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I own a small lake. Give or take it is about 50 meters wide and 100 meters long (or 160 feet wide, 320 feet long), with a max depth of around 2 meters / 6.5 feet. One third of the lake is surrounded by forest, the rest is grass / small enclaves of tress. The lake absorbes only rainwater from the surrounding residential area. There is a small outlet towards a much larger lake close by. The lake was recently dredge to remove many years of plantlife that had almost turned it into a swamp.

The lake has a large number of mosquitoes. There is also quite many frogs and a few birds that live near the lake.

What can I do to decrease the amount of mosquitoes in the lake?

For example, can I set up certain nesting boxes that will attract birds or bats that will eat a large number of mosquitoes?

Can I release certain fish that thrive on mosquites?

It is in Scandinavia.

EDIT

The shorelines are sharp and most of the vegatation has recently been removed. The lake was recently completely emptied and about 2 meters (or more) of vegatation and soil was removed. The lake has previously been contaminated which was cleaned away. I guess this means that a very large part of the ecosystem has been removed, a few fish did manage to survive, but not very many.

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  • $\begingroup$ Salitifing the water will terminate larva before developing into mosquitoes. $\endgroup$ – user5434678 May 24 '16 at 12:18
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    $\begingroup$ What does the shoreline look like? Is it "sharp" or does it merge into wetland areas that are flooded during spring? The latter will most often lead to more mosquites, since they are usually found in stagnant waters. $\endgroup$ – fileunderwater May 24 '16 at 13:22
  • $\begingroup$ Maybe you can find help at gardening SE? It's not about plants, but I would guess quite some gardeners have faced this problem before. $\endgroup$ – skymningen May 24 '16 at 13:22
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    $\begingroup$ If the lake was a lot smaller I could suggest tablets of bacillus thuringiensis that you can actually toss into the water and you let the bacteria control the mosquito larvae. For a lake that large though, this would entail quite a bit of control. You maybe want to try and get small top-feeding fish in there, almost like minnows, that will eat the mosquito larva. Other solutions I can think of are tightly regulated or available only to approved agencies/providers in the US or basically otherwise require special handling, or harm other insects/plants. $\endgroup$ – CKM May 24 '16 at 13:39
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There are a number of environmentally destructive methods that would be effective, including draining the lake, covering the surface with a continuous layer of oil, or adding toxins to the water, but I'm assuming you're looking for a method that will have the minimum possible off-target effect.

Different mosquito species breed in different habitats and are thus susceptible to different control strategies, so good photos of some adults might help, but I'm assuming here that you've correctly identified the lake as the breeding site. If there are any other possible breeding sites in the area please add them to your question.

If the assumptions above are correct then, broadly speaking, your best options are habitat modification or biological control. I'm not going to go into chemical control here since as stated above I assume you want low environmental and financial cost; if this is incorrect let me know.

  1. Habitat modification: regularly removing plant growth within the lake and trimming vegetation overhanging the edges will limit the ability of mosquito larvae to escape predators and may have some effect. However, the recent management you describe has probably gone some way towards this (did you notice whether the mosquito problem got any better after the plants were removed?)
  2. Biological control: introduce predators or parasites into the environment. Plenty of species eat adult mosquitoes but there aren't really any that primarily feed on them (unless they're the only food source available) and many adult mosquitoes will have laid eggs already - the most effective strategy will be to target the immature population. Larvivorous fish have been shown to be effective in many parts of the world, for example fish in the genus Gambusia ('mosquito fish'). I don't know of any that have been shown to be effective and can thrive as far north as Scandinavia, but if you stock the lake with fish (combined with managing vegetation as above) I'd be surprised if you don't see some improvement.
  3. Biological control, part 2: Use Bti (Bacillus thuringiensis var. israelensis). This is a bacterium that produces toxins which are highly specific to dipterans and don't really affect anything else. You can buy the bacterium in various formulations including sprays, tablets etc. I'm mainly familar with its use for controlling container-breeding mosquitoes, but this paper describes its use in large lakes so should give you some idea of dose required, optimal application method and time to reapplication.

Without knowing more about your budget, available time/manpower etc I can't really help you choose between these but I suspect #3 is your best bet.

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My suggestions, based on mosquitoe projects I've heard about in Sweden, would be to:

  • Thin vegetation along the shores
  • Remove accumulated detritus
  • Manage grasslands along the shore, by grazing if possible.

The idea here is to remove the stagnant pools where mosquitoes develop, either coming from rainfall or spring flooding/snowmelt. By removing pools and increasing sun exposture you will probably decrease habitats and larval survival. Continously managing grasslands usually also helps, since it removes excess plant materials and kills mosquitoe larvae by trampling (if grazing is used). To introduce grazing by e.g. sheep (if possible) will probably also have positive conservation effects. Stocking of fish might help, but I suspect that the mosquitoes mainly comes from adjacent waters and edge pools and not the main water body. I doubt that birds and bats can control the mosquito population to any larger extent, but don't have anything specific to back-up this statement.

Also note that actions depend on the types of mosquitoes found in the area. There are both "forest mosquitoes" and "flood mosquitoes" in Sweden, but most perceived problems come from the latter kind, and since you are referring to mosquitoes from the lake/pond I suspect you are dealing with the latter kind. You should try to determine the most common species found there though. Treatment with Vectobac G (Bacillus thuringiensis israelensis) might be an option, but I don't know what kind of retrictions there are on using it privately.

You might be interested in this report from a Swedish county board, since it mentions ways to control mosquitos (in Swedish):
Lundqvist et al. 2013. Förslag till hur myggproblemet vid Nedre Dalälven kan hanteras på lång sikt. Länsstyrelsen Gävleborg ("Suggestions for how to handle the mosquito problem at Lower Dalälven over the long-term" freely translated).

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Don't salinate the lake. It'll be a nightmare to maintain.

The 'standard' biocontrol measure is to release 'mosquitofish', but outside their native range they're almost as good at killing native mosquito-eating fish as they are at eating mosquitoes.

Find some native fish(perhaps in the bigger lake?) and stock your pond with them. Small fish are probably better for this purpose. If your pond freezes all the way through in the winter(it almost certainly does not, but it depends where you are in Scandinavia) you'll have to find something else. If it just freezes over lots of fish can handle being under an icecap for a few months.

If you have a local fish and game authority they will probably have advice about stocking your pond, or at least handled that question before.

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    $\begingroup$ And please: Do not release any fish and mess up the eco system of the lake. $\endgroup$ – Chris May 24 '16 at 13:39
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    $\begingroup$ There are some native fish in Scandinavia, such as the crucian carp (ruda in Swedish), that can survive even in ponds that freeze (nearly) to the bottom in winter. They'll eat pretty much everything, including insect larvae, so they should be at least some help in controlling mosquitoes. Do note that crucian carp tend to outcompete other fish in small ponds, and can be hard to eradicate once present. If your pond already has an existing fish population, think twice before introducing them. $\endgroup$ – Ilmari Karonen May 24 '16 at 21:11
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According to http://baliadvertiser.biz/mosquitoes/, more specifically the 3rd paragraph:

The next easiest solution is to add a submersible pump, a waterfall or fountain. Flowing water is pleasant and will also enhance the water quality. If it is possible keep the water flowing at all times. Mosquito larvae can only survive in stagnant water. Newly hatched mosquitoes must rest on the surface for a few minutes to let their wings dry, because of this they will not lay eggs in water that is constantly moving.

Mosquitos don't breed in moving water, because their larvae have to rest on stagnant water for a few minutes so their wings can dry. Flowing water will also improve the water quality by adding extra oxygen.

The trick is that you likely want a way to get rid of the mosquitoes that doesn't require maintenance, which rules out most mechanized methods that you could use to make the water move like wave generators, pumps or bubble generators. In theory, you can just let it run during the breeding season so they can't lay their eggs. The best solution will probably depend on how much effort you're willing to put into it. Maybe you can promote the pond as a nice place to play with remote controlled boats during the spring and early summer?

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protected by WYSIWYG May 24 '16 at 20:50

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