EFFECTS OF DREISSENA POLYMORPHA (PALLAS 1771), ZEBRA
MUSSEL, ON 1996 UNIONID MOLLUSK TRANSLOCATION
PROCEDURES: MISSISSIPPI RIVER MILE 697.5, HWY
14/16/61, CASS STREET BRIDGE, LA CROSSE, WI. 

Marian E. Havlik, Malacological Consultants, 1603
Mississippi Street, La Crosse, WI 54601-4969. 
E-mail: havlikme@aol.com 

In July and August 1996, prior to routine riprap
placement around three bridge piers, 12698 unionids
(27 species) including five endangered Lampsilis
higginsi (Lea 1857), and 580 other Wisconsin special
status unionids, such as 39 Ellipsaria lineolata
(Rafinesque 1820), 29 Arcidens confragosus (Say
1829), and 234 Pleurobema sintoxia (Rafinesque 1820)
were translocated, mostly from about 900 m2 around
each of two piers.  Densities and number of species
per pier were: Pier 6, 2.81/m2 (22 species); Pier 7,
0.07/m2 (12 species); and Pier 8, 10.65/m2 (27
species). The mean density was 4.65 mussels/m2.  This
unionid translocation was probably the first to have
been conducted in an area seriously impacted by
Dreissena polymorpha, 6-44 mm long.  Some unionids,
mainly from sand substrata, were devoid of this
exotic, although many showed evidence of previous D.
polymorpha attachment (byssal threads).  At Piers 6
and 7 there were a small number of D. polymorpha on
most unionids, but at Pier 8, on the left descending
bank and 320 m downstream of a barge facility, the
river substrata was blanketed with zebra mussels
25-75 mm deep, and up to 120 or more adults on 80% of
the unionids. Depths at Pier 8 were 6-9 m with very
strong currents, and a cobble, gravel, and sand
substrata.  The number of D. polymorpha greatly
slowed unionid collection by experienced divers. 
Many unionids were nearly covered with D. polymorpha,
but few appeared to have been killed by this exotic. 
The saving factor may be the strong current at Pier
8, on outside of a large, river bend. Processing time
was doubled because visible D. polymorpha were hand
stripped, like shucking popcorn, before unionids were
marked or numbered on each lower anterior valve.  A
local waste management company required D. polymorpha
to be bagged, and advance disposal notice given to
facilitate immediate landfill burial.  The first
records for La Crosse since 1907, and for the Upper
Mississippi River since 1930, included three
Alasmidonta marginata Say 1818 and four Lasmigona
costata (Rafinesque 1820). Depths of 6-9 m were
probably records for these two generally small stream
species.  Amblema p. plicata (Say 1817) was 39.75% of
the fauna.  The most suitable translocation area for
special status unionids was 4 miles upstream.  A
nearby site was used as the translocation area for
some common species.  Several species reported alive
nearby by Havlik (1983) were not found, including
Anodonta suborbiculata Say 1831, however the habitat
was not typical for this species.  All special status
specimens appeared to be doing well on 6 October
1996, except that #131, Ligumia recta (Lamarck 1819)
(160 mm long) was found dead with about 50 - 5 mm
long juvenile D. polymorpha attached to this shell. 
These small D. polymorpha represented the 1996
August-September cohort.

Platform; Ph/Fax 608-782-7958. 

KEYWORD: unionids, translocation, Dreissena
polymorpha, Mississippi River, endangered species.