1S93 ^*0 9 m'DR.-BIOL. STUDIES OF THE XORW. FISHERIES. 5I 



the Planktons Crustacea right up to the verj- surface of the water, 

 whilst on the slightest ripple they descend some feet beneath it.« 

 Dr. Petersen relates (1. c.) that by illuminating the surface on 

 calm nights with an electric lamp, he could perceive numerous spe- 

 cies which were not to be seen on the surface during the day time. 

 3. The ocean currents, and various degrees of salineness play, too, the 

 greatest part. To the Swedish scientist is due the honour of being 

 the first to have subjected this important question to systematic in- 

 vestigation. In August 1893, by the means of Pettersson's Plank- 

 ton apparatus, Aurhnllius and Cleve^ discovered that the Plankton 

 on the surface and that at a depth of 45 mètres was, essentially, dif- 

 ferent. Whilst Copepods and young Molluscs were chiefly found 

 on the surtace. the chief constituents at depth of 45 mètres were 

 Sagitta and Cilioflagellata. The contemporaneous hydrographie in- 

 vestigations now prove that the water on the surface was that of the 

 Balic of a ver\- low salineness\ whilst that, at a depth of 45 

 mètres was Bank Water. 



Dr. Petersen reports (1. c.) from the Danish Biological Station, that 

 the spread of the eggs of the pelagic fish is, to the greatest extent, depen- 

 dent on the currents. Xo eggs could be obtained in a strong North 

 going current, of low salineness, at the Danish Biological Station at Fano- 

 sund, whilst in a strong South going current, of great sahnenes, an e.K- 

 ceptionally large number of eggs were caught in, the Plankton nets. Sars 

 reports from the Northern Ocean Expedition that the amount of prey 

 (Calanus finmarchicus increased seawards from the coast, and was 

 greatest at a distance of upwards of 80 miles beyond the fresh layers). 

 As thus the diffusion and habits of the Plankton must, presumablj-, 

 be dependent on man}- factors, it would, without doubt, be as yet pre- 

 mature to la\- down detailed laws for the individual forms. In some 

 instances, however, it clearly appears from the materials at hand, that in 

 respect to a few individual tjpes, definite conditions can be indicated 

 under which they ne\er seem to appear on the Coasts of Norway. I 

 shall, subsequently, revert to this subject. 



The samples obtained during the Summer of 1894, shew that, both 

 on the West Coast, and in the Christiania Fjord, there exists an abund- 

 anc}- of Plankton consisting, chiefly, of animal life. The Crustacea play 

 the greatest part, especially numerous kinds of Calanida, both fully 



1 P. T. Qeve, Cilioflagellater och Diatomacéer. (Cilioflagellata and Diatomaceæ). Bihang 

 till (Appendix to) K. svenska vet akad. handl. Bd. 20. Afd. m. 



