CAUSES OF PATCHINESS 239 



do, pure cultures of euphausian species such as E. superba or Thysanoessa inermis. Aron's observa- 

 tions on feeding salmon (p. 148, note 3) also suggest that swarms of different species keep discretely 

 apart. 



The 'Walfischfrass ' (food greedily eaten by the whale) of Crantz (1765), which according to Massy 

 (1932) seems to have been the pteropod Scoresby (1820) figures under Clio borealis (now Clione 

 Umacijia Phipps), might also it seems be a species that lives much at the surface in swarms. Scoresby 

 lists it among the plankton devoured by the Bowhead or Greenland Whale, Bigelow (1926) calling 

 attention to the vast shoals in which it occurs in the Arctic seas and the ' bounteous food supply ' it 

 provides for right whales in this northern field. ^ 



In a note on the euphausian population of the Patagonian shelf, of which E. vallentini, E. lucens, 

 E. similis, E. longirostris, E. triacantha and Thysanoessa gregaria are the principal species. Hart (1946) 

 states that none ' have been observed to form dense swarms discolouring the surface of the sea, as 

 E. superba commonly does in the Antarctic '. It is perhaps too early yet, however, to accept this as 

 conclusive, for as yet, except for some of the larvae of E. triacantha (Baker, 1959), our extensive 

 collections of euphausians from this wide shelf region have not been worked up. 



In regard to the northern euphausians, Hjort and Ruud (1929) state that they are convinced from 

 their researches off More that they swarm only ' during the spawning time ', in other words only as 

 ripe adults. Einarsson (1945), however, questions this view, noting that immature Meganyctiphanes 

 norvegica have been found swarming in August, after the spawning is over.^ 'I am inclined to favour 

 the hypothesis ', he continues, ' that hydrographic conditions, especially current conditions, are mainly 

 responsible for the phenomenon of swarming, although I must admit that evidence is extremely scarce 

 and inadequate. Further investigations for the elucidation of the factors concerned with the swarming 

 are therefore warmly to be recommended'. 



Macdonald (1927) suggests that the swarming of M. norvegica at the surface is caused 'partly by 

 the presence of predatory fish and tidal currents ', and in an early account of swarm formation and the 

 occurrence of surface swarms Vanhoffen (1896) associates these phenomena with ' Stromkabbelungen ', 

 the turbulent and complex water movements found along the margins of great ocean currents or 

 where two converging currents meet. The German word can best perhaps be rendered as 'stream- 

 quarrelling '. 



The swarming of the Antarctic krill at least is clearly not confined to the spawners. Nor is it due 

 to any local or particular trick of the currents, for it occurs widespread throughout the circumpolar 

 sea all over the faces of its great surface streams. In other words it is not a manifestation of any 

 specialised or short-term behaviour. On the contrary, as Hurley (1959) has said of the dense con- 



^ I have never, however, been wholly satisfied that the Bowhead did (or does) in fact eat this swarming pteropod. Scoresby 

 himself, it is true (Scoresby, 1823), records it in 'vast quantities' off the Greenland coast, includes it (Scoresby, 1820) among 

 the ' Medusae and other animals, constituting the principal food of the whale ', and remarks on its regular occurrence in water 

 frequented by feeding whales. But, he continues, in the very few whale stomachs he had in fact opened ' squillae or shrimps 

 were the only substances discovered ', adding that in the mouth of a fresh-killed whale he once found ' a quantity of the same 

 kind of insect'. And the 'insect' he figures looks to me uncommonly like Thysanoessa inermis, the species devoured in such 

 quantities (p. 138) by the North Atlantic right. Professor Jameson of Edinburgh, where Scoresby was a student, also (Jameson, 

 1823) refers to the 'different species of shrimps' that with other animals 'seem to contribute the principal food of the mysti- 

 cetus', while the swarming 'insects' of John Gravill, captain of the 'Diana', convey the distinct impression that it is upon 

 A jointed animal, rather than a naked mollusc, that the northern right whales feed. Nothing however is known for certain. 

 Lilljeborg (1886) suggests that Calanus and Thysanopoda might be among the more important animals on which they live, 

 Southwell (1881) stating that it is 'a kind of shrimp, found in great abundance in the Arctic seas', that provides the major 

 portion of the plankton they devour. 



^ It will be recalled too (p. 137), that other observers have reported immense shoals of both M. norvegica and Thysanoessa 

 inermis at the surface in August, September and October. 



