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Under the ice: aquatic invasive species in winter

By: Emilie Scholie, Environmental Science and Sustainability (CFRES), (escholie@mtu.edu); Emily Klawiter, Keweenaw Invasive Species Management Area (KISMA), Assistant Coordinator (eklawite@mtu.edu); Dr. Sigrid Resh, KISMA Coordinator and Research Asst. Prof., CFRES, MTU (KISMA.up@gmail.com)

Calling all ice fishers! While this winter has had some odd weather, many lakes have frozen over and ice fishing season is in full-swing or will be again after the warming spell! If you fish in the warmer seasons, you may know that aquatic invasive species can easily be spread by leaving plant or animal residue on boats, trailers, equipment, and other surfaces. Invasive species can also be spread by leaving water in livewells, and the misuse of live bait. But did you know that aquatic invasives such as Eurasian watermilfoil (EWM), spiny and fishhook waterfleas, zebra mussels and even deadly fish diseases such as viral hemorrhagic septicemia can spread in the winter too? It might not seem as likely, but it’s very possible.

Eurasian watermilfoil thrives under the ice in winter. This aquatic invasive has been found in many inland lakes throughout the Upper Peninsula and northern Wisconsin. The rapid growth of Eurasian watermilfoil causes dense canopies which limit the growth of or kill off native aquatic plants. According to a Montana State University article about this invasive: “Eurasian watermilfoil can maintain a large amount of biomass throughout the winter which aids in rapid and early seasonal growth in the spring.” So, even though we can’t see it under the ice, this invasive species grows all throughout winter – jumpstarting the spread in spring and summer.

Looking out for EWM while fishing year round can greatly reduce the spread! The leaves are key for identification: they grow as groups of four up and down the stem, and each leaf has 12 or more pairs of feather-like leaflets that are usually limp when pulled out of the water. The stems can grow up to 21 feet, allowing the plant to grow in shallow and deep waters. As the stems reach the surface, they branch rapidly. Stems that reach the surface can flower from June to September.

Other aquatic invasives to look out for are spiny and fishhook water fleas, which are predatory zooplankton. Spiny and fishhook waterfleas are very small, but can cause considerable damage to fish populations, habitats, and harbor and gum up fishing equipment and other water gear. They originated in Europe and Asia and have been spread to the Great Lakes and inland bodies of water likely by the shipping industry through ballast water and then by recreational water users. As adults, they are unable to survive out of water, however, they lay eggs during the cold months and their eggs are resistant to drying and freezing. This means that under the ice, the eggs of waterfleas will persist through the winter and hatch in the spring, overwhelming the ecosystem with an exploding population. Waterfleas can outcompete native zooplankton, which are important sources of food for young fish in the region. Waterfleas can be difficult to identify since they’re very small, however, when their eggs accumulate on fishing equipment it looks like a large, white blob with black dots. Make sure to completely remove all residue by cleaning your equipment, and never release fish into a body of water different from where you caught them!

Zebra mussels are one of the most notorious aquatic invasive in the Great Lakes region. Similar to others in this article, zebra mussels are resilient in the winter. At cooler temperatures, a longer dry time is needed for equipment to properly dispose of zebra mussels and reduce spread. Studies have also shown that zebra mussels move to deeper water in the winter to avoid the colder water temperature and survive the season. Zebra mussels are present in Lake Superior and are found in every other Great Lake and many inland lakes throughout the midwest. To prevent the spread of zebra mussels, make sure to properly dispose of any leftover bait and inspect and clean your equipment well!

The first step to preventing aquatic invasive species in our waters is being educated about how to prevent their spread. Always remember to inspect all of your gear before and after ice fishing. This includes lines, hooks, augers, nets, and other equipment. If you are harvesting fish, make sure to drain the water from the container or bucket you’re transporting them in to reduce the spread of fish disease. And most importantly, have a safe, warm, and fun ice fishing experience!

What you can do. The Michigan Department of Natural Resources recommends these measures to prevent these and other invasive species from spreading:

• Clean boats, trailers and other equipment thoroughly between fishing trips to keep from transporting undesirable fish pathogens and organisms from one water body to another.

• Allow boats, trailers and other equipment to fully dry for 4 to 6 hours in the sun before use.

• Don’t move fish or fish parts from one body of water to another.

• Don’t release live bait into any water body.

• Handle fish as gently as possible if you intend to release them and release them as quickly as possible.

• Don’t haul fish for long periods in live wells if you intend to release them.

• Report unusual numbers of dead or dying fish to the local DNR Fisheries Division office.

• Educate other anglers about the measures they can take to prevent the spread of fish diseases and other aquatic nuisance species.

See the links below for more information on:

1. Eurasian watermilfoil: https://www.michigan.gov/invasives/-/media/Project/Websites/invasives/Documents/StatusStrategy/EWM.pdf?rev=de18752d421f42cc9cf427913d13a153&hash=76756E83A741BC350EAB56080AF77165

2. Spiny and fishhook waterfleas: https://www.michigan.gov/invasives/id-report/crustaceans/spiny-waterflea

3. Zebra mussels: https://www.michigan.gov/invasives/-/media/Project/Websites/invasives/Documents/Response/Status/egle-ais-dreissenids.pdf?rev=adbf892dee53497d8daf130c5c94d42b&hash=34C1ED86F5538377C40E5893981B12E3

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