Executive Summary 



Unionid mussels were collected by quadrat 

 sampling at three sites in Reach 15 of the Upper 

 Mississippi River between July 1994 and September 1995; 

 Sylvan Slough (rivermile (RM) 485.8) within a mussel 

 refuge (commercially unharvested since 1988); Case-IH 

 (RM 488.5) (commercially harvested); and Illiniwek (RM 

 492.4) (commercially harvested). A total of 7,107 

 mussels were collected representing 26 species, including 

 one federally endangered species (Lampsilis higginsi), two 

 state endangered species (Plethobasus cyphyus and 

 Cumberland ia monodonta), and one state threatened 

 species (ElUpsaria Uneolata). Illiniwek had the greatest 

 mussel diversity and abundance with 25 species and a 

 mean density of 118.3/m-, followed by Ca.se-IH with 23 

 species and 86.7/m", and Sylvan Slough with 20 species 

 and 53.4/m". Temp>oral trends in unionid abundance 

 (1985-95) reflect significant declines (p < 0.001) in 

 overall unionid densities as well as densities for individual 

 species at both Sylvan Slough, where mean unionid 

 density declined from 100. 1/m- in 1985 to 53.4/m- in 

 1994-95 with 6 species showing significant declines, and 

 Case-IH where mean unionid density declined from 

 139. 2/m- in 1985 to 89.8/m- in 1994 with 4 species 

 showing significant declines. The four mussel species 

 which dominated our collections were Truncilla truncata, 

 ElUpsaria Uneolata, Quadrula pustulosa, and Amblema 

 plicala. Recent recruitment estimates for the ten most 

 common sjjecies indicate a high degree of variability 

 among sf)ecies and study sites. Some species, such as 

 Megalonaias rierx'osa, exhibited very low recruitment 

 densities (0.00 to 0.03/m-), while others like Quadrula 

 pustulosa exhibited good recruitment densities (3.7 to 

 4.3/m'^. Temporal trends in recruitment evaluated by 

 recent recruitment densities and size-frequency histograms 

 demonstrate the unpredictable nature of unionid 

 recruitment; some species (i.e., A. plicata and Q. 

 pustulosa) were fairly consistent, recruiting young 

 individuals to their populations each year, while other 

 species (i.e., M. nervosa and Q. metanevra) were more 

 sporadic, recruiting young to their populations only once 

 or twice within a ten-year period. On average, 

 commercial mussel species {A. plicata, M. nerwsa, Q. 

 pustulosa, Q. metanevra, and Q. quadrula) reached sexual 

 maturity during their seventh or eighth year, with a range 

 from 5 to 12 years of age. The time required for a 

 commercial species to reach minimum harvestable size 

 ranged from 19 to 24 years; M. nervosa took the longest, 

 requiring 24 years to reach a shell height of 101.60 mm 

 (4 inch), A. plicata required 21 years to reach a shell 



height of 69.85 mm (2.75 inch), and the three Quadrula 

 species required 19 to 21 years to reach a shell height of 

 63.5 mm (2.5 inch). 



We believe the current mussel refuges only exist 

 on paper and subsequently do not provide the services for 

 which they were intended. Our studies in Reach 15 

 suggest illegal harvest has occurred in the Sylvan Slough 

 refuge, since all commercial species collected within the 

 refuge demonstrate a truncated size distribution at the 

 minimum commercial size limit, a characteristic of 

 harvested areas. In fact, individuals have been prosecuted 

 for harvesting mussels in Sylvan Slough and other UMR 

 mussel refuges. 



Zebra mussels {Dreissena polymorpha) were first 

 established in Reach 15 during late 1991 or early 1992, 

 but did not become abundant until 1995. Mean density at 

 Illiniwek increased exponentially from less than Mrcr in 

 July 1994 to 2,519/m- in July 1995. Similarly, zebra 

 mussel infestation (% unionids with 1 or more zebra 

 mussels) at Illiniwek increa.sed significantly from 1 % in 

 July 1994 to 48% in September 1995. Mean and 

 maximum degree of infestation increased from 

 0.00/unionid and 2/unionid in July 1994 to 2.3/unionid 

 and 37/unionid in September 1995. Length-frequency 

 histograms indicate that at least one and possibly two 

 zebra mussel recruitment events occurred in Reach 15 

 during 1994 and 1995. These histograms also indicate 

 that zebra mussels which settle in Reach 15 during May 

 or June can reach 15- to 20-nmi in length by the end of 

 their first growing season (October). Although rapid 

 increases in zebra mussel densities and infestation of 

 unionids represent an alarming trend, we did not observe 

 any negative zebra mussel effects on the Reach 15 unionid 

 populations (i.e., increased mortality, siphon occlusion, 

 etc.) during the current study. However, based on past 

 experience in the Illinois River, we predict that by 1996 

 or 1997 unionid mussels in Reach 15 of the UMR will 

 experience significantly greater infestations by zebra 

 mussels and will subsequently suffer reduced fitness and 

 increased mortality. 



Management recommendations concerning the 

 protection of mussel populations in Reach 15 and 

 throughout the Upper Mississippi River (UMR) include: 

 (1) closing the commercial harvest of live Megalonaias 

 nerwsa, (2) establishing entire reaches of the UMR as 

 mussel refuges, (3) developing population models to guide 

 and assist the management of mussels, and (4) monitoring 

 zebra mussel densities and impacts in the Mississippi 

 River. 



