68 V. S. BUEEAU OF FISHERIES. 



tected in fish that have been dead for more than a few hours. It forms 

 cysts and in this form may survive outside the fish for long periods. It 

 seems probable that the disease is transmitted by such cysts in tlie excre- 

 ment, which, if eaten by otlier fish, might become active and infect tlie new 

 liosts. It is evident that hatchery water should be protected from infection 

 by keeping it free from fish when this is possible. It was thought that it 

 might be derived from frogs, but, though these animals are plentifully 

 infested by a somewhat similar form, this particular one was not found in 

 them. 



COSTIASIS. 



Occurrence. — Costiasis, a disease rapidly fatal under hatchery conditions 

 to many species of fish and to which trout are particularly susceptible, 

 appeared at one of our hatcheries early in June. Early recognition enabled 

 remedial measures to be taken promptly and it was stamped out. Delay would 

 have doubtless cost us hundreds of thousands of young trout. This disease 

 was also found in other hatcheries. 



Symptoms. — Fish die suddenly at any point in the trough. Loss of appe- 

 tite and a frayed, slimy, and grayish appearance of the fins are the most 

 readily recognized symptoms. Viewed against a piece of glass the attacked 

 fin shows a clear area where the outer surface of the skin is sloughing away. 

 Unfortunately, when these symptoms become evident to the unaided eye, the 

 disease is far advanced. 



Costia necatrix. — Costiasis is caused by a small protozoan parasite {Costia 

 necatrix) which destroys the cells of the skin and other membranes. Once 

 established it rapidly covers the entire body of the fish. In the final stage 

 the gills are attacked and the fish dies of suffocation. Costia attach them- 

 selves under the edges of the epithelial cells and multiply with great rapidity. 

 An exudation of slime issues from the diseased skin, marking the affected 

 areas. 



Transmission. — Costia appear to have a free swimming stage, during 

 which they may pass from fish to fish. They also form resistant cysts, both 

 on the fish and on the bottom of the troughs. It has been suggested that 

 the use of fresh-water fish as food for hatchery trout may result in the 

 introduction of the disease. Further study is necessary. 



Remedy. — Remedial measures suggested in the literature on the subject 

 have been tried out with success. The process consists in placing the fish in 

 a 2^ per cent solution of salt in water (21 pounds of table salt to 100 gallons 

 of water). Fish are left in this bath from 10 to 1,5 minutes, or a less time 

 If distressed. This kills the attached and free-swimming forms. Four treat- 

 ments at intervals of three days are required to kill the young as they 

 emerge from the cysts. Best results are obtained by adding sufficient salt 

 to the water in the trough in which the infected fish are. This treatment 

 must be applied to both fish, infected troughs, and utensils. Additional work 

 on these measures and the life history of the caustive organism is needed. 



GYEODACTYLIASIS. 



Fin disease. — Two of our hatcheries were infected with fin disease, or 

 Gyrodactyliasis. This disease is probably widespread, frequently epidemic 

 in hatcheries among fish of all species and all ages. Trout, particularly the 

 brook trout, are especially susceptible. It is caused by a flatworm or fluke 

 (Gyrodactylus), so called from its resemblance to a gyrating finger as it 

 protrudes from the layer of slime over the affected skin. The animal itself 

 is equipped with an anchor disk, by which it attaches itself to the fish, and a 

 sucking organ for feeding. 



Symptoms. — Affected surfaces are covered with a bluish gray slime, most 

 conspicuous on the fins. This copious production of slime may cause con- 

 fusion between this disease and Costiasis. Fins are apt to be most severely 

 affected; in advanced stages they become frayed so that the rays project as 

 spines, or may be reduced to mere stubs. The disease is not always fatal. At 

 times open sores are produced at the bases of the fins, a condition which causes 

 death in a short time. With a microscope diagnosis is simple and certain, as 

 slime scraped from affected parts will contain adults, young, or both forms of 

 the parasite. The worm itself is transparent and exceedingly active, moving 

 backward or forward, looping along like an inch worm, or gyrating. 



