216 
BULLETIN OF THE BUREAU OF FISHERIES 
of mussels at all comparable to the fauna of Lake Pepin. The conditions of sedi- 
mentation in Lake Keokuk seem entirely different from those in Lake Pepin and 
quite likely are far less favorable for mussels. 
In 1926 the author was greatly impressed by what seemed a remarkable change 
in the aspect of the mussel fauna of the river as a whole between Lake Pepin and 
Keokuk. Twelve years before the niggerhead mussel had been the standard shell 
of button manufacture and was still abundant at many places in the river, although 
evidently in course of gradual depletion. It is a slow-growing species, and the 
mussel fishery was then pursued without check. There were many other species 
of commercial mussels occurring in varying abundance. One of these was the yellow 
sand shell, which was never found in abundance. Those that were taken along 
with other shells were sorted out on the banks if found in numbers to justify the 
effort. Those that reached the factories in mixed lots of shells were picked out there. 
The shell was considered too valuable for use in button manufacture and was gen- 
erally shipped abroad to be used for the making of novelties, such as knife handles 
and ornaments for umbrella handles. Its high value was based in part upon its 
exceptional qualities among fresh-water shells, in both form and texture, and in 
part upon the fact that it was never available in relatively great quantities. 
In 1926 the conditions had changed. Niggerhead mussels had become scarce 
in most parts of the river visited and very small shells of this species seemed especially 
rare. This information was gained by examination of many shell piles on the river 
banks (piles including all shells taken) and by inquiries of shellers and manufac- 
turers. The evidence was strong that the niggerhead mussel is not reseeding itself 
successfully in the river. There may be presumed to be several responsible factors, 
of which overfishing may be one, for the mussel is one of the slowest growth. A 
second factor is undoubtedly the depletion of the river herring, formerly abundant 
in the upper river but now relatively rare; for this the dam might be responsible. 
(See discussion of river herring, p. 165.) There may also be other conditions that 
affect this particular mussel more unfavorably than others. We present the appar- 
ent facts; a definitive explanation of causes is out of the question in view of the 
complexity of the conditions involved. 
On the other hand, the yellow sand shell, formerly relatively scarce, had become 
a very common mussel by 1926 and was extensively used in button manufacture. 
The shell is eminently adapted for the purpose, because it has the proper texture 
and is of such form as to yield a very high number of blanks per ton, although it 
lacks the qualities of fine irridescence possessed by the niggerhead shell in its hinder 
part. It is a very profitable shell to use when it is had in such abundance that the 
cutting machines can be set for it. Furthermore, it is a species of far more rapid 
growth than the niggerhead. To one who had known the conditions in former years 
it was an occasion for surprise when small boats were seen in 1926 containing only 
sand shells. At one factory the author was shown a single heap containing 200 tons 
of sand shells from the Mississippi River. 
For the increased numbers of sand shells there may be several causative factors. 
A greater quantity of gars, if prevailing, would be one factor. The chief cause may, 
however, be the development of a condition of stability in the areas between wing 
dams. These have been constructed by the Government in great numbers along 
almost the whole course of the upper river, with the idea of restricting the channel 
and causing the river to scour its main course. Away from the channel and between 
