Fisli Culture in. Inland Waters 277 



The deterioration and the decrease in numbers of fish will be dis- 

 cussed as follows : 



1. Concurrent decrease in size and number of fish may be due to 

 one or more of several assignable causes. 



In many instances the apparent decrease in size may be attribu- 

 table to over-fishing, especially in a comparatively small body of 

 water. To attain a large size the fish require time in which to grow, 

 and persistent intensive fishing tends to lower the size standard by 

 continuously removing the larger sizes, so that sizes previously re- 

 garded as small come to be the maximum size attained. 



If a size and quantity limit is strictly observed in the fishing (which 

 is seldom the case) the coincident decline of size and number may 

 be due to decrease of food supply, and cannibalism. 



There may be other factors than those mentioned which are operat- 

 ing independently. Whatever the cause may be, it can be positively 

 determined only by competent investigation. 



2. Decrease in size with no decrease in number may be attribu- 

 table to one or the other of two common causes. One is that of re- 

 duced food supply. The other is that of the larger fish having been 

 caught before they have had time to attain a large size. But the 

 most common cause is likely to be found to be that there are too 

 many individuals compared with the amount of food for them. In 

 other words there is lack of balance. 



3. Decrease in size with increase in number may be attributable 

 to inadequate food supply and exceptionally favorable breeding 

 conditions, with freedom from enemies, or it may be a case of over- 

 stocking. 



4. A decrease in number with no decrease in size, suggests as a 

 cause over-fishing and unfavorable breeding conditions. 



5. A decrease in number with increase in size suggests over-fish- 

 ing or other destructive agency, with no diminution of food supply, 

 so that the remaining fish are well fed. Or, if the water has been 

 artificially stocked, that the existing fish are merely survivors of 

 the original plants, and that they have not reproduced, at least 

 not extensively enough to maintain the stock. 



It has often been noticed that following the first, introduction of 

 some kind of fish in a body of water their growth is rapid and they 

 attain a large size in good numbers. Later it becomes evident that 

 the fish have quite generally fallen off in size, or newly planted fish 

 do not grow as rapidly or attain the size of those first planted. 



Such instances have been many, and have led the fish commis- 

 sioners, the fish culturists, or the interested public, to attribute the 

 facts to various causes other than the real one. Or, it may be that 

 while the cause was obscure, or without any consideration of the 

 possible or probable cause, the situation being unsatisfactory, there 

 is a demand for some other fish. If the Commissioner did not comply 

 with the demand, the new fish were sooner or later secured by first 

 getting a new Commissioner. In a few instances the new fish ap- 

 peared to be the solution, for it waxed great in size and was suftici- 



