by Cathy Higley – On July 24, 2019 graduate student Ben Martin pulled up a plankton sample from the deepest part Plum Lake that contained 2 live spiny water fleas – the new invasive species was soon verified for Plum Lake. At this time densities are expected to be low, but needs further monitoring.
Martin is working through the UW-Madison Center for Limnology and is researching invasive spiny water flea impacts on food webs in Northern WI. His work is also exploring how these food webs might be managed to mitigate impacts from spiny water fleas. Spiny water fleas are a zooplankton native to Northern Eurasia. They were likely brought to the United States through ship ballast water and were discovered in all Great Lakes by 1987. From there inland lakes began to be invaded.
Spiny water fleas can reproduce asexually during the summer, so only 1 individual is needed to start a population. In the fall, spiny water fleas will mate and lay “resting eggs”. These resting eggs fall into the sediment and are fairly durable – they have been known to survive being eaten by fish and are not effectively killed off by the DNR recommended 500 ppm bleach solution (however, this is effective on many other species). Martin says, “Often it is the resting eggs that are the problem. Boaters may not realize how important it is to keep their anchors, boat, and other equipment mud-free. Going from one lake to the next with a muddy anchor can transfer the resting eggs.”
How long have the spiny water flea been in Plum Lake? It is hard to say. These organisms are less than ½ inch long and translucent. Finding them in a lake is like finding a needle in a haystack. It can also take several introduction events before the organism is able to establish, so it makes sense to expect a time lag between when an invasive species like spiny water flea establishes until it is detected. Spiny water fleas are classified as a Prohibited invasive species and are currently in 11 inland lakes in Wisconsin, but the public access lakes of Vilas County are a hotspot. Trout Lake (verified in 2014), Star Lake (verified in 2013), Stormy Lake (verified in 2007), Ike Walton Lake (verified in 2015), and Butternut Lake (verified in 2014) are known to have spiny water flea. Other inland lakes that have verified spiny water flea are the Gile Flowage in Iron County (verified in 2003) and the Madison Chain of Lakes in Dane County (verified in 2009).
Impacts from spiny water fleas are varied, depending the ecology of the lake. The biggest known concern is that spiny water fleas are predators of algae-eating zooplankton like Daphnia, and if they eat enough Daphnia they can change the food web of a lake. If Daphnia (algae-eaters) densities drop, more algae in the lake is expected. Lake Mendota in Madison saw a loss of nearly 1 meter (3 ft) of water clarity that was attributed to dense spiny water fleas. In the case of the northern WI lakes, the spiny water fleas have not been as impactful. Martin hypothesizes this is because the northern lakes have less nutrients, therefore less algae and less Daphnia for the spiny water fleas to eat. This could mean spiny water flea populations remain limited. He further elaborates that since spiny water fleas seem to prefer cold, clear, large bodies of water, he is investigating whether plankton eating native fish populations, such as cisco, could be helpful in managing spiny water flea densities.
There is currently no effective strategy to control spiny water flea once they are established, so prevention is key.
All boaters in Wisconsin are required by law to inspect their boat and equipment for plants and animals, remove what they find, drain their boat and equipment, and to never move live fish. And while WI law does not require it, letting your boat and equipment remain dry for at least 6 hours will effectively kill adult and resting egg spiny water fleas. However, plenty of recreational and professional boaters go from lake to lake and do not take the time to wait 6 hours to kill off the spiny water flea.
This tendency was the primary driver for the UW-Oshkosh Decontamination program. This program allows boaters to decontaminate their boats and other equipment through a mobile hot pressure wash at no charge to the boater. The operators are trained to adjust temperature, pressure, and contact time as needed to carefully and effectively decontaminate without causing harm to sensitive areas of the watercraft such as motors, wiring, and decals.
The idea behind decontamination is that “you can’t remove what you can’t find”, and it is more effective at removing small bodied organisms like spiny water flea than manual removal.
Plum Lake has participated in the UW-Oshkosh Boat Decontamination program since it started the summer of 2018. This program targets 3 lakes with verified spiny water flea (Trout, Plum, and Star Lakes) and 1 lake with suitable conditions for spiny water flea (Big Muskellunge Lake). Upcoming fishing tournaments include Plum Lake as well as other lakes that have spiny water flea. If boaters plan to be lake hopping, make sure that lake water in livewells and bait buckets and bait are not transferred from lake to lake. Bring ice or freezer packs if you will need to keep fish fresh. Keep your anchor, boat, and equipment mud-free. Consider decontaminating your boat by using a hot pressure wash. To follow Martin’s research, check out his Twitter page @TinyTheSpiny. For more information on spiny water fleas, their impacts, and how you can help prevent their spread contact Cathy Higley, Lake Conservation Specialist at the Vilas County Land & Water Conservation Department at 715-479-3738 or email@example.com.