SEA-FISHERIES LABORATORY. 35? 



naturally exposed to many dangers. They are probably 

 eaten by many pelagic animals, though there is not much 

 published evidence on this point. In Loch Fyne, how- 

 ever, the floating fish eggs are closely associated with great 

 numbers of Copepoda, and this results in the eggs being 

 eaten by the herring in the search of the latter after Cope- 

 poda and other pelagic Crustacea, and pelagic fish eggs 

 have been found in the stomach of that fish. It is possible, 

 however, that physical events are at least as fruitful causes 

 of the destruction of floating eggs and larvae as predaceous 

 pelagic animals. The change from the pelagic to the 

 demersal mode of living, for instance, happening when the 

 larva is still in deep water. Remarkable conditions are 

 present in the Baltic. Petersen* has shown that young 

 Plaice (up to 2-3 inches in length) are entirely absent in 

 that sea, though spawning fish are abundant, and, as 

 Hensen has shown, fertilized eggs are there in enormous 

 abundance. The low specific gravity of the water affords 

 the explanation. At a specific gravity of 10140 (at 9°C.) 

 a great number of Plaice eggs sink to the bottom, and at 

 a specific gravity of 1*0120 (at 10°C.) all sink. The lowest 

 specific gravity at which the eggs can drift about without 

 any sinking is 10152 (at 9"8°), and if in their migration 

 they enter water of less than this density their destruction 

 follows. Now it happens according to Hensen that about 

 once every month there occurs such a low specific gravity 

 of the Baltic water that all the Plaice eggs sink. There is 

 of course a possibility that the eggs may go on developing 

 at the bottom, but this is unlikely. It is possible too that 

 a low salinity of the Water may prejudicially affect the 

 processes of development, but this subject has not been 

 adequately investigated. It is possible also that Plaice 

 eggs entering the estuaries on the Lancashire coast may 

 * Rep. Danish Biological Station, IV., 1894, p. k 27. 



