FISHERY BULLETIN: VOL. 72, NO. 4 



dure was efficient enough to retain all species of 

 Nematoscelis present in the whole sample. Only 

 26 archives were found to have contained one or 

 more species not present in their subsamples. 

 Therefore it is assumed that the subsampling pro- 

 cedure was usually adequate. 



The wind system over the Indian Ocean is mon- 

 soonal. The two monsoon phases are designated as 

 the Southwest (SW) and Northeast (NE) mon- 

 soons, indicating the predominant wind directions 

 in the northern Indian Ocean during each phase. 

 The bulk of the samples were grouped into two 

 categories: those collected during 16 April to 15 

 October falling in the SW monsoon and those col- 

 lected during 16 October to 15 April falling in the 

 NE monsoon. The period 16 April to 15 October 

 generally agrees with that of the wind regime of 

 the SW monsoon (Wooster, Schaefer, and Robin- 

 son, 1967). 



Of the 1,927 samples processed at the Indian 

 Ocean Biological Centre, 1,732 samples were ex- 

 amined for the study of Nematoscelis. Of these, 

 879 samples were taken during the SW monsoon 

 and 853 during the NE monsoon. Since a compari- 

 son of day and night catches of total euphausiids 

 showed differences in day and night estimates 

 (Gopalakrishnan and Brinton, 1969), it was de- 

 cided to group day and night counts separately for 

 each season. There were 401 night and 478 day 

 samples for the SW monsoon period and 413 night 

 and 440 day samples for the NE monsoon. 



In order to fill gaps in certain geographical 

 areas, some samples collected by using gear other 

 than the lOSN have also been used in the prepara- 

 tion of charts. Kistna cruises 2 to 6 used an or- 

 gandy net with 50-cm mouth diameter. Natal 

 cruises 1 and 4 used the N-70 net with 70-cm 

 mouth diameter. These samples were standard- 

 ized for comparing with lOSN samples: the or- 

 gandy net samples were multiplied by a factor of 

 5.1 and N-70 net samples by a factor of 2.6. In 

 addition to these samples, 26 nonstandard surface 

 samples taken during the Patanela cruise were 

 also examined for qualitative information. Argo 

 Monsoon Expedition stations 9-27 (ref Snyder 

 and Fleminger, 1972) have also been used in the 

 preparation of present charts. These samples were 

 collected using 1-m nets. 



The observed differences between day and night 

 catches of adult euphausiids are due to the fact 

 that during day time many species either migrate 

 to deeper layers or are able to dodge the net. Total 

 euphausiids were 1.5-2.0 times more abundant in 



night samples than in day samples (Gopalakrish- 

 nan and Brinton, 1969). The mouth area of the 

 lOSN is only 1 m^ and it is probable that many 

 large zooplankters can avoid the net. However, 

 most samples taken at night contained many 

 adult euphausiids, including some of the large 

 thysanopods that migrated into the upper layer. 

 All species of Nematoscelis are smaller than most 

 Thysanopoda species. It is reasonable to believe 

 that the lOSN tows taken at night would have 

 been adequate to representatively sample adult 

 species of Nematoscelis. 



The present distributional study of 

 Nematoscelis in the Indian Ocean is limited to the 

 upper 200 m only. The geographical distribution 

 of each species of this genus is compared with the 

 pattern of water circulation in the upper layer. 

 Larvae and juveniles of all Nematoscelis species 

 are confined to the upper 200-m layer during both 

 day and night, whereas adults undertake diurnal 

 vertical migration (Gopalakrishnan, 1973). Dur- 

 ing daytime the distribution of adults extends 

 from the upper 200-m layer to about 600-800 m; 

 but at night, because of their upward migration, 

 the range extends from the surface to about 

 400-600 m only, with the maximum above 200 m 

 in most geographical regions. It is therefore prob- 

 able that the lOSN would have caught adults bet- 

 ter at night than during the day. For this reason 

 only night samples were considered in charting 

 the distribution of adults. However, a few trawls 

 (Isaacs-Kidd Mid-water trawl) fished from depths 

 greater than 200 m, were also used for examining 

 overall geographical range. Station positions of 

 these are included in Figures 7, 11, and 18. Most of 

 these reached as deep as ca. 900-1,000 m, some- 

 what deeper than the lower limit of the vertical 

 range of Nematoscelis. The geographical coverage 

 of these trawls was not sufficient to examine the 

 influence of deep circulation on the distribution of 

 species. A total of 286 mid-water trawl samples 

 were examined from the Pacific, Atlantic, and In- 

 dian oceans. [Collections from the Pacific and In- 

 dian oceans are located at the Scripps Institution 

 of Oceanography. Collecting data regarding many 

 of these appear in Clarke (1963).] The approxi- 

 mate boundaries of distribution of each species are 

 shown on the basis of present evidence, including 

 literature records. 



The distributional ranges of species of 

 Nematoscelis in the Atlantic Ocean were based on 

 specimens sorted from fractions of plankton sam- 

 ples obtained by the Woods Hole Oceanographic 



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