Population size and structure of three mussel species (Bivalvia ...

175
Dominance [%]
80
70
60
50
40
30
20
10
0
Station I
II
III
N 985 465 156
Figure 5. Species composition at the sampling stations I, II,
and III. Aa ¼ A. anatina; Ac¼ A. cygnea; Ut¼ U. tumidus;
n ¼ number of collected mussels.
Table 3. Similarity of the dominance structures according to
Renkonen’s coefficient (Rc)
Stations I/II I/III II/III
Rc [%] 72.44 89.71 82.73
I, II, III = sampling stations.
survey. At all stations, one or more age classes
were absent (Fig. 6). The youngest age classes
reached relatively low dominance values in the
stocks, and the older ones were very numerous and
partly even predominant. This tendency to overaged
stocks became clear especially regarding
A. anatina at station III.
At the three sampling stations, different age
classes of the Anodonta species were absent or
dominating. At the stations I and III, for example
the age classes of 1987, 1989, and 1995 were very
Aa
Ac
Ut
rich in individuals. At sampling site II, the age
classes 1988–1991 were represented by only few
individuals or they lacked completely. These differences
between the age structures suggest that
similar living conditions for mussels cannot be
presumed in the whole upper course of the river
Ryck.
In order to assess the similarity of age structures
more efficiently, set quotient, Renkonen‘s
coefficient, and Wainstein’s index were calculated.
The results showed that there was a higher similarity
between the age structures of both species
from the same station, than between the age
structures of the same species from different stations
(Fig. 7). This similarity between the age
structures of A. anatina and A. cygnea was greatest
at the sampling sites I and II. Amongst the stations,
the highest similarity was detected between
the sampling sites I and III (comp. Table 3).
For U. tumidus the partly very low abundances
and especially the age structures (Fig. 8) indicate
an instability of the population. At station I, a
very low population density of this species was
detected. But it has to be underlined that in this
certain river section some juvenile mussels were
also found. At station II, the population virtually
consisted solely of two age classes, one of them
was considerably young. The stock at station III
appeared overaged since no juveniles could be
collected there.
Discussion
The species Anodonta anatina, Anodonta cygnea,
and Unio tumidus which were found in the upper
course of the river Ryck inhabit running and
Table 4. Population size and mean abundance of the Unionidae in the river Ryck
Taxon I II III
Abundance (N/m 2 ) Pop. size (N/rm) Abundance (N/m 2 ) Pop. size (N/rm) Abundance (N/m 2 ) Pop. size (N/rm)
Unionidae 33.6 311.3 34.9 350.5 10.9 216.2
A. anatina 23.2 214.6 18.8 188.8 7.0 138.5
A. cygnea 10.2 94.7 6.2 62.4 2.8 55.0
U. tumidus 0.2 2.0 9.9 99.3 1.1 22.7
I, II, III = sampling stations; N = number of mussels; rm = river meter (stretch of the river of 1 m length).