Overharvesting of stocks of pigtoe mussels in Wheeler Reservoir 

 in 1956 and 1957 resulted from the high rate of exploitation by commer- 

 cial fishing (23 percent both years) as compared to the low rate of 

 natixral replacement (less than 1 percent) . Major support to the fishery 

 is derived from stocks which are over 15 years old; some members appear 

 to be over 25 years old. 



The limiting factors in natural replacement appear to be related 

 to the complex life history of the pigtoe mussel and changes in the envir- 

 onment after the river was impounded. Although the pigtoe produced 

 glochidia in 1957^ their survival is uncertain because of apparent fail- 

 ure to find the proper host-fish for the parasitic stage of development. 

 Extensive silt deposits on bottom areas^ acciimulated since the river was 

 impounded, appear to have created an unfavorable habitat for the survival 

 of young mussels. Only juvenile sizes of the deertoe Trunci3J.a donacifor- 

 mis , a silt-tolerant species, were found in appreciable numbers on bottom 

 areas sampled. 



The slow growth of the pigtoe is not conducive to a rapid restora- 

 tion of the resource for commercial purposes even if a dominant year class 

 succeeded in expanding the size of the popiilation in the near future. 

 Approximately 8 to 9 years must elapse before an adeqxiate marketable size 

 of 50 mm. in length is reached. 



Findings described in this report do not indicate the need for 

 imposing specific regulations on the commercial harvesting of mussels, 

 which would conserve or increase the resoxirce in future years. There 

 is a possibility that more knowledge of the life-cycle of the pigtoe 

 mussel might lead to some means for increasing its abundance. However, 

 if a poor environment is the chief limiting factor, there is slight hope 

 of restoring depleted populations to a former level and the trend may 

 continue to be that of a vanishing resource. 



39 



