ON THE SAPROLEGNIE^. 195 



lium ramifies in the derma, or true skin, where the sea-water 

 cannot reach it, the disease may simply He dormant till the return 

 of the fish to fi-esh water. 



There are certain predisposing causes which bear an important 

 part in the propagation of this disease. One of these is injury to 

 the skin of the fish, in the way of wounds, or bruises, received in 

 fighting with one another, or in unsuccessful attempts to overcome 

 obstacles in the passage up and down the river, whether weirs or 

 dams. Again, while in fish in robust health a slight bruise would 

 not result in parasitism, a debilitated fish would, in all probability, 

 suffer. Instances of known debility, from retention of ova or 

 spawning, make the bulk of fatal cases, and the few which are 

 supposed to have been strong fish, may well be assumed to have 

 been exhausted by overcoming obstacles in their way up the river, 

 or otherwise to have been in a low and weak state. Shallow water 

 is a source of debility to fish in general, through lack of food, and 

 at this time fish congregate in the holes of the stream. 



If we accept the theory that the resting-spores of Saprolegnia, 

 which have previously been described, being heavier than water, 

 sink to the bottom, they would consequently be washed into the 

 holes, thus causing these hollows to become reservoirs for the 

 zoospores which are liberated in the spring, and evident centres 

 of contagion. When the water is high and plentiful, food is 

 washed down from the land, the fish are well fed, do not collect 

 in the holes, and are therefore not crowded together into the 

 midst of contagion. At such times there is less disease; this is 

 fully borne out by the evidence laid before the Commission. 



We cannot, however, forget that the fish-fungus is no respecter 

 of species or individuals. It attacks nearly all, if not all, fresh- 

 water fish, and it exhibited its predilection for carp before the 

 salmon-disease became prevalent. How is it that all the fish in 

 the infected rivers are not destroyed as well as the salmon ? The 

 only answer possible is, that in some condition of bodily health or 

 physical weakness, the salmon falls a prey, whilst the other fish, by 

 a more robust constitution, or some peculiar circumstances which 

 the salmon does not enjoy, escape with impunity. Some weight 

 must be attached to the argument which has been advanced, that 

 disturbing the natural conditions, by protecting the salmon, has 



