NATURAL HISTORY OF BRITISH SALMONID^. 167 



by them being actually less. On another occasion trout were 

 kept for many years in a store stream, and tested with various 

 kinds of diet, when it was ascertained that in some instances 

 the increase in weight was as much as nine pounds in four 

 years (or from one to ten pounds). 



It is evident from these experiments that fish and grubs 

 bear no comparison with insect food in point of nourishment, 

 in consequence, no doubt, of the amount of phosphate of lime 

 contained in the latter ; and of the insects specially contributing 

 to fish food, probably most nutritious of all is the May fly, 

 which, when in the larval state, works havoc amongst the trout 

 ova on the spawning beds. 



Recent piscicultural experiments have demonstrated the 

 great value of the fresh-water shrimp also as an article of fish 

 diet, and for feeding young fry on. 



It was once my good fortune to have an opportunity of 

 verifying the growth rate of trout when fed upon this insect 

 which abounds in almost every stream and ditch where the 

 water is not too turbid. In its general structure the fresh- 

 water shrimp bears some resemblance to the common sand 

 hopper to which it is closely allied, and its movements in 

 the water increase the similitude. The author of the ' Fresh 

 and Salt Water Aquarium' (the Rev. J. G. Wood, M.A.) says 

 they act much like fish in their habit of keeping their heads 

 up the stream, and in their general conduct look something 

 like the fry of various fish. 



Sometimes they make their way up the stream by clinging to 

 the stones and other objects that form the bed of the stream, 

 making quick darts forward, and then holding tightly to a stone 

 until they choose to make a second dash onwards. When they 

 have gone up the stream as far as they thmk proper they loosen 

 their hold and come drifting back again, sometimes rolling over 

 and over, but generally contriving to keep their heads pointing up 

 the stream. In fact, they appear to amuse themselves by this 

 action, just as the gnats amuse themselves by dancing up and 

 down in the air. 



The food of the fresh-water shrimp is usually decaying animal 



