A-3). Four live threatened or endangered species were 

 collected at one or more of the three sites: the federally 

 endangered Lampsilis higginsi (Higgins eye), the state 

 endangered Plethobasus cyphyus (Sheepnose) and 

 Cumberlandia monodonta (Spectacle case), and the state 

 threatened Ellipsaria lineolata (Butterfly) (Table A-6). 



The three most abundant species in Reach 15 

 were Truncilla truncata (Deertoe), Ellipsaria lineolata 

 (Butterfly), and Quadrula pustulosa (Pimpleback). In 

 combination, these three species constituted 53% to 73% 

 of the unionid populations at the three study sites (Table 

 A-6). Amblema plicata (Threeridge) ranked seventh in 

 overall abundance at Sylvan Slough (3.3/m-; 6.0%), third 

 at Case-IH (14.0/m-; 15.6%), and fourth at Illmiwek 

 (10.3/m-; 8.6%). Megalonaias nerwsa (Washboard) 

 ranked between sixth and eighth in overall abundance and 

 only accounted for 2-4% of the unionids collected at 

 Reach 15 sites between 1994 and 1995. 



Mean unionid densities at the three sites sampled 

 in the 1994-95 survey increased significantly (p < 0.001) 

 in the upriver direction (i.e.. Sylvan Slough (53.4/m-) < 

 Case-IH (86. 7/m^ < Illiniwek (1 18. 3/m=)) (Table A-12). 

 This is likely attributable to the similar trend in mean 

 densities of three of the more abundant species, E. 

 lineolata, T. truncata, and A. plicata (Table A-7). Only 

 two species, Quadrula metane\ra (Monkeyface) and 

 Truncilla donaciformis (Fawnsfoot), had densities which 

 increased significantly in the downriver direction 

 (Illiniwek < Case-IH < Sylvan Slough). 



Temporal trends in unionid abundance reflect a 

 significant decline (p < O.OOI) in mean unionid density 

 at both Sylvan Slough (refuge) and Case-IH (harvested) 

 over the past decade (Table A-12). Between 1985 and 

 1995, six mussel species {A. plicata, M. nerwsa, 

 Leptodea fragilis , Potamilus alatus, Potamilus ohiensis, 

 and U. imbecillis) showed statistically significant declines 

 (p < 0.001) in mean densities at the Sylvan Slough site 

 (Table A-9). During the same time period, four species 

 (L. fragilis, P. alatus, T. truncata, and T. donacifonnis) 

 declined (p ^ 0.01) at the Case-IH site (Table A-11). 

 No species showed a significant increase at either of the 

 two sites. 



Mean overall unionid density at Illiniwek (98 

 samples, 118. 3/m-^ and Sylvan Slough (116 samples, 

 53.4/m=) were likely (p = 0.05) within 10% of their 

 actual densities (Table B-1) based on the statistical 

 technique described by Green (1979). Greater sample 

 variance and fewer samples at Case-IH (72 samples, 

 86.7/m^ resulted in an estimate within 15 % of the actual 

 density. Although fewer samples were collected during 

 quantitative sampling m Reach 15 during 1983 and 1987, 



density estimates were still within 20% of the actual 

 density at Sylvan Slough and between 30% -40% at Case- 

 IH (Table B-1). We also applied this technique (Green 

 1979) to density estimates for individual species (Tables 

 F-2 to F-4). Density estimates for abundant species at 

 each site were the only ones which had a 95 % probability 

 of being within 20 % -30 % of their actual densities (Tables 

 B-2 to B-4). To estimate the uncommon or rare species 

 with the same level of precision would require an 

 unreasonably large number of samples (i.e., 1,000 to 

 61,000 samples). 



Density histograms based on shell height for all 

 commercial mussel species collected over the past decade 

 at Reach 15 sites exhibit a truncated distribution pattern 

 which coincides with the minimum commercial size limit 

 (Appendix D). Possible explanations for this trend are: 

 (1) shortly after reaching the minimum size limit all 

 commercial mussel species experience a period of near 

 complete mortality, (2) the minimum size limit is at or 

 near the maximum achievable size for commercial species 

 in Reach 15, or (3) commercial musselors are extremely 

 efficient at removing adult mussels from the population 

 once they reach the minimum size limit. Our data from 

 Reach 15 mussel populations over the last decade indicate 

 that the latter is the most likely explanation for the 

 truncated distributions. For example, at Sylvan Slough in 

 1983 and 1985 (Figure D-5) and at Case-IH in 1987 

 (Figure D-10) there were relatively large cohorts of adult 

 A. plicata in the 60- to 70-mm size intervals, just below 

 the minimum commercial size limit. Within 2 to 4 years 

 these apparently strong cohorts were missing or not 

 evident from the population as they were not identifiable 

 in subsequent histograms. The disappearance of these 

 cohorts probably occurred within a few years after they 

 grew beyond the minimum commercial size limit. Sylvan 

 Slough was designated a mussel refuge in 1988, and yet 

 seven years later (1995) the distribution patterns of the 

 primary commercial species remain truncated at the 

 minimum size limit similar to harvested beds, suggesting 

 that illegal harvest occurred within this reflige. In fact, 

 individuals have been prosecuted for illegally harvesting 

 mussels in the Sylvan Slough refuge (Scott Wright, IDNR 

 Conservation Warden, personal communication) 



The 1994-95 densities of commercial mussels 

 with heights greater than the minimum size limit (legal- 

 size) is extremely low at all three Reach 15 sites 

 (Appendix C). The mean density of legal-size A. plicata 

 ranged from a low of 0.24/m- at Sylvan Slough to a high 

 of 1.12/m= at Case-IH (Table D-1). Based on these 

 density estimates there are from 2,424 (Sylvan Slough) to 

 11,314 (Illiniwek) legal-size A, plicata/h&:taTe (970 to 



