212 BRITISH FRESH-WATER FISHES 
stock has brought about the disease. Ten per cent. of diseased 
fish upon a stock of ten thousand salmon will amount to one 
thousand—a shocking display of mortality ; but ten per cent. 
upon five hundred salmon will result only in fifty diseased fish, 
which a single flood may carry away to sea without attracting 
any notice at all. I have seen salmon as badly diseased in 
rivers where they were exceedingly scarce as in rivers where 
there was a heavy stock ; and, on the other hand, I have seen 
shallow Loch More, at the head of the Thurso, crowded with 
salmon throughout a hot summer, yet the salmon disease has 
never been reported from the Thurso at all.* 
Although all salmon spawn in the autumn and winter 
months, covering a period from the middle of October at 
ae ae earliest to the end of January at latest, there is a 
Late Salmonsingular and hitherto inexplicable variation in the 
rivers: time of year at which they begin to ascend different 
rivers. Into some rivers salmon enter in greater or less 
numbers in every month in the year, nor is this dependent 
upon the size of the river, because it is the case both in the 
mighty Tay and the puny Thurso. Into some rivers salmon 
begin to run early in spring, while in others fresh-run fish are 
never found till summer or even autumn; neither does this 
seem to be connected with the volume of the stream, as is 
shown in the case of the Bann and the Bush, rivers of Ulster, 
debouching at a distance of about ten miles from each other. 
The Bann is a noble river, flowing out of the Lough Neagh, 
the largest sheet of fresh water in the United Kingdom, and 
here, if anywhere, one might expect early salmon to run. Yet 
not a fresh fish may be looked for in the Bann before the 
month of May ; whereas in the Bush, an insignificant ditch 
compared with the other, flowing out of no great reservoir, but 
draining the peat-mosses of Antrim, salmon put in an appear- 
ance in February, and are caught in great abundance in March 
and April. 
* See Appendix, p. 305. 
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