popialation to maintain the same relative abundance^ disregarding natural 

 mortality. The significant decline in catch per unit of effort between 

 1956 and 1957 indicated a declining abiindance^ substantiated by the age 

 structixre of the available population in Wheeler Reservoir (see figure 9) • 

 Over 90 percent of the popiilation was older than 12 years and almost 20 

 years had elapsed since the dominant year class originated. Recruitment 

 to the population between 1956 and 1957 was less than 1 percent as com- 

 pared to the 23 percent loss by commercial exploitation. Since the beds 

 on Wheeler Reservoir had been exposed to commercial fishing for the past 

 12 years, it was suspected that harvest operations might have a harmful 

 effect on the survival of young mussels and therefore limit recruitment. 

 Further studies on Chickamauga Reservoir failed to justify this hypothesis. 

 The age structure of this relatively unexploited population showed that 

 over 95 percent of the population was older than 12 years. These old- 

 age stocks were dominated by the year classes of 193^-36. While protected 

 from fishing, there has been no recruitment to the Chickamauga population 

 during the past 6-year period as evidenced by the failure to find any 

 mussels younger than the 1951 year class. 



Age and growth studies of the pigtoe mussel showed that between 

 8 and 9 years is reqiiired to produce a desirable market size shell of 

 50 mm. in length. This also is approximately the age at which they reach 

 an optimum catchable size. The slow growth rate does not permit a rapid 

 restoration of catchable stocks to the fishery even if reproduction should 

 be successful at least once within the next few years. Since growth rates 

 were similar in Wheeler and Chickamauga Reservoirs, there is no evidence 

 that favors one location over the other so far as growth is concerned. 



Examination of the condition of gill marsupia in gravid female 

 plgtoes in 1957 indicated glochidia production to be satisfactory for 

 the pop-ulatlon in Wheeler Reservoir. Release of glochidia by parent 

 females occurred d-uring three distinct periods between May I5 and Octo- 

 ber k. However, since the parasitic phase in the life-cycle of the 

 pigtoe is unknown at present, the fate of the glochidia produced could 

 not be determined. Despite the fact that gills from 6l3 fish of I7 

 different species were examined during the summer months of 1956 and 

 1957^ "the Identity of the host- fish presumed necessary for complete 

 development of the pigtoe glochidia could not be determined. Because 

 of this complex life-cycle, the availability and habits of a host-fish 

 could have a great influence on the future status of the pigtoe stocks. 

 To accOTint for the natural abundance and present distribution of the 

 pigtoe in the Tennessee River, an excellent relation of coexistence 

 between the glochidia and its host- fish must have prevailed in past 

 years. Presumably, the success of this relation has declined because 

 of a change in the availability of the host- fish or poor survival of 

 the glochidlal stage in the present environment. 



That the silt deposits on recently examined bottoms in Wheeler 

 and Chickamauga Reservoir may be having a detrimental effect on the 

 mussels present, is indicated by the finding of large numbers of properly 

 oriented dead shells, young as well as old individuals, under heavy depos- 

 its of silt. Very few live young mussels, with the exception of the 



36 



