BOTTOM FAUNA OF BLACKWATER ESTUARY: 1970 269
Comparison of Table 2 with the data of Davis (1967) shows
that Cheirocratus intermedius, Nereis longissima and Nephthys
caeca, which were regarded as rare or extremely local by that
author, were found at between one-fifth and one-third of the
Blackwater stations in 1970. This, however, does not necessarily
indicate an actual extension of the range of these species, by the
same token as the new records noted above are unlikely to be
recent immigrants into the Blackwater. Both changes are most
probably the result of bias on the part of the personnel conduct-
ing the survey. Any survey operated by a small number of
personnel is strongly subjected to bias in the form of the
particular interests of the people concerned: this becomes more
severe with decreasing numbers of participants. The 1970 sur-
vey was biased in this way towards the Crustacea and the Poly-
chaeta, and therefore it is not surprising to find that the three
species mentioned above are members of those groups and that
53% of the new records were of polychaetes and 20% were of
crustaceans.
The other species listed in Table 2 are all previously recorded
"common" to "abundant" species.
No very close or extensive correlations between the substrate
and the animal distributions within the Blackwater were appar-
ent, although Cheirocratus was confined to areas of shell debris.
Many of the less common species were, however, confined to
certain regions within the estuary as a whole (n.b. since very few
samples were taken and since the survey lasted less than a fort-
night, the distribution patterns of the rarer species must be
regarded as extremely approximate, but since a number of
definite patterns in the data were apparent, these are given here
as guides). Several species, e.g. Dexamine spinosa, Microdeuto-
pus anomalus and Botryllus schlosseri, were recorded only from
the Thirslet region (Fig. 2a); others, e.g. Stylarioides plumosa,
Flabelligera affinis and Nereis pelagica, were found only in mid
channel (Fig. 2b); whilst Nephthys caeca demonstrated the con-
verse, being taken only to shorewards (Fig. 2c). Some, e.g.
Scoloplos armiger and Ciona intestinalis, did not occur to sea-
wards of Pewit Island, whilst a large number of species, includ-
ing Nucula nucleus, Cancer pagurus, Palaemon serratus and
Crangon vulgaris, were restricted to the offshore stations (Fig.
2d). It is interesting to note that Psammechinus was once plenti-
ful within the Blackwater, but possibly as a result of a series of
cold winters it died out, only occurring in the offshore regions
(Orton & Lewis, 1931). Davis (1967) only recorded it from off-
shore. The 1970 survey, however, recorded a number from
station S.18 within the Blackwater (Fig. 2d), and so perhaps the
species is effecting a slight recovery. Similarly, Alcyonium
digitatum had only previously been recorded from the offshore
waters, but in May 1970 it was present at HB.4 (Fig. 2d).
The most productive regions of the Blackwater, in terms of
number of species per station, were the mid channel and Thirslet