CLARKE: ECOLOGY OF LANTERNFISHES 



towing depth within a few minutes after cable 

 was paid out and usually stayed within 5 m of 

 this depth during the entire tow. For deeper 

 tows, the trawl frequently took some time to 

 descend to maximum depth and often moved up 

 and/or down gradually during the tow. In all 

 cases, however, a single, most frequently fished 

 depth was assigned to each tow. 



For some IK tows an acoustic telemeter 

 (Benthos, Inc.) was used to determine depth 

 while underway and to reach desired depth, but 

 in most cases the amount of cable required to 

 reach a given depth had to be estimated before- 

 hand. This method required some practice, and, 

 needless to say, depth coverage was more even 

 for the later cruises. 



Towing speed for the IK was, in some cases, 

 measured by a TSK flowmeter attached to the 

 trawl and rigged to signal, through the tele- 

 meter, every 500 revolutions. In all cases, 

 position of the ship was determined by visual or 

 radar fixes at V2- to 1-hr intervals during the 

 tow and speed calculated from these data. The 

 towing speed for the IK varied between 1.54 

 and 1.98 m/sec with most tows about 1.75 

 m/sec. The CT was towed at about 1.5 m/sec. 



Assuming 100% filtering efficiency and a 

 speed of 1.75 m/sec, the 10-ft IK sampled about 

 9.4 X 10^ m'^ per 2-hr tow. At 1.5 m/sec, 

 the CT sampled just over lO** m^ per 2-hr tow 

 or about 11 times the IK tows. Only fish over 

 10-mm long were considered sampled quantita- 

 tively by the IK since smaller individuals could 

 pass through the meshes. Larger fish, up to 

 25-30 mm long, appeared to escape through the 

 CT meshes. 



The July 1970 (5-10 July 1970) series of tows 

 with the 6-ft IK were designed primarily to 

 determine whether significant changes in verti- 

 cal distribution occurred in the upper layers 

 during the course of night and to obtain an 

 estimate of sample variability. At depths of 

 50 m and 100 m, five 2-hr tows were taken from 

 dusk to dawn on a single night, and one replicate 

 tow was made during the following night. Dusk, 

 dawn, and night tows were taken at other depths 

 in the upper 400 m. In addition, day tows were 

 taken between 500 and 1,125 m. 



The four quarterly series of lO-ft IK tows 

 were designed to sample for seasonal changes 



and were intended to be replicate surveys of 

 the upper 1,000 m both day and night. Since 

 preliminary day tows caught no nonlarval 

 myctophids above 300 m, no attempt was made 

 to cover the upper layers during the day. The 

 dates of the cruises were 14-17 and 20-24 

 September 1970; 8-10 and 13-17 December 

 1970; 26 February-3 March and 19 March 

 1971; and 8-11 and 15-19 June 1971. The series 

 will subsequently be called September 1970, 

 December 1970, March 1971, and June 1971. 

 Depths sampled for these series and the July 

 1970 series are given in Figure 1. 



In addition to some gaps in depth coverage 

 during September 1970, the shallow night 

 tows were made at both full and new moon. 

 Subsequent analyses showed that the depth 

 distribution and avoidance for many species 

 changed considerably with moon phase, and as 

 a consequence, some populations were sampled 



JULY '70 

 D N 



200- 



400 



600 



UJ 



o 



800- 



1000 



1200 - 



^AAA 



A 



-■CA. 



A 



- -A 



SEPT '70 DEC 

 D.N D 



0-- 



•70 

 N 



MAR 



D 



-A 

 A 



A 

 A 



OQ. 

 O 



O 

 O 



o. 



•71 JUNE '71 

 N D I N 



--i 



O -- 



o 



o 

 o 

 o 



o 

 o 



00 



o 



Figure 1. — Depths of Isaacs-Kidd trawl samples taken 

 during July 1970, September 1970, December 1970, 

 March 1971. and June 1971. Day samples are designated 

 by circles, night samples by triangles. 



403 



