An arial shot shows a collection of boats parked on a sandy point in the St. Croix River.

A drone shot show large river boats parked on a sandbar on the St. Croix River. PC Craig Blacklock

Zebra Mussels

The Invasive Zebra Mussel

Recognizable by their distinctive stripes and shells shaped like the letter D, zebra mussels are a serious threat to the St. Croix River’s native mussels. They take food away from other animals, reduce the oxygen in the water, and colonize areas on the river bottom so nothing else can live there. Zebra mussels also attach themselves to native mussels, preventing them from eating, breathing, and reproducing. Damage to boats, docks, and beaches can also occur.

Presence in the St. Croix

The first zebra mussels were found on boats in the Lower St. Croix in 1994. Currently, zebra mussel reproduction occurs below the Stillwater Lift Bridge. River currents appear to be preventing their upstream movement. But humans and their boats can move zebra mussels upstream and across land, and at this time, there are no foolproof ways to control or slow the spread of zebra mussels once they are introduced into a water body. This makes strong prevention efforts an essential requirement to stop the spread of zebra mussels. The St. Croix is at a juncture in its ecological history as current actions determine what will happen to native species threatened by zebra mussels.

How We Monitor Zebra Mussels

Wild Rivers Conservancy Summer Intern Ajoi takes a water sample from a boat

A cinder block sample is removed from a body of water. Wild Rivers Conservancy staff will count measure the amount of zebra mussels attached to it to estimate population density.

Veliger tows

Veliger tows are water column samples used to detect microscopic zebra mussel larvae and spiny water fleas. In total, 65 lake and river sites are sampled annually.

Cinder block and plate sampling

Cinder block samplers are deployed in areas of the Riverway where zebra mussels are present to calculate zebra mussel population densities. Plate samplers are deployed as an early detection method in lakes and sections of the river that are at high risk for zebra mussel introduction.

Dry dock boat counts

Dry dock boat counts are conducted to assess zebra mussel populations in the Lower St. Croix River, where zebra mussels are already present.

Focused Monitoring Efforts

lake and river sites monitored annually

for zebra mussel veligers

plate samplers deployed annually

for early detection in high risk waterbodies

hours spent inspecting watercraft

at launches in 2025

Inspect

Check your boat, trailer, and equipment for attached plants and animals, and mud.

Clean

Remove all attached plants, animals, and mud from your boat, trailer, and equipment.

Drain

This means all water, including pools of water in the boat, to eliminate tiny larval zebra mussels, and leave your drain plugs out during transport.

Dry

Leave your boat in the sun at least 5 days before moving between bodies of water.