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MR. Erasmus V. — Letter from Erasmus Wilson, Esq., F.R.S. 



WILSON. 



Gentlemen, 



I was very glad to have the opportunity of seeing the diseased salmon 



this morning ; their appearance convinces me more and more that the fungus 



is a morbid growth of the mucus produced by the skin of a diseased animal, 



and not a vegetable parasite. 



The appearance of the disease very closely resembles ringworm in the human 

 subject, in which also there is a fungiform growth, and I have no hesitation in 

 referring the cutaneous disease of the salmon to a similar cause. The parts 

 most affected were those most distant from the centre of circulation, namely, 

 the fins and tail, and of these the whole surface was coated over ; whereas on 

 the fleshy parts the disease was developed in circular discs, each about the size 

 of a shilling, while here and there a cluster of discs had become blended into a 

 broad irregular patch. Again, where the fungiform growth was scraped away, 

 the scales were imperfect or lost, and the growth had taken the place of the 

 scaly covering. The parallel with ringworm is very striking. 



lou mentioned that the fish were females exhausted by ovi-production and 

 ovi-position. They were much emaciated and their flesh was pale and flabby. 



The gills were sound and there was some excoriation about the snout and 

 head, resulting from forcible rubbing off of the fungous growth. 



It is clearly evident that the fish are debilitated, and out of condition, and to 

 this, and this alone, must be attributed the production of the fungous growth 

 on their integument. And their state of exhaustion is such that they must 

 necessarily die, unless means can be devised for supplying them with water of 

 a purer character, and also it may be presumed with more congenial food. 



I would venture to suggest that the rivers are in the first instance con- 

 taminated by excessive numbers of fish, and that the evil is multiplied 

 manifold when, in addition to impurity from numbers, there is superadded 

 the presence of diseased fish, and furthermore, of dead fish. It is possible 

 that certain seasons, or certain states of the atmosphere, combined with certain 

 peculiarities of soil, may predispose to impurity of the rivers, and that at such 

 times the water is less life-sustaining than at others. I mention this as a 

 probable explanation of some rivers escaping whilst others are attacked ; but 

 in reality no simpler or stronger argument is required than that of a surplusage 

 of fish. For many fish in a globe of water would share the same fate ; as like- 

 wise, too many children packed together within a too limited cubic space of air. 



The remedy which naturally suggests itself for this state of things is more 

 water ox fewer fish. A natural remedy would be to keep down the numbers of 

 the fish ; and the fact cannot be questioned that a river of given dimensions 

 may be ample to support a certain number of fish, but no more. Again, 

 every facility should be given to the fish to deposit their ova without fatigue 

 or exhaustion, especially those dreadful barriers, over which no fish-power can 

 accomplish the leap, should be removed. Exhausted by fruitless attempts, 

 the poor creatures become ill and die, and a beginning is thereby made for 

 the contamination of an entire river. 



Piscatorial experts ought to be able to judge of the numbers of fish in a 

 river, and when they are present in excess, means might be adopted for 

 thinning them out. Even plants fail to grow when packed together too 

 closely ; how much greater the evils must be where animals are concerned. 

 The first effect of overcrowding is to engender debility ; the sick fish renders 

 the water impure. Then otbers become sick and some die, giving rise to 

 further vitiation of the breathing element, until at last the evil becomes almost 

 too great to cope with. 



I am not an ichthyologist, but I would ask you, How about food for the 

 fish in a crowded river ? Is not the provision of food limited as well as of 

 air? If so, we then have two energetic causes of disease, both acting in 

 unison, — starvation and suffocation. I quite agree with you in your proposed 

 remedy : As we cannot renew the water of a stream nor add to its quantity, 

 the only alternative that remains is elimination of the dead fish, and, if 

 possible, the driving of the diseased fish out to sea. 



I hope the interesting and important nature of the subject will plead my 

 excuse for this long letter. 



Yours faithfully, 

 I/, Henrietta Street, Cavendish Square, Erasmus Wilson. 



February 12, 1880. 



