12
THE ESSEX NATURALIST
It is, however, very doubtful whether this cause could operate in
the case of comparatively open water. It is, of course, easy to
understand 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 starfish caught, and the various operations
of oyster culture, may materially affect the number of some
other animals, since the inter-relationships 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".
Up to the end of the last century it would seem that changing
sedimentation, perhaps related to a stage of subsidence of the land,
extreme climatic conditions, and the influence of fishing interests
were causing some changes in the fauna. Since 1900 there have
been extra factors to take into consideration, namely the intro-
duction of exotic species with American oysters and the loss of
the eel grass (Zostera marina) from the coastal waters. There are
thus five main factors which might cause the fauna to change and,
to some extent at least, these are inter-related. Keeping these
factors in mind, the course of change in the Blackwater area might
be given as follows : There is strong evidence for a slow subsidence
of the coast and it is to be expected that as this progressed there
would be changes in the erosion and sedimentation patterns with
some associated changes in the fauna. Severe winters and other
extremes of climate would, from time to time, cause decreases in
the populations of some species and these may be unable to fully
re-establish themselves in the face of new conditions. In the 1890's
American oysters were introduced to Brightlingsea and with them
came at least three notable introductions, namely, Crepidula
fornicata, Urosalpinx cinerea and Petricola pholadiformis. While
the oysters themselves were a failure, the accidental introductions
became well established and achieved wide distribution.
Urosalpinx has been much discussed as it is an important oyster
pest, but it is Crepidula which has proved to be of fundamental
importance in the Blackwater. The species was still rare in the
Colne in 1897 (Cole, 1897). In 1915 Cole reports that some 35
tons were dredged up in four days by oyster fishermen working the
Blackwater grounds, and he concludes his paper, " . . . . The causes
of its (Crepidula's) abnormal increase, and its influence on the
culture of oysters, is little understood and certainly deserves care-
ful study". Therefore it seems that the 'population explosion' of
Crepidula took place in less than twenty years and would most
certainly have had a fundamental influence on the nature of the
bottom fauna. In fact the Crepidula became so abundant that
their shells and the mud produced as 'pseudofaeces' created a new
type of bottom. Added to this, in the 1920's the eel grass, which
formed extensive beds on the shore and below low-water mark,
suffered more or less a complete extermination due to a widespread
disease. On-shore waves were then allowed to build up as never