In the commercial fishery, therefore, not only is much spawn destroyed when large gravid mussels are captured, but it is quite probable 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. When 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 hundred feet long. Nevertheless, 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 enforcement, 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 might be a substantial uncompensated loss to some communities, where there are factories employing labor to cut shells derived from that river. On the other hand, should we divide the river up into small sections of 2 or 3 miles in extent, some of which would be open while others would be closed under the law, it is apparent that such a plan would be almost impossible of enforcement. To prevent shelling from being carried on in all these little, closed areas would require a force of wardens and an expense entirely incommensurate with the object to be gained.