nenvsa, Q. quadrula, Q. pustuhsa, and Q. metanevra) 

 evaluated from Reach 15. Growth curves (Appendix G, 

 Part IV) and calculated size-at-age (Appendix G, Part III) 

 indicate growth rates of Reach 15 mussels are similar to 

 other UMR populations (Woody 1988; Heath et al. 1988) 

 and much lower than populations in the Illinois River 

 (Whitney et al., unpublished data). Growth rates 

 decreased sharply with increasing age. Annual increase in 

 length for the five species can be summarized as: (1) 

 mussels aged 1 to 5 grow greater than 7 mm/yr (up to 17 

 mm/yr for M. nerwsa), (2) mussels aged 6 to 11 grow 

 from 4 to 10 mm/yr, (3) mussels aged 12 to 17 grow 

 from 2 to 4 mm/yr, and (4) mussels aged greater than 18 

 grow less than 2 mm/yr. Species specific annual growth 

 rates can be determined from Appendix G, Part III. 



On average, sexual maturity of most commercial 

 mussel species (A. plicata, M. nenosa, Q. pustulosa, Q. 

 metane\ra, and Q. quadrula) occurs during their seventh 

 or eighth years, with a range from 5 to 12 years of age 

 (Table G-2). 



The time required for a commercial species to 

 reach minimum harvestable size ranged from 19 to 24 

 years; M. nenvsa 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). 

 These values are similar to the results from other growth 

 studies in the UMR (21 years for M. ner\-osa, [Heath et 

 al. 1988]) and neariy double the time required by the 

 same species in the Illinois River (e.g., A. plicata reached 

 minimum commercial size in 9 years in Peoria Reach and 

 13 years in the Alton Reach of the Illinois River [INHS, 

 unpublished data]). 



Mortality 



Mean unionid mortality at Sylvan Slough reached 

 30.4% in 1983; eight mussel species had mean mortalities 

 greater than 25%, including M. nervosa (45%), Q. 

 pustulosa (37.6%), A. plicata (34.9%), and T. truncata 

 (33.3%) (Table A-8). Since 1983, mortality rates have 

 generally declined; however, these high mortalities in the 

 eighties likely contributed to the significant decline in 

 abundance of many of these species during the past 

 decade (Table A-9). 



During the current survey (1994-95), mean 

 unionid mortality at the three study sites was estimated at 

 0.81% at lUiniwek, 1.15% at Sylvan Slough, and 4.27% 

 at Case-IH. We are concerned about the apparent 

 increase in mortality at Case-IH, which has increased 



from 1.88% in 1987 to 4.27% in 1994. Amblema plicata 

 showed increased mortality from 1.52% in 1987 to 6.67% 



1994 (Table A-10). Overall mortality rates of the other 

 two sites appear to be within acceptable levels, since they 

 are typical of most mussel populations we have sampled 

 (INHS, unpublished data). 



Zebra Mussels 



We believe zebra mussels first arrived in Reach 

 15 m late 1991 or eariy 1992. The largest individual 

 collected in July 1994 measured 28.35 mm. From growth 

 studies in the Illinois River, this individual would have 

 been from 2 to 3 years of age (INHS, unpublished data). 

 Length-frequency histograms (Figure H-1) indicate that at 

 least one and at most two zebra mussel cohorts settled in 

 Reach 15 in both 1994 and 1995. Growth rates of zebra 

 mussels in Reach 15 (determined from length-frequency 

 histograms) are similar to those observed in the Illinois 

 River in 1993 (INHS, unpublished data), with mussels 

 reaching a length of 17 to 20 mm in their first growing 

 season. 



Zebra mussel densities at Reach 15 study sites 

 have increased significantly between July 1994 and July 



1995 (Table H-1). The highest densities have consistently 

 been found at the Illiniwek site where mean density 

 increased exponentially, from 1.7/m- in July 1994 to 

 2,519/m- in July 1995. Dunng the same period, mean 

 zebra mussel density at Sylvan Slough increa.sed from 

 0.6/m- to 426. 0/m". Much or the rivers flow is diverted 

 away from Sylvan Slough by a rock seawall (Figure 3). 

 This diversion may be the reason why fewer planktonic 

 zebra mussel larvae (veligers) settled in Sylvan Slough 

 than at Illiniwek. 



The infestation of native unionids at study sites 

 increased from less than 1% in July 1994 to 48.9% at 

 Illiniwek and 40. 1 % at Sylvan Slough in September 1995 

 (Figure H-2). Similarly, the mean degree of infestation 

 increased from 0.0 to 2.31/unionid at Illiniwek and from 

 0.0 to 1.29/unionid at Sylvan Slough from July 1994 to 

 September 1995 (Table H-3 and H-4). The maximum 

 number of zebra mussels collected on an individual 

 mussel was 37 on a Q. pustulosa collected from the 

 Illiniwek site in September 1995 (Figure H-2). From our 

 experience with zebra mussels in the Illinois River, we 

 believe this degree of infestation is insufficient to create 

 negative effects (i.e., reduced growth, restricted mobility, 

 increased mortality) on unionid mussels. Ricciardi et al. 

 (1995) used linear regression models to predict the 

 intensity and impact of zebra mussel infestation on native 

 unionids from field densities; their models predict severe 



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