Where Have all the Mussels Gone? A Proposal to Address the Widesrpead Decline of Freshwater Mussels

We propose a large-scale, collaborative project to examine causes of mussel declines. The approach will involve a rigorous examination of a wide array of potential causal factors, including previously proposed and largely ignored factors. Instead of focusing on a single stream or region, the project will encompass a broad area of North America including up to about 40 streams. The streams will represent a diverse array of physiographic, ecological, and land use contexts including the Appalachian Region, other upland areas of the Southeast, Gulf and Atlantic coast river systems, and the former prairie regions of the Midwest. Within each region, we will choose representative streams including those that have experienced mussel declines and those in which the fauna is more intact. This design will allow us to assess potential causal factors while evaluating how they may be influenced by ecological and regional conditions. The project will consist of two research components, which will address the problem from different angles. Either component would provide important information as a stand-alone project, but their real strength is as a set of two complimentary approaches to the same problem. Conducting both components simultaneously also greatly increases the cost-effectiveness of the project. In addition, the project will be accompanied by communication efforts aimed at highlighting the mussel extinction crisis and efforts to address it. Component 1: Associations between mussel faunal health and potential causal factors. Associations between faunal health and potential causal factors form the basis of current ideas about causes of mussel declines, but this approach has not been conducted at a large scale or by considering a comprehensive array of potential factors simultaneously. For each study stream, we will assemble two data sets. The first data set will include information about the current health of the mussel fauna. From this data set, we will develop a metric that represents the health of the fauna in each stream. This metric will be a composite score based on four pieces of information: 1) the proportion of historical species richness surviving in the stream; 2) the proportion of surviving species with documented reproduction; 3) overall mussel abundance; and 4) documented changes in mussel abundance over time. The metric will be a continuous variable between 0 and 10, where 0 represents total faunal loss, and 10 represents an intact, stable fauna. The second dataset will include information about a wide array of physical and biological characteristics of each stream and its watershed, as described subsequently. These characteristics will include factors previously proposed as causes of mussel declines and other factors that have received little attention. For example, we will make direct estimates of sediment deposition and transport in streams so that we may evaluate the widespread supposition that increased sedimentation is a major cause of mussel declines. Conversely, we will estimate abundance of invasive bivalves in streams because this potential factor has received little attention. Component 2: Direct assessment of mussel health in response to potential causal factors. In addition to faunal health, we will make direct assessments of the health of individual mussels in response to ambient conditions in streams. We will do this by placing captively-reared juvenile mussels in concrete “silos” for 2–3 months in the summer. Silos are flow-through devices that provide food and oxygen to mussels but facilitate their retrieval at the end of the study. At the end of the exposure period, we will measure aspects of mussel health including survival, growth, and physiological status, and we will screen mussels for pathogens and parasites. We will then examine associations between mussel health and potential causal factors. We have pioneered this approach in previous work and have shown it to be a powerful way to evaluate, in real-time, the ability of streams to support mussels and the factors that affect this ability. This approach also provides important information for assessing the suitability of streams for mussel reintroduction. The results of pathogen screening will provide vital information about the distribution of potential disease organisms in the U.S.