REACTION OF KRILL TO SHIP AND NETS 267 



an estimate of the relative density or abundance of this species in one region or another and equally for 

 estimates of the limits of its optimum abundance and absolute geographical range. They could not 

 in fact be expected, the krill being what they are, to do anything more. It will be seen, too, in the 

 distributional part which follows, that at all material times our sampling or observational density in 

 the circumpolar sea is for the most part distributed fairly over the principal regions of euphausian 

 abundance, a fact which by itself contributes not a little to the reliability of such inferences as may be 

 drawn. 



In so far as other Antarctic plankton animals may be concerned in these phenomena, there are 

 ample grounds for supposing that Euphausia crystallorophias, the large swarmer of the coastal waters 

 of the continental land, will also be found to react to both ship and nets in much the same way as 

 the krill, presenting a parallel, but possibly far greater, distributional problem. For in the high 

 latitudes where this species is so abundant there are few dark hours in which vessels can work, 

 the bulk of the sampling having to be carried on in the continuous daylight of the very short periods, 

 amounting in some instances only to days, during which such regions remain open to shipping 

 in summer. For the rest of the year, however, the difficulties of sampling are even greater, for 

 such as can be accomplished, whether from land bases or from vessels frozen in and used as winter 

 quarters, has to be done (Hodgson, 1907; Murray, 1910) very largely by working through holes in 

 the ice. 



There can, I think, be little doubt that many other plankton animals, euphausians especially, must 

 react to ship and nets in much the same way as the Antarctic krill. Tattersall (1924), for instance, 

 in his account of the Atlantic euphausians of the 'Terra Nova' expedition, observes that specimens 

 occurred in 19 out of 30 night surface hauls and in only two out of 32 day surface hauls, adding that 

 'those found in the day nets were larval forms only'. He continues, 'taken as it stands and for what 

 it is worth, the evidence from this collection does suggest that, for some reason or other, Euphausians 

 are much more easily caught at the surface at night than during the day '. He suggests as one possible 

 explanation of this phenomenon that these animals, although present on the surface by day, are able 

 by some means to avoid the nets, adding that experience with a wide-mouthed net of coarse mesh 

 towed rapidly on the surface had shown that the large Meganyctiphanes norvegica and Thysanoessa 

 inermis and the smaller Nyctiphanes couchii could frequently be caught at the surface in daytime 

 ahhough 'an ordinary tow-net worked at the surface at the same time would fail to capture any 

 specimens'. He continues: 



That Euphausians do sometimes occur in large numbers at the surface in the daytime in coastal waters was made 

 clear to me by an observation which I made in the seas round Orkney and Shetland in August 1907. From the deck 

 of the boat I could see enormous swarms of Thysanoessa inermis actually at the surface ; apparently they were caught in 

 the surface film, and they were gyrating about in the manner familiar to those who have observed aquatic insects 

 and Entomostraca in freshwater pools. The day was hot, the surface of the sea was absolutely calm, and the sun was 

 shining brilliantly.^ I was able to satisfy myself as to the identity of the Euphausians, but it was not practicable to 

 try to catch them by the ordinary methods of tow-netting.^ These coastal species may be subject to different factors and 

 different conditions from oceanic ones; their reactions and their behaviour may be different; it by no means follows 

 that oceanic species occur at the surface in daylight in the same way. Nevertheless, these observations show that 

 some Euphausians do occur in the surface waters by daylight, and experience in the use of wide-mouthed nets towed 

 rapidly^ at the surface in daylight suggests that the avoidance theory cannot be rejected, even for oceanic species, 

 without additional evidence. 



1 In equally well illuminated conditions 'during the hottest part of the day' Calanus finmarchicus has also been reported 

 swarming at the surface (Gough, 1905) in the English Channel in July. 



^ The italics are mine. 



* With young plaice, little longer than the adult krill, Wimpenny (i960) has recently demonstrated that the small Danish 

 Young Plaice Trawl produces larger catches when moving fast over the ground than when moving slow. 



30-2 



