154 TENTH ANNUAL REPORT OF THE 



It is conjectural just what relation, if any, this accident may have had to 

 the disease. The deposit of earthy sediment in the ponds may have brought 

 with it the infecting organisms, in which case it increased the pre-existing 

 chances of infection, and perhaps determined the onset of disease at that 

 particular time. It is not likely that it did more than this, for trout of 

 proper resisting powers would not allow infection to gain a foothold within 

 their own bodies, unless it were present in overwhelming amount. One 

 must look for a deeper and more fundamental explanation. 



The other unusual condition was the presence of a greater number of 

 trout than usual within the pond space. It is the tendency of repeated sea- 

 sons of successful experience in raising trout to maturity, to increase the 

 scale of operations and make the same pond area and water flow bring forth 

 a greater and greater product; in other words to overcrowd. Crowding is 

 one of the necessary conditions of domestication, and it is of course domes- 

 tication itself which is at bottom responsible for all these infectious epi- 

 demics which attack trout, as it is of most of the cases of parasitism and 

 even of most of the losses among such fishes. The conditions making up 

 domestication, which are a departure from nature, and which, despite the 

 -fact that such departures need not necessarily be unhygienic, are usually 

 regarded as more or less unfavorable, may be divided into three, as follows: 



i. Artificial, only slightly varied, diet. 



2. Lack of range and exercise. 



3 . Crowding. 



In addition to these, fish cultural waters frequently are somewhat, 

 sometimes considerably, of a lower oxygenation than the natural habitats 

 of fishes; and sometimes they have slight excesses of dissolved nitrogen, 

 which are not sufficient to cause immediate trouble with the symptoms 

 which higher degrees of the same excess cause, but which do have an 

 unfavorable influence when long continued. 



Taking the subject of domestication in a general way, and speaking in 

 a broad sense, it is admittedly a fact that the intensive cultivation which 

 it necessarily implies is accomplished at the expense of an increase of sus- 

 ceptibility to disease. This is in general true of both the animal and vege- 

 table world. Confining the present consideration to fishes alone, and 

 particularly to the trouts, one finds that while domestication protects them 



