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Their arrival in our rivers varies very greatly ; which, I apprehend, depends 

 much on the temperature of the water. This is strikingly exemplified in the 

 Oikel and the Shin, in Sutherlandshire. The Shin joins the Oikel five miles from 

 the entrance of the latter into the sea ; it might, therefore, be presumed that tlie 

 salmon which entered the Oikel in the spring, direct from the sea, would continue 

 a straight forward and upward course. The chief body of them, however, turns 

 up the Shin. The temperature of the Shin is higher than that of the Oikel ; and 

 they do not journey up this latter river till the advancing season has raised its 

 temperature. Precisely the same thing occurs in the Esk and the Eden of 

 Cumberland. Though emptying themselves into the same estuary, salmon enter 

 the Eden many weeks earlier than the Esk— the Eden is the warmer stream. 

 For a similar reason, salmon remain some time within tidal influence. The water 

 there is two degrees warmer than either salt or fresh water separate ; chemistry 

 having proved that when two fluids of different densities come in contact, the 

 temperature is elevated for a time, in proportion to the difference in their densi- 

 ties ; and as this mixture is constantly occurring at the mouths of those rivers 

 which run into the sea, salmon remain there, a shorter or longer time, according 

 to the warmth or coldness of the pure fresh water. 



The salmon-trout and the sewin are such rare visitants to this county, and 

 their habits so closely resemble those of the salmon, that they only need incidental 

 mention as Herefordshire fishes ; indeed, of my own knowledge, I cannot vouch 

 for either of them being entitled to the honour. 



The most universally distributed of the salmonidae, is the common trout 

 (S. fario). There is no purely fresh water fish that varies more in appearance and 

 quality. From the varieties of colour— deviations in the spots— and external 

 dififerences of shape, a plurality of species has been frequently inferred. But it 

 seems now to be well ascertained that we have only one single species in our 

 rivers ; and my own e.tperience certainly confirms it. No matter how nature may 

 clothe the skeleton of certain specimens, so long as the osteology remains un- 

 changed, the species is unchanged. The apparent varieties are easily accounted 

 for. The different strata through which the rivers flow must impregnate the 

 water with their peculiar chemical properties, and thus affect both the appearance 

 and the quality of their inhabitants. I have witnessed this in some trout, taken 

 from holes whence peat had been extracted, and which had become filled with 

 water. These fish were not only dark, but very dark— scarcely a lighter colour 

 than the peat itself ; while in the neighbouring river, from which they had, no 

 doubt, been driven by floods, they were peculiarly bright and beautiful. The 

 variety and quality of their food will also make a difference. Yet neither the 

 food nor the water are to be exclusively charged with this ; for the brightest fish, 

 in the clearest stream, if he takes up his abode under a low bridge, or in a thickly 

 sheltered situation, becomes much darker than his brethren who sport in more 

 exposed positions. Hence it appears that light is essential to the brilliancy of 

 fish, as well as of plants ; and so is good condition, for we never see a bright 

 coloured fish immediately after spavirning — the only season when they become 

 thin and poor. Circumstances, therefore, determine these variations, and not 



