ABSTRACT 



Since 19^2, the mainstream reservoirs of the Tennessee River have 

 served as the nation's most important source of fresh-water mussel shell. 

 Mussels are harvested in the Tennessee River by the crowfoot hrail method. 

 Because of its natiiral abundance and good quality shell, the pigtoe mussel 

 Pleurobema cordatum is the most important species to the commercial fishery. 



During 1956 and 1957^ ^ study was undertaken to determine the level 

 of abundance, rate of exploitation by fishing, age composition and potential 

 for natural replacement of the pigtoe mussel on specific beds in the 

 Tennessee River. The distribution of mussels on bottom areas was not random 

 but concentrated on specific beds. Population estimates were made by the 

 tag and recovery method and by actual count in bottom samples using SCUBA 

 gear. Measured drags were made with a crowfoot brail over beds where the 

 numbers of mussels were estimated to determine the correlation between catch 

 per unit of effort and population density. Catch records provided a relia- 

 ble estimate of the number of mussels harvested by the commercial fishery 

 from beds in Wheeler Reservoir during 1956 and 1957* Catch per unit of 

 effort varied in direct proportion to the size of the available population 

 during a given season of the year; however, the catch rate differed on a 

 seasonal basis, being highest in April and declining as the season pro- 

 gressed. The brail method of fishing was selective for certain size groups 

 of pigtoes and for certain species of mussels. Total catch, catch per unit 

 of effort, and fishing effort decreased on Wheeler Reservoir between 1956 

 and 1957* This decrease was attributed to the high rate of exploitation in 

 1956 which reduced the population available to the fishery in 1957* The 

 age structure of the populations studied showed that old age stocks (over 

 15 years of age) supplied the major support to the commercial fishery. 

 The rate of exploitation on beds in Wheeler Reservoir during a two-year 

 period (1956-1957) accounted for almost 23 percent of the available popu- 

 lation. 



Recruitment to the population during this same period amounted to 

 less than 1.0 percent. Poor survival of young mussels appears to be the 

 chief problem in replacement of the populations. The pigtoe mussel repro- 

 duced successfully in 1956 and 1957° The spawning season extended from 

 May 15 to October k in 1957* Results of a search for a sioitable host fish, 

 which is presumed necessary to complete the life-cycle of the pigtoe, were 

 inconclusive. Survival of juvenile mussels was not found to be related to 

 the intensity of commercial fishing as low survival occurred on beds sub- 

 jected to fishing for many years, as well as on new beds which were recently 

 discovered. The life-history of the pigtoe mussel is complex and unfavor- 

 able environmental changes have occurred since the river was impounded which 

 may account for low survival of juvenile mussels. Extensive deposits of silt 

 covered most of the bottom areas studied. 



The results of this study do not indicate that restrictions on the 

 commercial harvesting of mussels in the study area would offer any promise 

 of success in restoring depleted mussel populations to previous levels of 

 abundance . 



IV 



