4,526/acre) (Table G-8). There are even fewer legal-size 

 M. nervosa, with densities from 0.06/m- to 0.24/m- or 

 606 (Case-IH) to 2,424 (Illiniwek) legal-size mussels per 

 hectare (242 to 970/acre) (Table G-8). 



Recruitment 



Recent recruitment estimates from the ten species 

 we evaluated in 1994-95 indicate a high degree of 

 variability among species and study sites (Table F-1). 

 Mean density of recent recruits ranged from a low of 

 0.01/m- (M. nerwsa) to a high of 3.97/m- (Q. pustulosa). 

 Densities of recent recruits were similar among sites for 

 six mussel species (A. plicata, M. nerwsa, Q. metanevra, 

 Q. pustulosa, Q. quadrula, and O. reflexa). The 

 remaining four species (£. lineolata, L. fragilis, T. 

 truncata, and T. donaciformis) showed significant 

 differences in recruitment among sites; for example, the 

 density of recently recruited E. lineolata was significantly 

 greater at Illiniwek (3.34/m-\) than at the two other sites 

 (Case-IH = 0.56/m* and Sylvan Slough = 0.65/m-) 

 (Table F-1). Sf)ecies with the highest mean recruitment 

 densities, Q. pustulosa (4.0/m-^, E. lineolata (1.5/m-), 

 and T. truncata (l.S/m^^, were also the three most 

 abundant species collected from Reach 15 in the present 

 survey (Table A-5). Some species showed little evidence 

 of recent recruitment at one or more of the study sites; we 

 did not collect any M. nerwsa or Q. metanevra 

 (Monkeyface) less than 30 mm in length from either of 

 the two harvested beds (Case-IH and Illiniwek) and only 

 one M. nervosa and two Q. metanevra at the refuge bed 

 (Sylvan Slough). 



Recruitment information from Sylvan Slough 

 (Table F-1) and Case-IH (Table F-3) between 1983 and 

 1995 demonstrates the unpredictable nature of mussel 

 recruitment. Some species exhibit fairly constant 

 recruitment (e.g., Q. pustulosa) with relatively high 

 densities of young mussels in most years, while others 

 show evidence of sporadic recruitment (e.g., T. truncata, 

 T. donaciformis, and M. nervosa). Interpretation of 

 recruitment information is difficult since we know very 

 little about the natural reproductive patterns of mussel 

 species and how they are affected by environmental 

 conditions (i.e., water temperature, floods, turbidity, 

 etc.), biological factors (i.e., mussel abundance, host 

 abundance, peak gravidity, etc.), or anthropogenic 

 stressors (i.e., commercial harvest, recreational or 

 commercial boat traffic, pollutants, etc.). 



Density distributions based on shell length 

 (Appendix C) and age (Appendix F) facilitate the 

 identification of strong or weak cohorts which can be used 



to ascertain long-term trends in recruitment. For 

 example, most mussel species typically showed a modal 

 age distribution produced by years of significant 

 recruitment (strong cohorts) and years with poor 

 recruitment and/or survival (weak cohort). Two mussel 

 species, M. nervosa and A. plicata, which show very 

 different recruitment patterns are discussed in greater 

 detail. 



(1) M. nervosa exhibited relatively low densities of recent 



recruits (< 1.0/m^ in most quantitative surveys 

 conducted by INKS in Reach 15 in the last ten 

 years (1985-1995) (Tables F-1 to F-3). Density 

 distributions based on age and shell length from 

 1994-95, show few young (age < 7 years) 

 (Figures E-1, E-3, and E-5) or small mussels 

 (shell length < 85mm) (Figure C-2). In fact, 

 the last significant recruitment by M. nervosa at 

 our sites probably occurred in 1984-85 and can 

 be identified in nearly all density distributions 

 (age, length, and height) since 1987; for 

 example, the density distribution (based on age) 

 for M. nerwsa collected in 1987 Sylvan Slough 

 shows a strong (6.88/m-) age 2-3 cohort (Figure 

 E-2), which can be identified in the density 

 distribution from 1994-95 as an age 9-10 cohort 

 (Figure E-1). This indicates M. nervosa 



experiences infrequent recruitment success, 

 possibly only once during this ten year period. 

 Heath et al. (1988) suggested an approximate 7- 

 year recruitment cycle for M. nervosa in the 

 Wisconsin portion of the UMR. 



(2) A. plicata exhibited a more consistent recruitment 



pattern, with recent recruit densities typically 

 between 0.23/m- and 0.68/m- in 1985 and 1995 

 (Tables F-1, F-2, and F-3). Density 



distributions from all study sites (1983-1995), 

 based on age (Figures E-1 to E-6), shell length 

 (Figures C-10 and C-20), and shell height 

 (Figures D-1, D-5, and D-10), show that A. 

 plicata successfully recruit individuals to their 

 population each year and occasionally produce an 

 especially abundant cohort (1985-86, see 

 Appendix E). 



Age and Growth 



We observed strong non-linear age-size (Tables 

 G-3 and G-4) and size-weight (Table G-7) relationships 

 for the five commercial mussel species (A. plicata, M. 



10 



