Fish Culture in Inland Waters 253 



to the surface as " floating islands " (figure 30). Many small ponds 

 have been filled with such accumulation and become mere frog 

 ponds or even quaking bogs. The outlets of lakes or ponds of such 

 character are almost always lowered to such extent that they are 

 practically dry in summer. 



In time the animal life of these ponds, particularly the lower forms, 

 also increases in quantity and changes in character. But unless by 

 the instrumentality of man the fish life has seldom been changed 

 in character, although it might be in quantity in one way or another. 



The kinds of fish suitable for such ponds vary with the character 

 of the pond, as in the case of natural ponds. So in stocking 

 the physical and biological conditions must be ascertained 

 just as in the case of natural ponds. In restocking, guidance may 

 be provided by a knowledge of the original inhabitants, but as in 

 the case of natural ponds, this is not always a safe criterion, for the 

 pond may have greatly changed since it was occupied by the original 

 stock, and even the original stock may have existed only by tolerance 

 and perhaps in much smaller numbers than some new forms would 

 provide. So as in the case of natural ponds, each sort of pond fur- 

 nishes its particular problem. 



Streams. The term stream is a comprehensive one, even when 

 restricted to water courses. A river, brook, creek, run or rivulet, 

 is a stream. But among our common names for streams there is 

 no definite rule to enable us to distinguish by name a river from a 

 brook by the size. Webster defines a river as a natural stream of 

 water larger than a brook or a creek, and brook as a natural stream 

 of water smaller than a river or creek. The creek is an inland 

 stream, stated to be smaller than a river, larger than a rivulet or a 

 run. A rivulet is " a small stream or brook ; a streamlet," and is 

 synonymous with rill. A run as applied to streams is " a brook, a 

 stream, a watercourse." Thus only in a general way does the 

 dictionary enable us to distinguish one from the other. Common 

 usage also is about as obscure. 



Few streams are relatively non-fluctuating in volume through the 

 dififerent seasons (figure 33) or uniform in character throughout 

 their course, and therefore vary more or less in suitability or capa- 

 city for any one kind of fish. Such variations may be permanent 

 or temporar^^ Permanent variations in a measure determine the 

 fish fauna of the dififerent areas. Thus only one or two species mav 

 occur in the upper and several in the lower waters, as for example 

 brook trout in the upper portion and pickerel and suckers or other 

 species in the lower section. Then again a stream is often inter- 

 rupted by rapids or riffles and intervening still water sections, as 

 so-called "dead waters," and pools. Such localities often afiford 

 the occasion for seasonal movements of fish for breeding, feeding 

 or safety. 



Temporary variations are such as are caused by seasonal changes 

 (figures 32, 34, 35), as in height of water, temperature, etc. These 

 variations may also be the occasion of more or less migratory move- 

 ments of the fish inhabitants of the stream or a section of it. 



