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THE AMEBIC AX NATURALIST [Vol. LV 



nomic importance. When once they make their appear- 

 ance in our hatcheries they cause a great deal of damage 

 and loss amongst the fish. Here conditions are ideal for 

 parasitism. The ponds are small and large numbers 

 of fish are crowded into them. Because of this situation 

 the parasitic larvse have very little trouble in finding their 

 hosts. At the same time the current of water which cir- 

 culates through the hatchery ponds is not swift enough 

 to interfere with the movements of the parasitic organ- 

 isms. It is therefore a matter of a short time before 

 most of the fish become heavily infested with copepods. 



While the young fish as well as the adults are attacked 

 in the hatchery ponds, nevertheless it is mainly the adult 

 fish which are most heavily parasitized. These are 

 attacked by so many of the copepods that they are ulti- 

 mately killed. It is by no means uncommon to find as 

 many as two hundred and fifty copepods on one trout. 

 Eecently I found around five hundred copepods on the 

 gills of a single chinook salmon. In such cases of para- 

 sitism the injury to the host is considerable. In the first 

 place, the parasites suck enormous quantities of blood, 

 thereby depriving the host of a large amount of nourish- 

 ment. Secondly, when the copepods attach themselves, 

 they injure the tissues of the host, thereby making it 

 possible for injurious spores and bacteria to enter and 

 set up secondary infections of a serious nature. And 

 lastly, the injured tissues swell and develop into so-called 



scar tissues," which interfere with the normal functions 

 of the host. Taking all these facts into consideration, 

 there is little wonder that fish succumb under the attacks 

 of these parasites, particularly in hatchery ponds where 

 conditiojis arc just right for parasitism. In one Wiscon- 

 sin liarclici y tlie author found that in a single year about 

 twelv.. tlu.usnn.l adnlt trout out of fourteen thousand 

 kept ill outdoor j.onds died from the attack^ of these 

 copcpo.l^. 



Many <tates hav,. had thi> trouhle IV.r xcai-. uith wry 

 serious losses. The writer has devoted considerai)le 



