consider interspecific relation- 

 ships with respect to time and space and to 

 estimate the importance of myctophids and 

 other micronekton in the tropical, open ocean 

 ecosystem. 



MATERIALS AND METHODS 



A total of 221 mid-water trawl samples were 

 taken for this study. Fifty-four were taken with 

 a 6-ft Isaacs-Kidd (IK) mid-water trawl, 157 

 with a 10-ft IK trawl, and 10 with a modified 

 Cobb pelagic trawl (CT). All trawls were fished 

 without opening-closing devices. Thirty-seven 

 IK tows were taken during September and 

 November 1969 to provide preliminary data. 

 The remaining tows consisted of four general 

 types: a series of 31 6-ft IK tows taken in 

 July 1970 to check various aspects of sampling, 

 four series of 26-35 10-ft IK tows each taken 

 at 3-mo intervals from September 1970 to 

 June 1971, 17 10-ft IK tows taken in Sep- 

 tember and October 1971 to determine effects 

 of moon phase on sampling, and 10 CT tows 

 taken in conjunction with the 10-ft IK tows 

 during February-March 1971. 



All tows except those of November 1969 were 

 taken west of the island of Oahu, Hawaii, 

 roughly along a line between lat. 21°20'N; 

 long. 158°20'W and lat.21°35'N; long. 158°35'W. 

 The depth of water for all tows was 1,800 m 

 (1,000 fm) or more. BT casts to 300 m made 

 during the study indicated that seasonal tem- 



perature changes in the study area were nearly 

 the same as for a nearby station (lat. 22°10'N; 

 long. 158 °W) studied more thoroughly by Gor- 

 don (1971). Presumably the depth profiles of 

 other physical-chemical factors given by Gordon 

 (1970, 1971) are also representative of the study 

 area. The November 1969 tows were taken near 

 Gordon's station. 



The IK trawls were of standard dimensions 

 and were lined with 6.35 mm {V4 inch) knotless 

 nylon mesh anteriorly and 4.75 mm (3/16 inch) 

 knotless nylon mesh posteriorly. The 10-ft IK 

 terminated with a 1.0-m diameter plankton 

 net of 333 id nitex; the 6-ft, with a 0.5-m diameter 

 net. The CT, described in detail by Higgins 

 (1970), had 19-mm stretch mesh in the main 

 body and a cod end lined with 6.35-mm mesh. 

 Diver observations of the CT have indicated 

 that the mouth opening under tow is about 12 m 

 wide by 8 m high (Higgins, 1970). 



IK trawls were fished from the University 

 of Hawaii RV Teritu. Unless otherwise noted, 

 the procedure was the same for all tows. The 

 trawl was shot with the ship moving about 1.75 

 m/sec (ca. 3.5 knots), and the cable was paid 

 out so that just enough tension was maintained 

 to keep the net from fouling. For deeper (over 

 300 m) tows, the ship was slowed to ca. 1.0 

 m/sec for a few minutes after the cable was 

 paid out to allow the trawl to sink farther. These 

 procedures were designed to minimize forward 

 motion while the trawl descended to towing 

 depth. The trawl was towed at depth for 2-3 

 hr at a speed of about 1.75 m/sec. For retrieval, 

 the ship was slowed to ca. 1.0 m/sec, again to 

 minimize forward motion while the trawl was 

 above sampling depth. Cable was retrieved as 

 fast as possible; the average rate varied between 

 30 and 60 m/min. The rate was considerably 

 faster than 60 m/min when there was more 

 than 1,000 m of wire out. The CT was fished in 

 a similar manner from the National Marine 

 Fisheries Service (NMFS) RV Townsend 

 Cromivell. 



Depth of tow was determined by time-depth 

 recorders (Benthos, Inc.)- attached to the trawl. 

 For depths of less than 300 m the trawl reached 



^ Reference to trade names does not imply endorsement 

 by the National Marine Fisheries Service, NOAA. 



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