Some of the conditions that make a system of closed regions
particularly advisable for the conservation of fresh-water mussels may
be briefly mentioned:
1. It has been previously stated that some of the mussels are spawning,
or with spawn, during any period of the year. Many of the most
important species are spawning during the late spring, early and mid
summer; other equally important species form their eggs in the late
summer, when they become fertilized and develop into the glochidium
stage, but the mother clam retains them in marsupial pouches within her
shell during the entire winter and even into the summer. All species of
mussels carry the eggs in the marsupial pouches during the process of
development to the glochidium stage 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 observed that when gravid mussels are
disturbed they frequently discharge the young, regardless of whether
these are mature enough to be liberated from the parent or not; certain
species, such as the niggerhead, are particularly likely to do this.
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.