CHAPTER XIX 



DISEASES OF FISHES 



ONTAGIOUS Diseases. — As compared with other ani- 

 mals the fishes of the sea are subject to but few 

 specific diseases. Those in fresh waters, being 

 more isolated, are more frequently attacked by contagious 

 maladies. Often these diseases are very destructive. In an 

 "epidemic" in Lake Mendota, near Madison, AA^is., Professor 

 Stephen A. Forbes reports a death of 300 tons of fishes in the 

 lake. I have seen similar conditions among the landlocked 

 alewife in Cayuga and Seneca Lakes, the dead fishes being 

 piled on the beaches so as to fill the air with the stench of their 

 decay. 



Crustacean Parasites. — The external parasites of fishes are of 

 little injury. These are mainly lerna^ans and other crustaceans 



^.>^ ^ 



Ftg. 22.5.— Menhaden, Brevoortia tyrrinnus (Latrobe). Wood's Hole, Ma.ss. 



(fishdice) in the sea, and in the rivers different species of 

 leeches. These may suck the blood of the fish, or in the case 

 of certain crustaceans which lie under the tongue, steal the food 

 as it passes along, as is done by Cymothoa prccgiistator, the 

 "bug" of the mouth of the menhaden (Brevooriia tyrannus). 



340 



