PARASITES 115 



when the fish are indisposed, that they are thus 

 infested. Any salmon fisherman should at once 

 detect the difference between fi-esh-water parasites 

 and sea lice, for they are as different as chalk and 

 cheese. It is generally believed that sea lice can live 

 no more than twenty-four hours at the utmost in fresh 

 water. But there have been instances of salmon 

 being caught forty miles up the river with sea lice 

 upon them. If so, and if the theory be correct, they 

 must have travelled these forty miles in twenty-four 

 hours. That appears to me impossible. I need not 

 repeat my remarks on the rate at which salmon travel. 

 I will only say that, in my belief, sea lice live in fresh 

 water a much longer time than twenty-four hours. 

 Fish infested with sea lice have been killed at 

 Grandtully, on the Tay, and at Meggernie Castle, on 

 the Lyon, a tributary of the Tay. The former 

 locality is at least forty miles, and the latter eighty 

 miles above the tideway at Perth, I did not ascertain 

 the exact date these fish were caught ; but assuming 

 that they had travelled at their maximum pace in 



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