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

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Figure 7. Similarity of the age structures of both Anodonta species according to set quotient (Sq), Renkonen’s coefficient (Rc), and
Wainstain’s index (Wi). Aa ¼ A. anatina; Ac¼ A. cygnea; I, II, III ¼ sampling stations.
40
Ut (stn.I 96)
N =10
80
Ut (stn.II 97)
N =131
30
60
N [%]
20
N [%]
40
10
20
0
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
Age [years]
Age [years]
90
Ut (stn.III 97)
N =17
60
N [%]
30
0
0
2
4
6
Age [years]
Figure 8. Age structures of Unio tumidus (Ut). I, II, III ¼ sampling stations; 96, 97 ¼ years of surveys (1996 and 1997); n ¼ number of
collected mussels.
8
10
12
14
(1997) and Hochwald & Bauer (1990) verified a
reducing percentage of fertilized eggs in the clutch
during a period of a decreasing population density.
Downing et al. (1993) proved for Elliptio complanata
(Unionidae) that no successful fertilization is
possible at an abundance less than 10 mussels/m 2 .
Therefore, the stock at station I is possibly unable
to reproduce and depends on an import of glochidia
from other stretches of the river Ryck. In
the river section at station II, the stock was mainly