NOTES ON THE SALMON DISEASE. 351 
ment, and as it gradually drops down seaward again helps itself 
largely (as a kelt) tothe young of the Salmonidze—in the absence 
of more congenial food. The abstinence from its natural food 
while pushing upwards in a river to spawn must lower the fish’s 
vital powers greatly. 
Then it is to be borne in mind that the salmon is in finest 
condition and greatest vigour when it first leaves the sea—fat, 
plump, and gleaming with its coat of burnished silver. How is 
this? It is simply because the sea covers its natural feeding 
grounds, where it can fatten on numberless crustaceans, molluscs, 
and small fish, which appear to suit its constitution admirably. 
Besides proper food, it gets more orygex into its system in the 
sea. This is a matter requiring more investigation than has yet 
been devoted to it. Certain gases, it appears, are found secreted 
in the air bladder of all fishes furnished with that organ. Dr. 
Giinther says :—“ The gas contained in the air-bladder is secreted 
from its inner surface. In most fresh water fishes it consists of 
nitrogen, with a very small quantity of oxygen, and a trace of 
carbonic acid : in sea fishes, especially those living at some depth, 
oxygen predominates, as much as 87 per cent. having been found. 
Davy found in the air-bladder of a fresh-run salmon a trace of 
carbonic acid, and 10 per cent. of oxygen, the remainder of the 
gas being nitrogen.” We know also in fish culture that well 
grated, or oxygenated, water is a necessity to the life of the 
young of Salmonideze. Seeing then that the salmon is in its finest 
~ condition when in the sea, that is when its air bladder has most 
oxygen in it, and in its lowest condition when in the river after 
spawning, that is when we may believe least oxygen is in its air- 
bladder, I think it a fair inference that these states of the air- 
bladder may be taken as indices of the health of this fish. A 
gaseous analysis of the waters of affected and clean rivers, and 
of the contents of the air-bladder of salmon taken in these waters, 
would at this point beof great value. The only analysis in my 
possession is one made by the late Dr. Penny many years ago 
(1852), and I give it for what it may be worth now :— 
Carb, Acid. Oxygen. Nitrogen, Parts. 
Loch Lomond water at Balloch 5.10 31.18 63:72 = 100 
The Tay at Perth ‘e im eb25O. 25.00 62,50 a 100 
Loch Ness ie ee aie Bris 31.47 64.80 = 100 
Now, it is a curious coincidence at least, and favourable to my 
theory, that the Tay salmon are diseased, while those of Loch 
Lomond and Loch Ness are not. For the Tay water not only 
contains the smallest amount of oxygen of the three waters 
examined, but also the ratio of nitrogen to oxygen in it is highest. 
In other words, the waters of Loch Lomond and Loch Ness have 
not yet deteriorated so far as to encourage the outbreak of the 
fungoiddisease. And in these lochsit appears there ismoreoxygen 
than in the water of the Tay, the absence of the necessary pro- 
portion of which I consider the principal cause of disease. 
This loss of oxygen, as before stated, is consequent on the 
progress of agriculture, which has reduced the average flow of 
