Returning the Higgins eye Pearlymussel, Nature’s Silent Sentinel

Freshwater mussels are often referred to as ‘silent sentinels’ in streams and rivers. Their abundance indicates water quality is good, while their decline or absence sends an alarm that something in the ecosystem is seriously wrong. Threatened by poor water quality and the construction of locks and dams, Higgins eye pearlymussels were listed as endangered throughout their range in the Upper Mississippi River and its tributaries in Illinois, Iowa and Minnesota in 1976,. Early recovery efforts by the U.S. Fish and Wildlife Service and partners focused on reducing these threats.

Then Higgins eye recovery shifted during the 1990s when the invasive zebra mussel wreaked havoc on native mussel beds in Higgins eye range.  As the zebra mussel threat grew and the water quality in the Upper Mississippi River improved, the recovery efforts shifted focus  to mussel propagation and reintroduction.  The goal: restoring Higgins eye pearlymussels into their historic habitat.

Propagating Higgins eye, as with any freshwater mussel, is not as straightforward as it might seem. Mussels have complex life cycles, and depend on one or more species of host fish for their tiny larvae, called glochidia, to attach to while they develop. Growing mussels in a hatchery involves raising host fish (smallmouth bass, largemouth bass and walleye for Higgins eye), using scuba divers to collect adult gravid (pregnant) Higgins eye from the wild, collecting glochidia from the mussels, inoculating host fish (placing glochidia on host fish), and maintaining host fish until the glochidia grow into tiny juvenile mussels. 

Adult Higgins eye are collected from healthy mussel beds by divers and taken to Genoa National Fish Hatchery in Wisconsin, where biologists collect glochidia and place them with host fish. In large aquariums, the glochidia attach to the gills of host fish and begin their transformationinto juvenile mussels.

The potential numbers of mussels produced using this technique is startling.  In 2011, the seven donor female mussels collected from the Mississippi River produced an estimated 168,270 juvenile Higgins eye; 20 mussels collected from the St. Croix River females produced an estimated 841,117 juveniles.

To get Higgins eye pearlymussels back into rivers, glochidia-inoculated fish and cages are transported by boat to a relocation site where divers secure cages on the river bottom. Depending on their size, 30 to 50 host fish are placed in each cage. After 3 to 4 weeks, the glochidia mature and fall off the fish gills onto the river or the bottom of cages. Divers then return after another 3 to 4 weeks to release host fish to the river and then return again 4 months later to check results.  It is sometimes difficult to find young mussels, as glochidia may be swept downstream. Any subadults mussels found are used to restock other areas.

Sometimes, juvenile mussels grown at Genoa NFH are used for stocking. With this method, divers place the juvenile mussels into wooden-framed, screen covered trays that are anchored to the bottom of the river. Another relocation technique is stocking host fish that have been inoculated with glochidia. Both hatchery-raised and wild fish are used.  The wild fish are collected by electrofishing, inoculated onsite and released back into the wild.

Finding suitable places with the right conditions for Higgins eye to survive and thrive is a challenge. Sites with good water quality must have appropriate flow, river bottom substrate and water depth. They must be in areas not heavily colonized by zebra mussels or likely to fill with sediment. And for practical reasons, they must also be accessible to divers.

Growing young Higgins eye pearlymussels and reintroducing them back into waterways within their former range is a key and ongoing recovery task. Since 2002, the Service has released about 42,000 sub-adult Higgins eye at nine reintroduction sites within the Mississippi River watershed in Iowa, Wisconsin and Minnesota. Long-term monitoring will help guide efforts at restoring this silent sentinel in the Upper Mississippi River basin.