MARINE PISCICULTURE 315 



of sea-fish culture : ( i ) the argument a priori^ from 

 what we know of the life-histories of fishes ; and 

 (2) the argument a posteriori^ from what we have 

 been able to discover of the actual results of the 

 method. The first line of argument may be stated 

 in the following manner : — ^ 



Sea-fishes producing pelagic eggs are enor- 

 mously prolific, but the destruction in nature of 

 the eggs produced is just as enormous. This 

 destruction is due to three main causes — 



1. The physical causes. The eggs may drift 

 out to sea and be lost to the fisheries, or they 

 may be stranded or drifted on to an unfavourable 

 shore, or they may drift into an estuary, where the 

 low specific gravity of the water may destroy them. 

 But in artificial cultural work they are liberated at 

 such a place and time as to avoid these causes of 

 destruction. 



2. Many eggs spawned in natural conditions on 

 the fishing grounds fail to be fertilised by the 

 males of the same species, and are thus destroyed. 

 But in the hatchery we can be sure that practically 

 all are fertilised. 



3. There is an enormous destruction of pelagic 

 eggs in natural conditions, because they are eaten 

 as food by other pelagic animals. But in the 

 hatchery we can exclude these natural enemies, and 

 so avoid this destruction. 



4. There is most probably a great mortality at 



1 The case of such a fish as the plaice is considered here. 



