MARINE ANIMALS ON THE ESSEX COAST.
23
localities. In some places the water is much more muddy than
it used to be. Another probable cause of change in the estuaries
is the improved draining of the land, which necessarily tends to
increase the amount of fresh water at the time of floods. I have
been told of a case in which such an event killed thousands of
pounds worth of oysters, and this effect would explain why so
many full grown shells of clams and other species are often seen
farther up the rivers than living ones now occur. It is, however,
very difficult to believe that this cause could operate in the case
of comparatively open water. It is, of course, easy to under-
stand that a permanent diminution of fish may have resulted
from over fishing, and we may well believe that the great
number of fish, shrimps, and star-fish caught, and the various
operations of oyster culture, may materially affect the number of
some other animals; since the inter-relations of those living
together are probably sufficiently intimate to produce such an
effect. We must also bear in mind possible cyclic changes in
the weather, and the effect of a series of hot and cold, dry and
wet years.
Though thus calling attention to various possible explana-
tions of the facts I have described, I feel that they are little
better than suggestions, and that possibly the final results may
be due to the variable combined effect of many such temporary or
permanent causes. It seems to me very desirable that those who
have constantly studied other districts should record their experi-
ence, since that might throw far more light on the question, and
eliminate various sources of error which beset this subject on all
sides.
Note.—For the identification of the species named in this
paper, I am to a great extent indebted to Mr. Walter Garstang,
the Rev. T. R. R. Stebbing, F.R.S., and Professor Herdman,
F.R.S., so that I believe they may be relied upon as correct.