Laboratory procedures 



Laboratory processing included both weight and 

 age determination of individual mussels from selected 

 species (Table 1). Frozen mussels were rinsed in warm 

 water to remove ice from the exterior of the shell then 

 weighed to the nearest 0.01 g (live weight). Next the 

 mussel was forced open with a scalpel and soft tissue was 

 removed; we determined wet weights for both tissue and 

 shell. They were dried at 105°C to a constant weight and 

 re-weighed to determine dry weights. Shells will be 

 archived in Illinois Natural History Survey's mussel 

 collection at the University of Illinois 

 Urbana/Champaign. 



The estimated age of an individual mussel was 

 determined by counting growth bands on the external 

 surface of the shell (Chamberlain 1931, Stansbery 1951) 

 and within thin radial cross sections of the shell and hinge 

 ligament (McCuaig and Green 1983, Neves and Moyer 

 1988). In 1994, from 10% to 30% of each of the 

 following commercially harvested species were aged using 

 both techniques: Amblema plicata, Megalonaias nenosa, 

 Quadrula pustulosa, Quadrula quadrula, and Quadrula 

 metanevra. The two methods of age analysis yielded 

 comparable results (± 1 year); however, preparation of 

 thin radial cross sections was very time consuming, 

 requiring from 20 to 30 minutes per mussel compared 

 with 1 to 2 minutes per mussel for external counts. 

 Therefore, only external ring counts were used to age 

 mussels collected in 1995. 



The age at which an individual mussel became 

 sexually mature was estimated by recording the age at 

 which a marked decrease in distance between external 

 growth bands occurred on the external shell surface of 

 adult mussels (Stansbery 1961, Stein 1973). This 

 technique was performed after shells and tissue had been 

 separated and dried, therefore, we were unable to validate 

 this method by examination of the gonads for maturity 

 and ripeness. The age of sexual maturity was determined 

 from a subsample of randomly selected shells of adult 

 mussels of the five commercial species, A. plicata 

 (n = 78), M. nerx'osa (n = 29), Q. metanexra (n=12), Q. 

 pustulosa (n = 38), and Q. quadrula (n = 28). The mean, 

 standard deviation, and range of sexual maturity age(s) 

 were calculated for each species. 



Daia Analysis 



Data recorded in the field and laboratory during 

 1994-95 was analyzed in accordance with the five primary 

 objectives of the study: (1) species abundance and 



richness, (2) recruitment, (3) age and growth, (4) 

 mortality, (5) status and impacts of newly introduced 

 zebra mussels. In addition to the information collected in 

 the present survey, we also analyzed data collected from 

 three previous quantitative surveys at the Sylvan Slough 

 and Case-IH sites (Sparks and Blodgett 1983, Blodgett 

 and Sparks 1987a and 1987b) to identify temporal trends 

 in these mussel populations. We also reviewed two 

 mussel survey reports from Sylvan Slough conducted by 

 private consultants within the past decade (Stanley 

 Consultants, Inc. 1993, Cawley 1989). Annual 

 commercial harvest reports from Illinois, since 1963 

 (Fntz 1988, Williamson 1995) and Iowa smce 1984 

 (Ackerman and DeCook 1995) were used to evaluate the 

 effects of long-term commercial harvest on mussel 

 populations in Reach 15. 



Species richness and abundance 



Species richness was determined by tabulating the 

 total number of species collected from quantitative 

 sampling at each of the study sites. Abundance, typically 

 referred to as density (number of individuals/m-\), was 

 determined for each quantitative sample; data from all 

 quantitative samples collected at each site from July 1994 

 through September 1995, were averaged to determine 

 overall unionid and species specific density means. 

 Statistical comparison of density means among and within 

 the three study sites was conducted using an extended t- 

 test designed for comparisons of means obtained from 

 unequal sample sizes. Statistically significant difference 

 between means was determined at the p < 0.05 level. 



Mean densities were used to classify each unionid 

 species as very abundant (> 20.01/m"), abundant 

 (10.01 to 20.00/m=), common (1.01 to 10.00/m^, 

 uncommon (0.34 to 1.00/m-), or rare (< 0.33/m^ 

 (Table A-5). This arbitrary classification system was 

 designed specifically for Reach 15 mussel populations to 

 categorize species with similar abundance; it may or may 

 not apply to other reaches of the Mississippi River. 



Using a technique described in Green (1979) we 

 computed the number of samples required to estimate 

 unionid density within 10%, 20%, 30%, 40%, and 50 % 

 of the actual density with a 0.05% probability of being 

 incorrect using the following equation: n = [(2SD) -f 

 (xM)]", where, n = number of samples required, SD = 

 standard deviation, x = desired level of accuracy (i.e., 

 10% = 0.1), and M = mean unionid density based on 

 samples collected 



Density distributions based on mussel age, shell 

 length, and shell height were used to evaluate spatial and 



