Flexin’ our Mussels: Successful reintroduction of Louisiana Pearlshell Mussels

Jan 24, 2023

Written By

Freshwater Mussel Lifecycle — A diagram of the lifecycle for freshwater mussels showing how larva from a fertilized adult female needs to use a host (fish gills) in order to metamorphose or transform into a juvenile adult. Without this parasitic stage juvenile, thus adult mussels would not be created.

Freshwater mussels are some of the most endangered organisms on Earth, much of this due to pollution, excess sedimentation, and habitat fragmentation (dams and failing infrastructure). Efforts to restore freshwater mussels have been on the rise and Natchitoches National Fish Hatchery (NFH) is fortunate to be involved in this recovery effort. Natchitoches NFH has been working with the Louisiana Pearlshell (Margaritifera hembeli) mussel in some capacity for nearly two decades. This federally threatened species is unique, as they can only be found in cold, headwater streams within Grant and Rapides Parish’s. Louisiana Pearlshell watersheds exist in rural areas which have seen an increase in logging and farming, activities that can increase sedimentation, disrupting mussels through all stages of life.

Tagged Sub Adult Louisiana Pearlshell Mussels — Louisiana Pearlshell mussels with freshly glued, yellow, Hallprint or PIT tags. Pit tags are covered sealed with gray marine epoxy to increase retention. These tags provide a unique identification for each individual, each presenting an different advantage. HallPrint tags are labeled directly on the shell and can be easily read while PIT tags need a special reader but can be detected without seeing or handling the mussel.

Freshwater mussels have a unique parasitic live stage where the larvae, called glochidia, need to attach to a host (various fish, hellbender, and mudpuppies). Glochidia are typically found on the host’s gills but can be found attached to other structures such as fins. Each species of mussel will have a its own host(s), some are generalized having several hosts, while others have a specific one or two. Glochidia will stay attached to the host until fully transformed into a juvenile mussel and will drop off. These are all early life history traits that need to be learned about each mussel species for a successful program.

After looking at what fish coexist with Louisiana Pearlshell, a series of host fish trials were completed, and the Grass Pickerel (Esox americanus) was identified as a viable host and is now used for the program at Natchitoches NFH. In 2018, the first official year class of Louisiana Pearlshell was produced for stocking and reintroduction. After 2 years of grow-out, individuals reached a length of 20mm or more and were tagged with either a Passive Integrated Transmitter (PIT) tag or HallPrint tag. PIT tags are the same technology used when pets are microchipped and allow us to use a waterproof scanner to effectively detect and recapture mussels in the wild. Both PIT and HallPrint tags offer unique identification so in the event of recapture we can identify what year it was created, date stocked and growth since last capture. All are vital to determining a successful program.

Juvenile Louisiana Pearlshell Mussels being measured after capture during monitoring. — Staff from Natchitoches National Fish Hatchery and the US Forest Service work together to complete monitoring of Louisiana Pearlshell Mussel that were stocked in 2020.Several mussels were found and measured using a set calipers. Two team members can be seen in the background searching for an individual after a PIT tag what identified by the Biomark tag reader.

Nearly 1,000 Louisiana Pearlshell were stocked into historically occupied areas in July of 2020.Altogether, 300 PIT tagged mussels were stocked between 3 water bodies and 12 sites. Since stocking, hatchery staff has teamed up with the US Forest Service and completed 2 monitoring events with the goal of recapturing as many live PIT tagged individuals as possible. A total of 63 individuals were found in 2021 and 50 individuals were found in 2022. Of the 50 found in 2022, 19 were recaptured from 2021. This means that 31 (62%) were never found in 2021 suggesting that detection rates can be underrepresented, likely based on how far below the surface an individual burrows.

Horizonal movements, or burrowing, are well documented throughout freshwater mussel species. Burrowing is seen in response to changes in flow (discharge), reproductive activities, water temperature fluctuations and perceived threats of predation. Individuals can also be covered in fresh deposits of sediment after large storms and rain events as the dynamic system fluctuates and finds new stability.

There was a lot of uncertainty whether any mussels would be found during the first monitoring event and even less certainty that they would be alive. Within 2 months of stocking, the area was decimated by Hurricane Laura (Category 4, nearly 5), followed by Hurricane Delta (a weaker Category 4) and several rounds of severe weather producing multiple tornadoes. Sites were hard to find and access but we started detecting tags and mussels that were alive!

Finding new individuals in 2022 and seeing an average of 15mm of growth from the recaptured individuals shows the resiliency of this species.It offers great promise that the Louisiana Pearlshell Controlled Propagation and Reintroduction Program at Natchitoches NFH will be very successful long-term!!