CHANGES IN MICRONEKTON FROM DAY 

 TO NIGHT ON SUCCESSIVE DAYS IN 

 THE SAME AREA 



Table 2 gives the standardized volumes from 

 standard hauls made alternately about local noon 

 and midniffht during a 20-day period of tracking 

 a drifting surface-current drogue off the coast of 

 southern Mexico (cruise TO-62-1 or TE:\IP0, 

 August 1962). The hauls were encompassed by a 

 rectangle boimded by lat. 14°58.0' N. and 15°17.8' 

 N. and long. 99°49.7' W. and 100°48.0' W.; the 

 same body of surface water was sampled through- 

 out the period. Table 2 shows the expected striking 

 difference between noon and midnight volumes of 

 total micronelvton, which have ranges of 0.4 to 2.1 

 and 4.3 to 14.3 ml. per 1,000 m.^ respectively. The 

 main reason for this difference is that several 

 families of mesopelagic fishes — Myctophidae, 

 Gonostomatidae, Stomiatidae, Bathylagidae, and 

 Melamphaidae — occurred frequently in midnight 

 catches although they were practically unrepre- 



sented in noon catches. Bregmacerotidae also oc- 

 curred exclusively in the midnight catches, and 

 Carangidae and Leptocephali were taken in larger 

 numbers at midnight than at noon. The principal 

 crustacean groups in this series of micronekton 

 samples were hyperiid amphipods and stomatopod 

 (squillid) larvae, both of which tended to be more 

 abundant in the noon catch than in the midnight 

 cat«h on any particular day. These groups were 

 the most abundant in the noon samples, which con- 

 tained comparatively few fishes. Cephalopods 

 occurred in small, broadly similar, amounts in 

 both noon and night samples. 



The noon and night series are, thus, greatly dif- 

 ferent quantitatively and qualitatively; on the 

 other hand, the hauls in each of these series have 

 a great deal of similarity, especially between those 

 made on consecutive days. Consecutive night hauls 

 seldom differed in total volume by a factor of >2 

 and consecutive noon hauls seldom differed by a 

 factor of >3. Consecutive samples in either series 



Table 2. — Micronekton from standard 1.5-m. net hauls made alternately about local noon (D) and local midnight (N) during a 

 20-day drogue-tracking experiment on cruise TO-62-1 (TEMPO), August 11 to 30, 1962 



[Asterisk means <0.1 ml. per 1,000 m.^] 



' Haul omitted. 



MICRONEKTON OF THE EASTERN TROPICAL PACIFIC OCEAN 



81 



