DO JUVENILE UNIONIDS LIKE SAND?  A 1996 UNIONID TRANSLOCATION AT  A
SITE WITH A MODERATE DREISSENA POLYMORPHA (PALLAS 1771) INFESTATION, 
MISSISSIPPI   RIVER   MILE   725.8,   T.H.   43   BRIDGE,   WINONA,
MINNESOTA/WISCONSIN

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

A unionid translocation from 9 bridge piers (total area 3221 mư), September 1996, 
yielded 6129 unionids (23 species, mean density 1.9/mư) in a primarily sand 
substrata.  No federally listed unionids were found but 2% represented 7 Minnesota 
and Wisconsin endangered, threatened and special status species.  Most of 82 
Obovaria olivaria (Rafinesque 1820) were juveniles.  Ellipsaria lineolata (Rafinesque 
1820) (1), Pleurobema sintoxia (Rafinesque 1820) (4), Ligumia recta (Lamarck 1819) 
(12), and Utterbackia imbecillis (Say 1829) (11), were also represented by juveniles. 
Other special status species were Arcidens confragosus (Say 1829) (2), and 
Megalonaias nervosa (Rafinesque 1820) (2).  Moderate numbers of visible Dreissena 
polymorpha (Pallas 1771) were removed prior to translocation.  

Translocation follow-up 18 September 1997, showed an overall survival of 93.98% 
(94.74% of numbered special status unionids, and 91.18% of hash-marked common 
unionids).  Spring 1997 high water apparently caused habitat changes since 5-7.5 cm 
of sand had accumulated over much of the sand, gravel, and mud substrata found at 
the Translocation Site in 1996.  In 1997 most unionids were nearly buried in packed 
sand.  The 1997 mean density of hash-marked common unionids from 32-0.25 
quadrats at the translocation site was 4.25/mư (11 species); the density of unmarked 
unionids was 17.25/mư.  Besides hash-marked unionids in quadrats, 38 numbered and 
185 hash-marked unionids were found during random searches, for a follow-up total 
of 250 live and 16 dead marked unionids (4.34% of translocation total).  Twelve 
unmarked special status unionids were found during follow-up: Arcidens confragosus 
(1), Pleurobema sintoxia (5), and Obovaria olivaria (6).  Both at this translocation site, 
and at a similar translocation site near La Crosse WI, the species that seems most 
affected by translocation is Ligumia recta).  We have 7059 the impression that this 
species would do better if it were placed on the substrata rather than being planted, 
since there was no other obvious cause of the mortality seen for that species (2 or 4 
numbered specimens dead).  Others have reported seeing L. recta lying of the 
substrate.  Many marked special status unionids, particularly young Obovaria olivaria 
(Rafinesque 1820), clearly showed growth, with 5 mm of growth after the 1996 
translocation (a false rest or interruption ring).  In 1997 there was another 5mm of 
growth after the winter rest ring.  There was good recruitment and growth in spite of 
obvious impacts from high water, spring 1997.  Only small numbers of D. polymorpha 
were found in 1997.  This exotic species does not appear to have seriously impacted 
the Translocation Site, possibly because of the sand substrata and the large number 
of buried juvenile unionids.

Keywords:  juvenile unionids, unionid habitat, unionid translocation, Mississippi River, 
Dreissena polymorpha
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