PROTECTION OF FEESH-WATER MUSSELS. 19 



or longer, whether the period be for a few weeks or for a few months. 

 In this condition the mussels are said to be gravid. It is readily ob- 

 served that when gravid mussels are disturbed they frequently dis- 

 charge the young, regardless of whether these are mature enough to 

 be liberated from the parent or not; certain species, such as the nig- 

 gerhead, are particularly likely to do this. 



In the commercial fisheiy, therefore, not only is much spawn de- 

 stroj^ed when large gravid mussels are captured, but it is quite prob- 

 able that other mussels, disturbed on the bottom, though not captured, 

 are caused to abort the young in an immature stage when they are 

 entirely unable to complete the development without the parent. 



2. In the stage of existence immediately after liberation from the 

 parent, the young mussels are parasitic upon fish. We are not here 

 concerned with them during this period of the life history. AYlien 

 they are dropped from the fish many of the young mussels do not at 

 once take up life in the sand or mud of the bottom, but we find them 

 forming delicate threads by which they hang from plants or sticks 

 or stones or from clam shells, and thus are kept from being washed 

 away or smothered in the mud of the bottom. "We may imagine the 

 harm to these little mussels that is unavoidably wrought when the 

 beds are continually dragged over. In like manner, the little shells 

 that are just beginning to take hold in the bottom may be torn out by 

 the rake or hooks, to be smothered or washed away to less favorable 

 bottoms. It will be remembered that when mussels first begin life in 

 the thread stage or in the bottom if the thread stage is omitted, they 

 are too small to be found without a microscope. 



3. One of the principal methods of capturing mussels is with the 

 bar and hooks dragged over a large area of mussel bed in taking a 

 relatively small number of shells. There is chance for these hooks to 

 injure many little shells when each drag, requiring a period of only a 

 few minutes, covers a space of bottom 16 feet wide and several hun- 

 dred feet long. Xevertheless, it is not certain that there is any 

 method to take its place, and any implement used will accomplish 

 some injury to the very youngest mussels. 



CONSIDERATIONS DETERMINING SIZE OF CLOSED REGIONS. 



In planning for the closing of portions of rivers for periods of 

 years consideration should be given to community needs as well as 

 to general economic and biological conditions. On the one hand, the 

 closure will be more effective in result, as well as easier of enforce- 

 ment, if the regions of closure are made very large; while, on the 

 other hand, making the closed regions smaller might cause less 

 economic inconvenience. If, for example, the entire Illinois River 

 should be closed to mussel fishery for a period of several years, there 



