Milfoil in Lake Fairlee

Memorial Day — Monday, May 28th.

Today while many of us are relaxing around a barbecue, our divers are working. This week they will continue hand picking isolated plants and smaller patches near the shoreline. Working in dry suits against the still cold water and wearing scuba tanks, the divers swim a patrol along the shore removing individual plants, roots and all, and collecting them in their catch bags. This morning they were picking in front of the Treasure Island swimming beach.

One of their goals is to clean out the waterfront areas of the summer camps around the lake before the counselors and campers arrive. Horizons day camp has a sandy swim area, in which milfoil does not grow. Aloha Hive and Billings both have milfoil growing in their swimming and boating areas. Lochearn poses an additional problem because they operate water ski boats from their shore, whose propellers can fragment and disperse milfoil.

While hand picking is appropriate for smaller and less dense patches, there are other control techniques we can use for thicker growths. Suction harvesting is basically an improvement on hand picking. The suction harvester is a boat mounted pump with a six inch wide hose on the intake side and a large screen basket on the output. It works like a huge vacuum cleaner. The divers still remove milfoil plants by hand, then they direct the harvested plants into the mouth of the suction hose, which carries them away.

One of the persistent problems when picking milfoil is visibility, as the bottom of the lake is covered in many places with silt, which is easily stirred up. Since it is important to remove the milfoil plants roots and all, the process of picking quickly clouds the water and lowers the visibility to near zero, making it hard to see the plants to pick. A major benefit of the suction harvester is that the silt is pulled away and the water remains clear enough to see the plants.

The suction harvesting process also necessitates the use of a silt screen. The pump that moves the water also breaks up the plants, and some smaller fragments can escape the collection screen. Also quite a lot of silt is pulled up with the plants. This long, floating silt screen hangs down from the surface, encircling the operation. It captures the fragments and reduces the spread of the silt.

Because use of the suction harvester is more disruptive to the natural state of the lake, its use requires a special permit from the state, and we are limited in where and when we can use it. It is only appropriate where the milfoil has completely taken over, and there are virtually no native species present. In addition, we may not use it until after the indigenous fish have spawned, determined to be June 22nd.

The other milfoil control technique we employ uses Bottom Barriers. Where the milfoil infestation is thick and extensive, sheets of dark plastic are used to cover the bottom of the lake. They are held in place by plastic coated steel rebar and J-hooks. We install them in September and leave them in place for a year. By cutting off the sunlight they effectively kill all plant life where they are used. As with suction harvesting special permit for the use of bottom barriers is issued by the state.

I plan to write more about these techniques as the season progresses and our dive crew starts to use them.

At this time of year the milfoil plants are weak and brittle. Rather than making them easier to deal with, their very fragility creates problems. Simply swimming through a patch of the plants can cause fragments to break off. Each fragment is a potential source of new growth.

For this reason the divers begin picking where the pickings are slim. They start in areas where the plants grow in “ones” and “twos.” These plants are often found growing among native species. They do not pose the nuisance to swimmers and boaters that a thick patch does, but if left unchecked they can spread to become one.

The divers are working in “dry” suits. The water (at least in my cove, near the surface) is about 55 degrees. That doesn’t sound so bad, but yesterday while anchoring my floating dock I had my arms in the water for just a few minutes and it was cold! They wear scuba gear and carry “catch” bags, made of heavy netting. These bags are flattened cylinders about two feet long and ten inches across mounted on a metal frame that both holds the mouth opened and closes it securely.

They pick the plants individually, removing the entire plant with its roots. The plants’ fragility again causes problems, as they must be careful not to fragment the plant accidentally, and to retrieve any parts that do break off.

Yesterday (Monday, May 21st) they were working near the north shore of the lake in sector O-2 (see map). In the short term they plan to concentrate on smaller areas of growth at various places near the shoreline, continuing to pick what they call “onesies and twosies.”

On Thursday I accompanied the divers on their first trip around the lake to survey the extent of the milfoil this year. The rain had stopped for a while, but it was quite chilly, and after two hours on the lake I was very glad I wore my long johns. I stayed in the boat (don’t expect otherwise) but the divers took turns in the water taking a closer look.

Each spring brings surprises in the milfoil growth. Its extent is influenced by many things, including the amount of sunlight (which depends on when the ice went out as well as sunny days) and the temperature and the success of the last season’s eradication efforts. Besides looking for milfoil we were checking the status of the bottom barriers installed last September. Decomposing milfoil releases gas (methane) which makes large bubbles under the plastic barriers that have to be let loose so the sheets stay in place.

Ira seemed generally pleased with what we saw. There were a few heavy growths of milfoil, and many scattered “onesies=twosies,” which is what they call isolated or scattered plants. There were no big surprises. The heavier patches are where they were last year, and we found no new infestations.

We spoke generally about the order of activities. The divers will first revisit every piece of bottom barrier and re-situate them as necessary. Then they will begin hand pulling, treating the summer camp’s swimming areas early. Suction harvesting cannot begin until after June 22nd, because of the spawning cycles of several native fish species.

I came away from the morning with a much better understanding of where milfoil grows in Lake Fairlee and with increased appreciation for what the divers’ jobs entail. I plan to write up a summary of our survey later this week.

So I have added gridlines to the USGS maps, and am figuring out how to best incorporate them here. The map of the lake has been divided up into sections (quadrants?). I am hoping these can clearly convey the location of milfoil infestations and control activities.

I have split the map up into three segments covering smaller areas. They show detail of the west end of the lake, the center of the lake, and the north end of the lake. Take a look:

Clicking on any of these maps links to a larger image. You can always return to the main page by clicking on “Milfoil Control Activities” in the banner at the top.

There is a larger map, which might be the most useful here. And and even bigger one here. I am still trying to figure out how best to do this. Maybe it depends on how big a screen you have . . . .

So I have downloaded some USGS maps and will be using them to show where our milfoil eradication efforts are taking place. Here they are without gridlines:

Click on an image above to view a larger version. Use the BACK button on your browser to return.

The lake is warming up and the divers are set to arrive next week. We have lots to do.

Our first big task is to survey the current incidence of milfoil. We will travel the entire lake checking for emerging patches and examining old ones. We will verify the placement and condition of our bottom barriers.

Meanwhile we will get our boats and engines out of winter storage and make sure that are in good repair. We will inventory equipment and

As the divers get in the water and begin harvesting milfoil, I plan to use this space to report on their progress and announce planned activities and locations.

Questions? Email me at skip@lakefairlee.org