Illinois Natural History Survey - University of Illinois

INHS report January February 1995

Long-term Monitoring of Freshwater Mussels

The freshwater mussel fauna of North America has undergone an alarming decline over the past century. The geographic range of most species has decreased significantly, many species face extinction, and the entire fauna appears to be declining. Concern for this declining resource led federal and state governments to enact endangered species legislation. In addition to the protection afforded these rare species, state and federal conservation agencies have recommended that significant mussel populations be protected. The most effective way to protect these sensitive species is to preserve their habitat. However, in the face of growing urbanization, localized impacts (such as the building of bridges) on the fauna appear unavoidable.

In 1987, the Washington Avenue bridge in Kankakee was scheduled for demolition and replacement. Preliminary surveys by INHS staff at the bridge site revealed one of the richest mussel beds in the Kankakee River. We developed a plan to relocate all mussels in the vicinity of the bridge and to monitor these populations over time. Our objectives were to evaluate growth, movement, and mortality over the long term. The purpose of this monitoring, supported by the Illinois Department of Transportation, was to assess the effectiveness of mussel relocation as an acceptable mitigation strategy.

Survey staff monitoring mussels in Kankakee River.

The first phase of the project involved moving over 4,000 mussels, representing 20 species, from the bridge site. This was accomplished in 1987 under the direction of INHS biologists Jeanine Berlocher and Mark Wetzel. Live mussels were collected, identified, weighed, and measured, and a sequential number was etched into each shell. Mussels were then transported to sites upstream and placed into a series of plots. The transplant sites had been chosen previously because they had water depth and substrate similar to those of the bridge site. Mussels that already occurred in these locations were also marked so that the existing (controls) and new (relocatees) populations could be compared over time.

The next phase of the plan involved monitoring the mussel populations on a regular basis. Transplant sites were visited in 1988, 1989, 1991, and 1994. A subsample of the plots was checked both inside and outside the delineated areas. The locations of recovered mussels were noted, all live mussels were weighed and measured, and all dead shells were collected.

An initial assessment of mussel survival indicated there was no observed mortality resulting from handling or transporting to the sites. The percentage of dead marked shell recovered from the plots over time was used as a general indication of mortality. Although not accurate, this amount was used to compare mortality of relocatees with that of controls at a location. After 1 year, approximately 7% of the individuals recovered was from marked dead shell. This percentage has been amazingly consistent over time, varying by only 1 to 2 % in 7 years.

Individual mussel growth, as measured by increased shell length and width, was evaluated over time. As might be expected, smaller individuals grew substantially, but larger mussels did not appear to grow much annually. Comparisons between 1987 and 1994, however, demonstrated growth even in very old individuals. Overall, moving mussels did not appear to reduce or arrest their growth.

Mussel samples from monitoring project.

Percent recovery of marked mussels declined with each subsequent year. After the first year, overall recovery was estimated at 57%, varying from 24% to 79% between plots. Seven years later a recovery rate of only 5.6% was estimated. These rates were based on recovery of individuals found in subsamples from both inside and outside the plots. Loss of mussels may be a result of upstream, lateral, or downstream movement, which may also account for the origin of unmarked mussels collected in the plots.

This project offers evidence that mussels will survive and grow after being relocated. Low recovery of individuals may be a function of normal downstream attrition, a highly mobile mussel community, or mortality with shells swept far away from the site. Even with low recovery rates, relocation as a mitigation measure may be used to reduce the damage to mussel communities from localized site impacts. Although strategies to protect mussel populations have not been clearly formulated, mussel transplants may be a reasonable conservation strategy to assist mussel populations if specific threats to the fauna are imminent.

Helen Elise Kitchel, Center for Biodiversity

Charlie Warwick, editor