Higgins Eye Pearlymussel
5-Year Status Review

Synthesis

The Higgins eye population in the Mississippi River at Prairie du Chien, Wisconsin, has crashed as a result of skyrocketing zebra mussel populations at that site. What was once the stronghold for the species now holds only a few pockets of Higgins eye, while the main portions of the habitat at this site now consist of layers of zebra mussel shells. As a result, the population in the Mississippi River at Cordova, Illinois, is likely the stronghold for the species.

Recent reintroductions of Higgins eye outside of areas heavily infested by zebra mussels have been conducted by the Corps of Engineers to counteract the effects of operation and maintenance of the nine-foot channel project in the Mississippi River in Minnesota, Wisconsin, Illinois, Iowa, and Missouri. These efforts have been ongoing since 2000, and if successful they will likely ensure the species is established in areas protected from the threat of zebra mussels.

The primary threats to Higgins eye are habitat changes (chiefly in the form of impoundments), water quality problems, and non-native species, both as predators (carp) and competitors (zebra mussels and Asian clams). In particular, zebra mussels pose the most immediate threat to the species by colonizing and rapidly increasing populations in the majority of reaches inhabited by Higgins eye. No control measures are yet known, although the Corps’ reintroductions will enable the species to persist while zebra mussel eradication efforts are researched. In 2006, the Corps will initiate a federal feasibility study on zebra mussel management on the upper Mississippi River (Dennis Anderson, U.S. Army Corps of Engineers, pers. comm.). The feasibility study was recommended in a federal reconnaissance study that evaluated potential management measures for zebra mussels (U.S. Army Corps of Engineers 2003).

Sedimentation and contaminants also pose a large threat to Higgins eye. Dredging, other channel maintenance activities, and upper watershed practices greatly increase sediment load, which interferes with feeding and breeding activities and also degrades substrate conditions. Contaminants are often bound to fine sediment particles, exacerbating the effects of sedimentation to the species.

Since the Revised Recovery Plan was issued in 2004, the species’ status has not changed. The threats to Higgins eye remain prominent, and in the case of zebra mussels, threats are worsening. Without the protections of the Endangered Species Act and all of the recovery and reintroduction activities it has fostered and continues to promote or mandate, Higgins eye would be extremely close to extinction. The remaining stronghold for the species in the Mississippi River at Cordova, Illinois, must be surveyed annually and Higgins eye collected and scrubbed of zebra mussels in order to persist. No populations are thriving independent of protection or conservation actions by state or federal agencies. Higgins eye continues to meet the definition of endangered and will do so until the threat of zebra mussels is alleviated either through zebra mussel eradication or successful population establishment outside of zebra mussel-infested waters within Higgins eye’s historic range. Population viability, water quality, harvest restrictions, and other threats of lesser magnitude will also need to be addressed for the species to be downlisted or delisted.

At this time, it is unclear whether the recovery of the Higgins eye population in the Essential Habitat Area (EHA) at Prairie du Chien should be included as a requirement for downlisting or delisting. While this population used to be the most productive throughout the species’ range, it may not be recoverable as it seems to have the characteristics of good habitat for zebra mussels, as well. It is possible that this area is more susceptible than other Mississippi River EHAs to zebra mussel invasion; if that is the case, ensuring this site’s freedom from future zebra mussel population increases may not be attainable. However, because this population was once the most productive, its recovery could herald the recovery of the species. The Recovery Team held a conference call about this issue on January 31, 2006, and decided that no decision should be made for five years while the Higgins eye and zebra mussel populations at Prairie du Chien are monitored and more information is gathered.

Entire Higgins eye pearlymussel 5 Year Status Review (PDF; 25 pages)

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May 2006

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