22
T. I3. R E T N O L D S O N
TABLE I V
d Tgp'icul Y v i c l d E'uicnu C'onipwr~d with thosc of Auglcsey plus Isla?/, and
Scotlad lVorlh of the Fwult (CaledoniarL Canal)
Species
Absent
Polycelis rcigra
Polycelis tenuis
Dugesia lugubris
Dendrocoelum Euctrii ?ti
Total lakes
Lakes with > 7.5 mg/litre calcium
Typical
Anglesey & Islay
N. Scotland
_ _ _ ~ - ~
24 Yo
21%
50 Yo
56%
79%
10%
2OYO
0 Y O
40 YO
7 %
0%
0%
4 Yo
0 Y O
0%
54
14
10
Lakes with > 7 *5 mgjlitre calcium
-
_
_
_
~
~
Absent
0%
0 Y O
75 %
Polycelis nigra
74%
87 %
0 YO
Polycelie tenuis
94%
26%
25 %
Dugesia lugubrie
52 %
0 Y O
0%
Dendrocoelum lacteum
52 %
4%
0%
Total lakes
50
23
12
supported P. tenuis are examined, we find that they are all well-known
trout-fishing lakes which have been stocked from time to time with
mainland trout. The four Islay lakes have not only been stocked
frequently (Reynoldson, 1958a) but also they lie close together, within
a radius of 2 miles.
The data for the extreme north of Scotland give an entirely different
result. Here, in lakes with G7.5 mg/litre calcium the outstanding feature is the large proportion of lakes which did not contain triclads, and
this was even more pronounced in the calcium-richer lakes where 75%
lacked a population. It might be emphasized at this point that the same
criteria for lake selection were used here as already described (p. 8). The
other equally striking feature of these northern lakes is that in the small
proportion (seven lakes) which did contain triclads, P . tenuis was the
species recorded, a reversal of the situation on the western islands. As
noted for the islands, several of the lakes containing P . tenuis are noted
trout-fishing places, e.g. L. Heilin (Caithness) ; L. Hempriggs (Caithness); L. Ussie (Ross and Cromarty); and furthermore the other four
are grouped closely around L. Garve (Ross and Cromarty). This evidence
for the scarcity of triclads from the extreme north of Britain was supported by data from islands of corresponding latitude. For example,
lake-dwelling species were scarce on Rhum, where one lake had a small
population of P. nigra and another a population of P. tenui8. None were
T. I3. R E T N O L D S O N
TABLE I V
d Tgp'icul Y v i c l d E'uicnu C'onipwr~d with thosc of Auglcsey plus Isla?/, and
Scotlad lVorlh of the Fwult (CaledoniarL Canal)
Species
Absent
Polycelis rcigra
Polycelis tenuis
Dugesia lugubris
Dendrocoelum Euctrii ?ti
Total lakes
Lakes with > 7.5 mg/litre calcium
Typical
Anglesey & Islay
N. Scotland
_ _ _ ~ - ~
24 Yo
21%
50 Yo
56%
79%
10%
2OYO
0 Y O
40 YO
7 %
0%
0%
4 Yo
0 Y O
0%
54
14
10
Lakes with > 7 *5 mgjlitre calcium
-
_
_
_
~
~
Absent
0%
0 Y O
75 %
Polycelis nigra
74%
87 %
0 YO
Polycelie tenuis
94%
26%
25 %
Dugesia lugubrie
52 %
0 Y O
0%
Dendrocoelum lacteum
52 %
4%
0%
Total lakes
50
23
12
supported P. tenuis are examined, we find that they are all well-known
trout-fishing lakes which have been stocked from time to time with
mainland trout. The four Islay lakes have not only been stocked
frequently (Reynoldson, 1958a) but also they lie close together, within
a radius of 2 miles.
The data for the extreme north of Scotland give an entirely different
result. Here, in lakes with G7.5 mg/litre calcium the outstanding feature is the large proportion of lakes which did not contain triclads, and
this was even more pronounced in the calcium-richer lakes where 75%
lacked a population. It might be emphasized at this point that the same
criteria for lake selection were used here as already described (p. 8). The
other equally striking feature of these northern lakes is that in the small
proportion (seven lakes) which did contain triclads, P . tenuis was the
species recorded, a reversal of the situation on the western islands. As
noted for the islands, several of the lakes containing P . tenuis are noted
trout-fishing places, e.g. L. Heilin (Caithness) ; L. Hempriggs (Caithness); L. Ussie (Ross and Cromarty); and furthermore the other four
are grouped closely around L. Garve (Ross and Cromarty). This evidence
for the scarcity of triclads from the extreme north of Britain was supported by data from islands of corresponding latitude. For example,
lake-dwelling species were scarce on Rhum, where one lake had a small
population of P. nigra and another a population of P. tenui8. None were