EXTERNAL AGING OF UNIONIDS REVISITED: HEIGHT VERSUS AGE OF
4800 MEGALONAIAS NERVOSA (RAFINESQUE 1820), MISSISSIPPI RIVER
REACHES 9-19, LANSING - FORT MADISON, IA, JULY-SEPTEMBER, 1997 

James A. Frink and Marian E. Havlik, 
Malacological Consultants, 1603 Mississippi Street, La Crosse, WI 54601-4969  

The Shell Exporters of America (SEA) sponsored an extensive Mississippi River
commercial unionid mollusk survey using modified 0.25 m2 Surber samplers.  We
sampled from River Mile 663.0-386.0, Reaches (Pools) 9-19, with an emphasis on
M. nervosa.  Over 20,400 unionids were processed during 252 random and 692
quadrats samples (944 dives).  Over 4800 living M. nervosa were scrubbed to
remove Dreissena polymorpha debris, measured, and aged.  The mean
density/Reach was 2.7-7.6/m2.  The mean height/age class/Reach, and the mean
age/height class/Reach were analyzed.  Age does not equal size.  The large 10-13
year old age classes probably represent the species' response to the mid-1980's
die-off, or else indicate a peak period of reproduction, although this was not
apparent in a 1986 study. Commercial M. nervosa (101.6 mm height) ranged from
9-38 years of age.  In most reaches 2.4-10.8% (mean 7.8%) of a bed is of
commercial size with age ranges from 9-38 years.  The youngest M. nervosa was
2 yrs (10 mm height).  The most legal unionids were in Sylvan Slough Sanctuary
(18.5%) indicating little illegal harvest.  3.18% of M. nervosa were less than 50
mm (2-7 years). Growth rates generally increase further south.  The 6 year class
had the largest height range; 130-6 year olds varied 65 mm in height; 118-21 year
olds varied 37 mm in height.  The mean difference in each size class was 31 mm
in height, and 54 mm in length.  Number/size class for M. nervosa showed bell
curves, but every reach showed a marked shift to the left with the number/age
class, usually with peaks around 10-13 years of age.  Reach 14, and to a lesser
extent Reach 19 had bimodal age distributions.  

Keywords: unionid age, unionid density, unionid height, commercial mussels,
Mississippi River.


POSTER.  Phone/Fax: 608-782-7958; email: havlikme@aol.com