40 U. S. BUREAU OF FISHERIES 



4. There is an important and specific relation between light and 

 the nature and extent of the fouling of ships' bottoms. 



Experiments have been conducted to determine the reactions to 

 light of the organisms commonl}- causing fouling. These show 

 that for certain species, both of barnacles, bryozoa, and tunicates, 

 the larvae are negative to light at the time of attachment. 



Collections of adult barnacles and of various data regarding 

 breeding habits, at various stations on the Atlantic coast and at 

 different periods throughout the year, have been made to determine 

 the seasonal occurrence and distribution of the " infective stages " 

 of the barnacles. 



Studies of the life histories and of the factors determining dis- 

 tribution of several species of barnacles have been made. 



INVESTIGATIONS PERTAINING TO FISH-CULTURAL OPERATIONS 



PATHOLOGY OF FISHES 



Investigations on Octormtus sahnonis, a parasitic flagellate of 

 trout, were continued during the year by the pathologist. Dr. H. S. 

 Davis, and the life history has now been almost completely worked 

 out. It has been found that this parasite is often associated with a 

 parasitic amoeba, and that the cysts of the latter have been mistaken 

 by some investigators for those of OctomituSi. The vegetative 

 stages of the amoeba are found only in the stomach, and the cysts 

 formed here pass into the intestine, where their common occurrence 

 along with Octomitus has led to their being identified as cysts of 

 the latter. As a matter of fact the cysts of Octomitus have a very 

 different appearance and when once seen can never be confused with 

 those of amoeba. After becoming encysted the organism divides 

 into two equal parts, each of which shows the essential features of 

 the flagellated forms except that they lack flagella. The later his- 

 tory of the cysts has not yet been worked out, but there can be little 

 doiibt that it is by this means that infection is spread from one fish 

 to another. 



The amoeba that develops in the stomach, where it is often very 

 abundant, does not appear to appreciably injure the host, but its 

 life history, which has been only partially worked out, exhibits some 

 very remarkable and interesting features. 



A severe epidemic among the fish at the Wytheville (Va.) station 

 during March and April, which was accompanied by heavy mor- 

 tality, was found to be due to Octomitus, the parasites being ex- 

 cessively abundant in the intestines of the infected fish. Ordinarily 

 only fingerlings from about II/2 to 3 inches long appear to be se- 

 riously injured by the presence of the flagellate, although they may 

 interfere more or less with the growth of larger fish. While these 

 parasites are undoubtedly present at nearly all trout hatcheries, it is 

 believed that the outlook is distinctly encouraging and that as a 

 result of improved methods of caring for the fish the loss from this 

 source need not be serious. 



Very encouraging results have been obtained in preventing thyroid 

 tumor in trout by the administration of iodine. In one lot of oyer 

 1,200 rainbow trout at the Wytheville station, which were badly in- 

 fected with this disease, no new cases have appeared since beginning 



