THE FAUNA OF THE CROUCH/ROACH ESTUARY 133
study. Samples have been collected by grab and dredge to find the inter-
actions of other fauna with oysters (Mistakidis, 1951), or to record effects
of the Bradwell Nuclear Power Station on the Blackwater fauna (Davis,
1965; Barnes and Coughlan, 1971). This difference in sampling methods
mainly accounts for the higher proportion of nekton (especially fish)
against benthos (especially the molluscs and polychaetes) in the present
work. The bias is accentuated by the fact that these samples come from
mid-channel, so that many of the littoral animals are unrepresented. The
ecological significance of the benthic species is considered in the papers by
Mistakidis and Davis and need not be repeated here. However, some of the
present records are interesting in themselves, while others appear to differ
from Davies's (1967) observations in the Blackwater and deserve comment.
The Roker (Raja clavata) is common in the estuary in the warmer
months of the year. Most of the records in fact are of recently hatched
young but large specimens are taken by anglers, and reputedly by spearing
in hot sultry weather, when the fish sometimes swim at the surface.
This presumably indicates a critical oxygen level away from the
air interface. Davis (1967) described the long-clawed Porcelain Crab
Porcellana longicornis as still undetected despite three consecutive years of
survey in the Blackwater following a population slump of this species in
1961. Barnes and Coughlan (1971) were unable to find Porcellana in 1970,
but it was present in the Crouch, during this survey. The Sea Snail (Liparis
liparis) is described by Davis (1967) as being occasionally recorded in the
Blackwater, whereas it is a prominant member of the Crouch fauna. Those
caught in the January survey were adults approaching spawning, but none
were caught anywhere during the April survey. By October, there were
large numbers present again, but all were small and presumably had
hatched earlier in the same year.
The Lemon Sole (Microstomus kitt) also occurred in greater numbers in
the Crouch than in the Blackwater, where Davis (1967) described it as
'occasional in summer'. The present survey showed it to be present during
January and quite common during April at Station III, although all
specimens were juveniles. In addition to this specific limited survey, a
general fauna list is maintained at Burnham on which all interesting
records of marine animals occurring in the area are recorded. This is
necessarily haphazard, containing only records that happen to have been
made as a by-product of some other activity, but recent notable species in
the Crouch which are absent from Davis's (1967) Blackwater list include
Sardina pilchardus, Pollachius virens, Spondyliosoma cantharus, and
Pomatoschistus pictus.
In summary, most of the animals caught are those that would be ex-
pected to be found in a North Sea estuary like the Crouch. Significant
exceptions are the Lemon Sole (Microstomus kitt) whose presence indicates
good water quality on the river bed, and the warm water species Sardina
pilchardus and Spondyliosoma cantharus indicating that water conditions
have allowed them to extend their range from the English Channel.