"The nature of the trawling gear * * * did not per- 

 mit or justify an exact quantitative evahiation of 

 the catcli." The Isaacs-Kidd and simihir niixed- 

 mesh midwater trawls seem to be much more use- 

 ful for qualitative than for quantitative sampling. 

 Pearcy and I.aurs (1966) used an Isaacs-Kidd 

 trawl lined with material of uniform small mesh 

 size (5 mm. square). 



The main difficulty with the 1.5-m. net is that 

 some organisms, presumably the larger and more 

 mobile ones — e.g., flying fish (Exocoetidae) — 

 probably avoid the net or escape through the 

 mouth after entry; doubtless similar escapement 

 also occui"s from the Isaa^^s-Kidd trawl and othei-s 

 of similar size. Apart from such unknown losses, 

 standardized volumes from the 1.5-m. net esti- 

 mate real concentrations of micronekton in the 

 water. 



The 1.5-m. net has proved to be easy to operate; 

 it can be fished by two men, plus a winchman, ex- 

 cept in rough weather or heavy swells, when it is 

 prudent to employ a third man. Specimens caught 

 generally are in good condition. 



The standard 1.5-m. net hauls were made at 

 night — usually about midnight — because most 

 kinds of micronekton are more readily caught in 

 near-surface waters by night than by day (Aron, 

 1959, 1962a ; King and Iversen, 1962 ; Pearcy, 1964 : 

 !Uid table 2 of this paper) . It would be necessary to 

 lower the net to greater depths, at appreciably 

 greater cost in ship time, to obtain similar samples 

 in the daytime. Pearcy (1964) foimd no obvious 

 diiference in numbers of mesopelagic fishes caught 

 with the Isaacs-Kidd midwater trawl at different 

 (imes of night at the same station, and I assmne 

 that time of night is equally imimportant for the 

 other animals sampled. 



HIGH-SPEED NET: DESCRIPTION, OPERATION, 

 AND PERFORMANCE 



The high-speed net is an elongated cone with a 

 base (the mouth of the net) 70 cm. in diameter 

 and measures 2.6 m. from the center of the base 

 to the apex (fig. 4). The base, or mouth is sur- 

 rounded by a narrow selvage laced to a circular 

 frame of galvanized iron. The opening at the apex 



Figure 4. — Views of the high-.'ipeed net, showing attachment of towing bridles and depressor. 



MICRONEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN 



323-489 O - 69 - 6 



79 



