Refroduction and Growth 165 



egg develops in the water. It is because of this that fish are 

 practical subjects for large-scale artificial propagation. Such 

 propagation falls into three general categories. The first, in 

 which the parents are killed, is the simplest. This is used 

 especially in connection with commercial fisheries, many of 

 which concentrate on the fish at the time they are gathered 

 together for spawning. "Ripe" females caught in the nets — 

 meaning those in which the sex products are mature — are 

 opened and their eggs removed and fertilized with sperm- 

 bearing milt taken from the males. Sometimes this is done 

 on board the fishing-boat, either by government agents or 

 by fishermen who receive extra pay for the work. If the eggs 

 are of the kind which float, they are returned to the sea to 

 develop while drifting naturally j otherwise they may be sent 

 to some kind of hatchery. There has been much debate as to 

 the value of this work, its opponents maintaining that the 

 number of young it produces is so small in comparison to the 

 enormous numbers provided by nature that it is not worth 

 while. When you realize that a single school of herring has 

 been estimated to contain three billion individuals, while 

 scores of such schools exist in the Atlantic and the North 

 Sea; that commercial fishermen take from the seas of the 

 world some forty billion pounds of fish yearly; that this 

 catch is only a fraction of those which are left in the water; 

 and that of those left in the water at least half are females 

 each producing from twenty thousand to two and a half 

 million eggs per year, you begin to understand the validity 

 of the argument. The fact is that this type of work is now 

 carried on less extensively than it used to be, except in cases 

 where restricted numbers make it of value. 



A variant of the system is applied to the salmon of our 

 Pacific coast, especially in restoring the supply in streams 

 where the natural run has been endangered by dams or other 

 man-made interference. All these salmon die as soon as they 



