About 7,500 captive-reared snuffbox mussels have been released into three waters in south-eastern Michigan, in a stocking announced by the Michigan Department of Natural Resources in late September 2026. One thousand of them carry passive integrated transponder (PIT) tags, so much of the follow-up survey work on this endangered species will be done with an RFID reader held in the water rather than by eye alone.
The snuffbox is one of 44 freshwater mussel species native to Michigan and is listed as endangered both by the state and by the United States federal government. The Michigan Department of Natural Resources (DNR), the state agency responsible for Michigan’s fisheries and wildlife, assisted the US Fish and Wildlife Service (USFWS) and the US Geological Survey (USGS) on the project, and local DNR fisheries biologists worked with the USFWS Michigan Ecological Services Field Office to choose the release sites.
Where the mussels went
The Huron River in Livingston County took more than 4,100 mussels across several sites. The Clinton River in Oakland County took approximately 2,800. Cass Lake, also in Oakland County, took 500. The Huron and the Clinton both hold existing snuffbox populations, and Cass Lake held one historically. Every release site sits in state or municipal ownership, away from infrastructure and private interests, which matters for a programme that needs repeat access to the same patches of riverbed for years. The site figures are approximate, which is why they add to roughly 7,400 against a headline total of about 7,500.
The mussels were reared by the USGS Columbia Environmental Research Center, a federal laboratory that propagates freshwater mussels for recovery programmes. The broodstock, meaning the parent mussels, was collected from the Clinton River, so the stocked animals are the offspring of local stock rather than an introduction from a distant population. Most are still juveniles, but they have reached adult size, which is what made it safe to put them out.
“It is exciting to take a positive action to help benefit snuffbox populations,” said Cleyo Harris, a DNR fisheries biologist. Harris noted that mussels cannot swim, so they “need to be placed a bit more strategically” than a fish stocking would require. A mussel set down in the wrong substrate or the wrong flow has no way of correcting the mistake.
Three marking methods, one of them RFID
The mussels were marked in three ways. One thousand received PIT tags. Another thousand received small physical tags carrying a unique identifier, glued to the outside of the shell. The balance, which the DNR did not put a figure on but which works out at roughly 5,500, received colour markings on their shells. The redundancy is a hedge rather than duplication: numbered tags and colour marks need the mussel in hand or at least in sight, whereas a PIT tag can be detected in situ without lifting it, and a glued tag can in turn be abraded, fouled or lost.
Why mussel PIT work is low frequency and glued to the shell
The DNR release gives no technical detail about the tags, so none of what follows is confirmed for this particular deployment. It is, however, what PIT tagging in freshwater mussel research almost always looks like, and it is worth setting out for an audience that mostly works in UHF.
PIT tags in fisheries and aquatic conservation work are overwhelmingly low frequency devices operating at 134.2 kHz and conforming to ISO 11784 and ISO 11785. ISO 11784 defines the structure of the 64-bit identification code; ISO 11785 defines the air interface, including the two signalling schemes in common use. In full duplex B, or FDX-B, the tag is energised continuously by the reader field and modulates its code while sitting in it. In half duplex, the tag charges a capacitor from the field and transmits during a deliberate gap in it. FDX-B glass capsule transponders, the same family used to identify livestock and pets, are the usual choice for mussels.
On a mussel the tag is bonded to the outside of the shell with cyanoacrylate or a two-part epoxy rather than implanted. A bivalve offers no soft-tissue site where a glass capsule can go without injury, but it does offer something a fish does not: a hard, clean external surface that takes adhesive well. The trade is that an externally bonded tag is exposed to abrasion, biofouling and adhesive failure in a way an implanted one is not.
UHF does not work underwater, and this is why
For readers used to RAIN RFID at 865 to 868 MHz in Europe or 902 to 928 MHz in the United States, the frequency is the interesting part. UHF RFID is a far field technology: the reader radiates a propagating electromagnetic wave and the tag harvests energy from its electric field component. Water ruins that twice over. It is a strongly polar molecule with a relative permittivity of around 80 at low frequencies and a dielectric relaxation up in the gigahertz region, so at UHF the molecules are driven hard enough to convert a large share of the incident energy into heat, which is the mechanism a microwave oven exploits a few gigahertz higher up. On top of that, even fresh river water carries dissolved ions, so it has a finite conductivity and the passing wave induces currents that dissipate as heat. Seawater is far worse, but freshwater is quite bad enough: the useful range of a UHF link in water is measured in centimetres, not metres.
The air to water boundary makes it worse again. The permittivity mismatch reflects much of any wave arriving at the surface, so a reader held above the water struggles to get energy into it at all. And a UHF tag antenna is a resonant structure tuned for the permittivity of air. Submerge it, the surrounding permittivity rises by a factor of roughly 80, the antenna’s electrical length changes with it, and the resonance shifts well clear of the operating band. The tag detunes and stops matching the reader even where the field does reach it.
Low frequency inductive coupling sidesteps all of that. At 134.2 kHz the reader antenna and the tag coil couple in the near field through a magnetic field, behaving as a loosely coupled transformer rather than a radio link. The relative permeability of water is essentially 1, the same as air, so the magnetic field passes through almost unaffected, and the skin depth in fresh water at 134 kHz runs to metres, making conductive loss negligible at these distances. Read range is limited by the geometry of the near field, which falls away roughly with the cube of distance, and by the size of the reader antenna and the tag coil, not by the water. In practice a hand-held or pole-mounted LF antenna worked over a riverbed reads from a few centimetres out to a few tens of centimetres, which is the right scale for locating tagged mussels in substrate. UHF buys metres of range in air and loses nearly all of it on contact with water; LF never had that range to start with and keeps what it has when the antenna goes under.
What the monitoring is meant to answer
Researchers will use the tags and markings to follow survival, reproductive success and movement, and to establish whether invasive zebra or quagga mussels affect the stocked animals. Those invasives colonise hard surfaces, native mussel shells among them, which can interfere with feeding and movement, so telling a mussel that simply died from one that was smothered is a live question here. Harris said the work will help advance conservation actions in the Michigan Wildlife Action Plan and in the federal snuffbox recovery plan.
Freshwater mussels are among the most vulnerable groups of species in North America, under pressure from habitat loss, pollution, invasive species and climate change, and they do more than their profile suggests: they filter water, stabilise the substrate they sit in, feed other animals, and their shells provide habitat for small fish, insects and crayfish. The DNR described the stocking as “a unique opportunity to stock endangered native mussels in Michigan waters”.
Further coverage of RFID in animal tagging and in environmental and conservation work is collected on RFID News, and the terms used above are defined in the RFID glossary.
Read more at https://michigan.gov/dnr/about/newsroom/releases/2026/09/24/endangered-snuffbox-mussels
