deertoe, a mud-loving species, were found in SCUBA samples. Silt 

 deposition has long been regarded as a destructive factor on mussel 

 beds. Ellis (l936) reported that lethal limits of silt coverage on 

 experimental beds of mussels varied from l/k inch for the least 

 resistent species to 1 inch for some of the most resistent species. 

 Death was attributed to gradual smothering as the silt accumulated 

 over the mussels. 



Siltation in the reservoirs of the Tennessee River was inten- 

 sified after impoiindment of the stream was completed during the years 

 1936 to 19ij-4. On the basis of the age structure of the population of 

 pigtoe mussels in Wheeler Reservoir, the dominant year classes of 

 1936-39 were produced during the years before the reservoir system 

 was completed. On Chickamauga Reservoir, over 90 percent of the pig- 

 toe population was preimpoundment stock. This indicates that success 

 of population expansion and the change from a stream to a reservoir 

 type habitat are related. As the present age structures show, recruit- 

 ment to the populations by subsequent year classes has steadily declined 

 diiring the elapsed span of years since habitat conditions changed. 



As previously stated, the results from these studies apply to 

 specific areas in the Tennessee River. Because of physical limitations, 

 it was impossible to carry out a comprehensive study program on all 

 parts of the river simultaneously, which would be necessary to determine 

 annual variations in population structure. Further research is needed to 

 understand the complete life-history of the pigtoe mussel. This is impor- 

 tant since the economic level of the commercial fishery is primarily 

 dependent on the abundance of this species. 



CONCLUSIONS 



The pigtoe mussel Pleurobema cordatimi is the most abiindant com- 

 mercial species taken by the mussel fishery on all parts of the Tennessee 

 River. On Wheeler Reservoir, the pigtoe comprised over 80 percent of 

 the commercial catch in 1956 and 1957 • 



The commercial fishery on Wheeler Reservoir declined between 1956 

 and 1957 as evidenced by the significant reduction in total catch, catch 

 per Tonlt of effort, and fishing effort during the latter year. This de- 

 cline was attributed to the high rate of exploitation during 1956, and 

 presumably earlier years, which reduced the size of the population avail- 

 able for the fishery in 1957. 



Catch per unit of effort by the crowfoot brail varies in direct 

 proportion to the size of the available population for a given area. 

 Catch rates also vary on a seasonal basis for a given area, being highest 

 in April and declining as the season progresses. The brail method of 

 fishing is selective for certain species and sizes of mussels in a popu- 

 lation. Pigtoe mussels over 50 mm, in length are especially vulnerable 

 to fishing by the brail method. 



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