Table 1. Number and percent of the indi\iduals from each mussel species collected in Reach 15 of the UMR which were 

 aged and weighed in 1987 and 1994-95. 



polynomial regression equations (calculated). In 1987, all 

 mussels were aged, therefore these histograms represent 

 only estimated ages. In 1994-95, only from 7.3% to 

 36.9% of the individuals from each species were aged 

 (Table 1), therefore these histograms consist primarily of 

 mussels with calculated ages. Ellipsaria lineolata and O. 

 reflexa were not aged in the current study, instead we 

 used 1987 regression formulas to calculate their ages 

 based on observed shell lengths. 



Mortality 



Mortality estimates were based on the percentage 

 of recently dead mussels in our quantitative samples. 

 Some researchers contend this method results in 

 overestimation due to the misidentification of old dead 

 shells as recently dead. In the present survey, we used 

 the same method and definition we used in 1983, 1985, 

 and 1987; thereby at least allowing comparisons among 

 mussel surveys conducted by the INHS in the past decade. 

 We have also conducted field trials to validate this method 

 in the Illinois River: these data indicate our assessment of 

 mortality is conservative and actual mortality rates are 

 likely higher (INHS, unpublished data). This is due to 

 the rapid discoloration, breakdown of nacre, and 

 separation of the hinge ligament which often resulted in a 

 mussel being identified as old dead when it had died 

 within the past three months. 



Zebra Mussels 



Mean zebra mussel densities (by site and date) 

 were determined from either the same quantitative 

 samples from which unionids were collected (July and 

 August 1994) or from a separate set of samples in which 

 only zebra mussels were counted (July 1995) (Table H-1). 

 Length-frequency histograms of zebra mussels collected 

 at the Illiniwek site (RM 492.4) on five dates between 

 July 1994 and September 1995 were used to evaluate 

 population size structure and recruitment events. 



Zebra mussel infestation of Unionid mussels was 

 reported as % Infestation (the number of unionids with 

 one or more attached zebra mussels) and Degree of 

 Infestation (the number of zebra mussels attached to an 

 individual unionid). The degree of infestation of all 

 unionids on each sample date was used to calculate a 

 mean, standard deviation, and range for each study site 

 and unionid species. 



Results and Discussion 



Species Richness and Abundance 



From July 1994 through September 1995* we 

 collected a total of 7,107 native mussels representing 

 twenty-six species from the three study sites in Reach 15 

 (Table A-3 and A-4). Illiniwek had the most species (25) 

 followed by Case-IH (23) and Sylvan Slough (20) (Table 



