FISHERY PROBLEMS 171 



quarter. 1 The tunny is found principally in waters whose 

 temperature varies between 55*4 and 68. When I was 

 visiting Bergen, Herr Nordgaard told me that those fjords 

 whose bottom temperature was less than 43 contained 

 many animal species proper to the glacial seas, while in 

 others the fauna was of a more southern character. 



It is true that oceanic conditions do not always act 

 upon fish in a direct manner. They act indirectly, the 

 intermediary being plankton. Indirect action is far 

 more frequent, for plankton being more simple is more 

 sensible to external agencies. Plankton, if I may so 

 express myself, is less remote from mineral matter than 

 the fish, and is in closer communion with it. To expound 

 the whole truth of this assertion it would be necessary 

 to expound the whole science of oceanography. I will 

 rather cite a very precise and plainly synthetical instance 

 described by Herr Yolk. In 1904, at the end of a long 

 period of drought, the waters of the Elbe were low. 

 The hydrographic conditions of the estuary were per- 

 ceptibly modified ; the average temperature was increased, 

 and the salinity also. The river had scarcely commenced 

 to rise once more when prodigious quantities of organic 

 substances of all kinds began to flow down to the sea. 

 Vegetable plankton immediately appeared in the estuary ; 

 it assimilated the nutritive substances, developed, and 

 multiplied itself in dense, thick layers. The quantity 

 per litre was three times as much as in the preceding 

 year. In the lower reaches of the river a cubic metre of 

 water contained 460 grammes of vegetable plankton 

 (Eurytemora), or 45 grammes of dry material. In short, 

 at any given moment there was, between Hamburg and 



1 The sardine fishers of Provence have a saying, "All currents 

 that flow from the land are bad ; all currents that flow towards 

 the land are good." 



