18 



Tliere are people, perhaps, wlio have no doubt about it; but I cannot help 

 doubting, and I regret that I have not formerly been as consisteut in niy 

 doubt also with respect to the Cattegat, a faet to wliieh Ehrenhaum and 

 StrocJtniavn, 1. c. page 123, justlj' call attention. For it is quite possibie, 

 indced, that the Cattegat does not get its main supply of young plaice 

 through fry of its own eggs, but tliat the greater part of the fry, perhaps, 

 comes from eggs shed in the NorthSea, the Baltic, or the Belt-Sea. 



For oue tliing is certain, that the eggs in the long period before the 

 hatching, and the fry in the still longer pelagic period (from October till 

 May — June, as we may suj^pose for mauj' of them), must be carried along 

 with the eurrent — in most cases we scarcely know where. According to 

 m}' earlier investigations the eggs have such a specific gravity that they 

 can only float in water which at 10" C. has a salinity of 14,4 7oo' oi" higher. 

 Henscn has fouud that somewhat higher salinities are necessary. But in 

 open nature I have often fouud eggs of plaice in water of c. 15 "/ooi ^^^ 

 Ehrenhaum, 1. c, thinks he has fouud such an egg at a salinity of 13,73 "/oo 

 and a temperature of 4,5-"; I do not, however, attach great importance to 

 this one egg. By my experiments I have shown ("Report of the Biol. 

 Station, IV") that there is a considerable variation in the fioating-power of 

 the eggs; but as a rule they will scarcely be found, I think, in water of a 

 salinity lower than c. 14,5 "/oo- They are able to float, consequently, onl}' 

 where there is such water, and if they cannot float, I must suppose that 

 they die; for it is not very likely that they can be developed in the oozy 

 particles of the bottom, and if they only toucli the latter, they will probably 

 be devoured in the course of a very sliort time by the animals that live there. 



Tiie existence and development of the eggs is, consequently, only pos- 

 sibie, where there is for some length of time water of a sahnity of c. 

 14,5 "/oo, or higher. We do not know more exactly what salinity the pelagic 

 young tish require, what temperature, light etc. ; but the very necessit}' of 

 a salinity of a least 14,5 "/q„, through rather a long time, reduces the distri- 

 bution of the eggs cousiderably. 



1 have before me a map of the Danish seas on which a nuraber of 

 piaces have been marked down where pelagic eggs were not found at all 

 in the surface in March & April 1902, and other piaces where they were 

 found. These piaces, altogether c. 20, have been stated by Lieutenaut C. J. 

 Hansen, R. N., a gentleman wliose name is often mentioued, e. g. in "Report 

 of the Biol. Station, IV". While in command of the schooner Argns, he 

 fished for pelagic eggs with a silk net of the usual form and size, in the 

 Belt-Sea, in the above mentioned months. If we draw on this map a line 

 from Cape Kullen to the southern point of Langeland, all the stations where 

 eggs have been caught fall north and west of this line; no eggs have been 

 caught south and east of the same. All these eggs were taken in the sur- 

 face. The said line is very nearly identical with the boundary line betvveen 

 water of 10 — 14 "/oo salinity and water of 14 — 21 "/oq or higher salinity, 



