Catastrophic Decline of Mussels 5 



Table 2. Comparison of mussel and Corbicula densities (No./m 2 ) estimated 

 from quantitative samples taken in 198 l a and in 1987. 



Family 













and 









Station 





year 



8 



13 



16 



Unionidae 













1981 







7.5 



7.2 



2.9 



1987 







0.0 



1.1 



0.0 



Corbiculidae 













1981 







46.6 



43.7 



10.6 



1987 



nd 



Bogan 



366.0 

 (1982) 



363.8 



115.5 



1 198 1 data from Starnes a 





DISCUSSION 



Downstream from site 7, unionid populations have been devastated. 

 Although remnants of the fauna persisted in this section, we saw few 

 live mussels. The decline in these populations is further illustrated by 

 comparing estimated densities with those of Starnes and Bogan (1982) 

 (Table 2). During our survey, only site 13 had mussel populations that 

 were plentiful enough to enable us to estimate population density — 1.1 

 mussels/ m 2 compared with 7.2 mussels/ m 2 estimated by Starnes and 

 Bogan (1982). Despite the disappearance of unionids, the estimated 

 populations of C. jluminea increased about ninefold at all three sites 

 from 1981 to 1987. Although not quantitatively sampled in our study, 

 pleurocerid snail populations were apparently healthy and often dense. 



Although the exact cause of the mussel kill in the lower third of 

 LSF is unknown, it is apparent that not all species have been affected to 

 the same degree. In 1981 P. fabula was found at densities of 2.2/m 2 at 

 site 8 and was the most abundant mussel at site 16 (Starnes and Bogan 

 1982). Although we found shells of P. fabula at all sites from site 8 

 downstream to site 15, we failed to find living specimens at these sites. 

 Similarly, V. trabalis, which was previously reported alive at two sites, 

 was not found alive in this area. In contrast, Ptychobranchus subtentum 

 was the most abundant mussel in both 1981 and 1987. Although it was 

 the mussel most frequently seen alive in the lower section of the stream, 

 its densities were much lower in 1987 than in 1981. 



The increase in density of C. Jluminea in the lower section of LSF 

 and its upstream range extension are in marked contrast to the unionid 

 populations. Corbicula jluminea has been invading the Cumberland 

 River and its tributaries for many years (Counts 1986). Apparently it is 

 still colonizing the LSF; however, part of the observed increase in 



