THE MARINE FAUNA OF THE BLACKWATER ESTUARY 11
Brittle Starfish (Ophiothrix fragilis) declined to an undetectable
level during the severe winter of 1962-63, but was recorded in
small numbers in the autumn of 1964 and almost re-established
to its original status in the summer of 1965. The Tingle, Nucella
lapillus, shown by Orton and Lewis (1931) to have suffered high
mortality in the winter of 1928-29, is now common on parts of the
lower shore of the Blackwater. The related species, Ocenebra
erinacea which suffered at the same time, though at times regain-
ing some ground (Mistakidis & Hancock, 1955), has not recovered
to establish a breeding population. Psammechinus miliaris, once
common in the Blackwater, apparently suffered continual setbacks
as a result of cold winters (Orton & Lewis, 1931), and is now only
found in offshore areas. Also the observations made on some
Blackwater species in the winter of 1962-63 tend to contradict
observations from other parts of Britain reported in Crisp et al
(1964). It also seems unusual that Nucella lapillus did not suffer
so obviously in 1962-63 as it did in 1928-29. Therefore, although
severe winters can cause setbacks in animal populations in certain
circumstances, other factors must be responsible for the con-
tinued decline and inability of species to recolonize lost ground
lying adjacent to an established population. The appearance of
rare species, as juveniles, in the Blackwater benthos, indicates
that planktonic larvae can be carried in from offshore waters.
An example of such a rare introduction is the bivalve Phacoides
borealis of which only two living juveniles have been recorded.
Sorby (1901 and 1903) remarked upon changes in the Essex
marine fauna that he observed between 1890 and 1903, par-
ticularly around Brightlingsea and also in the estuaries of the
Stour and Deben. In summing up the changes that he observed,
Dr. Sorby writes: —
"It will thus be seen that, even in the short period of 10 or 12
years, many noteworthy changes have occurred, and, in some
cases at all events, it seems doubtful whether the original condi-
tions will be restored in years to come. This is certainly indicated
by the fact that in so many cases vast numbers of well preserved
dead shells are met with in situ in places where the same species
are seldom, if ever, met with alive. Taking all into account, it
certainly appears to me that a considerable number of interesting
animals have become more and more rare, whilst but few have
become more abundant. The first explanation that suggests itself
is that the changes may in some way have been due to human
agency, like so many changes in animals and plants inland. In
some localities an increase in the amount of sewage discharge
may have been the cause, but, though in this respect the worst
place I ever stayed at is the water at the back of Walton-on-Naze,
yet I never saw a better collecting ground for numerous interest-
ing animals .... 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 time of floods ....