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MOSQUITO PREVENTION CONSIDERATIONS FOR FISH PONDS
 

INTRODUCTION

 
A backyard fish pond can be an enjoyable addition to your landscaping and provide a nice point of interest. Please bear in mind that fish ponds can become major localized producers of mosquitoes. Mosquito problems from a backyard fish pond can be prevented or controlled by proper construction and maintenance. The Alameda County Mosquito Abatement District will be happy to provide advice and information to you before you build your pond or on your existing pond. If you have a fish pond in your yard or are considering building one, there are a number of ways you can prevent or reduce mosquito production.
 
 
Mosquitoes and their predators have co-existed for many millions of years. Mosquitoes will seek out places in a pond where the possibility of survival for the next generation is the highest. Egg laying female mosquitoes will seek sheltered areas in any pond to lay eggs. Areas such as water trapped in the top of planter pots, very shallow margins, water in gravel or between rocks, algae mats or thick emergent aquatic plants and areas sheltered by overhanging plants provide protection against predators (fish and insect). By managing these areas to minimize opportunities for the mosquitoes you can prevent any mosquitoes from hatching in your pond.
 


FISH POND CONSTRUCTION

See your local nursery for advice on materials, available books and construction. You may also find a lot of information on the internet and seek advice from other pond owners, (see Internet Resources for Pond Owners). Check with your City or County jurisdiction for any special considerations before starting construction. Below are some general principles for constructing a mosquito-free pond.
Construct the pond using a plastic or vinyl liner
These are sold premolded for a pond or in sheet material for you to use for your own design or layout. Concrete reinforced with wire or steel rod may be used, but in California earth movement from earthquakes or soil expansion and contraction may cause cracking and leaks. If you do use concrete, put plastic sheeting under this to help prevent leakage.
Choose a sunny, sheltered location
The pond location ideally should receive about a half day or more of sun and be sheltered from prevailing winds. This will provide sufficient sunlight for water plant growth and reduce evaporation and damage to plants from winds.
Avoid locating under trees or large shrubs
Leaves from trees and shrubs can drastically increase the amount of maintenance needed. Large amounts of leaves can cause a drop in oxygen available in the water and create anaerobic conditions (indicated by pink or purple color in water and foul odors). Some leaves may be directly toxic to fish so especially avoid pines, redwood, eucalyptus, fruit trees (fruit rots quickly in a pond), acacia, oak, walnut and pittosporum and remember that the little one foot tall tree you just purchased may grow 100 feet tall over the years.
Construct pond as large as possible
A pond should be large enough to support fish and resist rapid temperature changes. Most of the oxygen for your fish will come through the surface of the water, so the greater the surface area exposed, the greater the Oxygen absorption (fountains, waterfalls or aerators increase the surface area). Larger volume ponds require less routine maintenance and resist rapid temperature changes, allowing fish to adjust. Larger ponds can also absorb larger amounts of organic matter (e.g., leaves) and remain healthy.
Construct pond as deep as possible
A pond should be at least 18" deep (deeper if possible) to provide protection for the fish from temperature changes and predators. The deeper you are able to construct your pond, the more resistant it will be to temperature variations. Deep areas in the pond will provide shelter for your fish. A shallow pond will heat up fast on sunny summer days and can become too warm for healthy fish. A shallow pond will expose fish to predation by raccoons, opossums and occasionally egrets or herons. Fish are an easy meal in shallow water. Check with local government agencies for any restictions or limitations on water depth.
Avoid constructing too much shallow area (less than 6 inches) in the pond
A well designed pond may have some shallow margins, but generally you should avoid having more than 10-20% of the total surface area in shallows (6 inches deep or less). Shallow areas provide warm sheltered areas for mosquitoes to lay eggs and for larvae to grow. Small fish or fry utilize shallow margins to keep warm or avoid larger fish and these margins also provide areas for some aquatic plants. Surface feeding fish such as the mosquitofish (Gambusia affinis) will feed in areas as shallow as 1 inch deep. If you have potted plants in your pond, be aware that shallow water in the tops of pots can provide an area for mosquito larvae to escape from fish.
Construct the sides of the pond to be as steep as possible
The steeper the sides, the less sheltered area available for mosquito larvae to live. Mosquitofish and aquatic insects such as back swimmers (Notenecta) can be effective predators when the sides are vertical or steep.
For Cleaning purposes provide for drainage or removal of the water
At some time in the future you may need to drain your pond for cleaning and if you plan for this you can minimize the work that will need to be done. You can build in a drain in the lowest section of the pond, or create a deep area for use of a sump pump to remove the water.
Avoid metal to water contact
Many metals, especially copper, can react with chemicals found in the water and produce compounds toxic to fish or aquatic plants so it is preferable to use plastic pipe.
Provide protection from Predators
Protection for fish from predators such as raccoons, opossums, egrets or herons can be built into the design of your pond. Overhanging rock edges can provide cover from sun as well as hiding places from predators. Anchoring ceramic drain pipes, large rocks or cinder blocks to the bottom of the deepest part of the pond can provide protection, as will large planters for lilies or other aquatic plants. Generally items weighing over 10 pounds cannot be removed by an animal.
Special considerations for artificial streams or waterfalls
Many pond owners like to have artificial streams and/or waterfalls leading into their ponds. These require some additional considerations for a mosquito-free pond. Water should not stand in small pools when the stream/waterfall is not running (calm pools can be major mosquito producers). Small tube drains can be built into the lowest points in these pools to drain rain water or remaining water between uses. The course can also be designed not to hold water when the pump is off. Leaves, dirt, organic material and other contaminants can be picked up along the course and delivered into your pond.

MOSQUITO PREVENTION

Once the pond is built there are some things you can do to insure that you do not develop a mosquito problem. Most mosquito problems can be prevented by taking the following steps.
Stock your pond with goldfish, koi and/or mosquitofish
A pond with a healthy and hungry fish population will eat most mosquito eggs, larvae and pupae. At no cost to the owner, the District provides mosquitofish (Gambusia affinis ) for ponds. You can pick them up at our office (See map) or call us and they will be delivered to your pond. Smaller goldfish or koi, if hungry enough, will generally eat mosquito larvae and pupae. If you have fish and notice larvae, stop feeding your fish for a few days and see if the larvae disappear. If they do not call the District for advice. Note: If your pond drains into or is connected to a natural waterway such as a creek or lake, contact the District or California Department of Fish and Game for advice before introducing any fish.
Keep landscape plants trimmed away from pond edge
Ground cover plants or other landscaping plants that touch the surface of the water can provide a base for algae growth and shelter for mosquito larva. Keep surrounding plants trimmed away from the water surface so this will not be a problem.
Remove excess organic material from pond
Organic matter such as leaves, fruit, and dropped flowers or buds should be periodically removed from the pond. Large ponds can absorb a lot of material, but excess amounts or rapid deposit can quickly cause a pond to go"bad". Excessive organic matter can require more oxygen than the pond has available for decomposition. Anaerobic bacteria grow in this material (water turns pink to purple and odiferous) and fish can die off rapidly when a pond turns anaerobic.
 
Thin or Remove excessive aquatic plants
Aquatic plants that are in contact with the surface of the water may provide shelter for mosquito larvae. Thin or remove plants so that fish can swim around and through the vegetation. Thin leafted plants such as milfoil, parrot's feather, provide excellent shelter for larvae.
Trim or prune landscaping plants
All ponds need some sunlight. Prune landscaping shrubs or trees to thin the shade cover and provide some light to reach the pond. Depending upon your choice of aquatic plants, you may need more or less direct sun to reach your pond. Check with your nursery or garden books for specific plant requirements.
Avoid contamination of your pond
Avoid contamination from fertilizers, pesticides, herbicides or muddy runoff. If your yard is to be treated for pest control, protect your pond by covering or not treating that part of the yard. Fertilizers can cause algae to grow rapidly in your pond. Many pesticides and herbicides used for yard pests can be very toxic to fish. If you are refilling or adding large amounts of water to your pond, consider whether there are chlorine or chloramines added to the new water. Check with a local pet store that sells fish or you nursery for advice on neutralizing these compounds (generally you can add water to you pond in small amounts - less than 10% of the volume - without problem).
Do not overfeed your fish
Hungry fish make better mosquito predators and you will not have left over food to encourage algae growth.

FISH POND REMOVAL

If you have a fish pond you no longer use or want, it may be permanently removed to prevent mosquito problems. Unwanted, neglected fish ponds are a problem for you and your neighbors. If you are going to remove your pond, or fill it with soil, the goal is to have no standing water.
Completely remove the pond
To completely remove a fish pond, break up and remove the bottom and edge material. Fill the hole with soil.
Provide for complete drainage
A pond may be prevented from causing a problem by breaking large holes in the bottom or removing the bottom. Drainage is successful if water does not stand for more than a few days. Check after rain to be sure drainage is sufficient to remove all the rainwater.
Fill the Pond with soil
Provide drainage as above then fill the pond with good soil and landscape it. If you desire to grow plants that prefer continuously wet soils, leave pond intact and fill with soil. Allow for shrinkage of the soil and fill above the edges of the pond so that water will not stand on surface. Use plants preferring these wet sour soil conditions. Consult a horticulturist at your local nursery for advice.
 
CHECKING FOR MOSQUITOES IN YOUR POND
 
You can check suspect areas in your pond for the presence of mosquito larvae by looking closely at areas where larvae may find shelter from fish. You may also use a white cup or bowl to dip water from these areas (mosquito control professionals use what looks like an old fashioned dipper attached to a long handle), any larvae present will be easier to see against the white bottom. Larvae can be very small and range up to 1/8 inch long. Generally they swim in a snake-like sideways motion. Pupae are rounder and "tumble" as they move. Additional information on general mosquito biology may be found on our life cycle page.


 
MOSQUITO CONTROL
 


If you have a pond and find that you have mosquitoes, there are some methods of control available. If you are a resident of the District, you can call our office and our field personnel will be happy to assist you at no charge.
Plant fish
If you do not have fish (or the ones you have are not controlling the mosquitoes) consider planting mosquitofish (Gambusia affinis) or other top feeding minnows. A few mosquitofish can consume large numbers of larvae and prevent the emergence of adult mosquitoes.
 
Physical Control
Mosquitoes can be eliminated by exposing them to predation by your fish or aquatic insects. Keep overhanging vegetation trimmed back from the edge of the pond. Remove or thin algae mats and emergent aquatic plants from the surface of the pond. Periodically (whenever larvae present or once every two weeks) drain or allow evaporation to lower the pond level to remove water from very shallow margins or rocks and gravel.
 
Bti
Bacillus thuringiensis israelensis is (Bti) is used to control mosquito larvae. This has no effect on eggs or pupa but is rather effective against larvae and is very specific to mosquitoes and a few aquatic insects (blackflies and some midges). The homeowner can obtain Bti for use in ponds in the form of floating "donuts" at many pet stores or nurseries.
Methoprene
This is a insect growth regulator used to control late stage mosquito larvae. It is not yet available to the homeowner, but information from suppliers indicate that it will probably be registered for sale soon. This is very specific to mosquitoes and a few aquatic insects (some midges) and while it does not kill the larvae, it prevents hatching into viable adults. If and when it becomes available for pond use, it will probably be in a briquet or pellet form that will prevent emergence of mosquitoes for up to 30 days. The District currently uses this material for control of mosquitoes where fish are not feasible.
Oil
Oil is an old time control technique. Mosquito larvae and pupae breathe through air tubes, and contact with a oil film prevents them from obtaining oxygen. Unfortunately, the only oils registered for mosquito control in the USA are obtainable only by mosquito control agencies. Oils can cause some "burning" of surface aquatic plants and can be very damaging to other aquatic insects that are dependent on the surface film (water beetles, back swimmers, water striders, etc.) so they should be applied only by mosquito control personnel. Do not use cooking or vegetable oils on your pond, as these will not provide effective control and will only cause a mess.
 


MOSQUITOES FOUND IN PONDS

In the northern California Bay Area there are a number of mosquitoes that are found in backyard fish ponds. Similar mosquito species are found all over the world. Below is a listing of the species found in the Bay Area:
Culiseta incidens
(The fish pond mosquito) This is the most common mosquito found in fish ponds. The larva can be found year round. The adults are moderately aggressive and attack in the evening or in heavy shade. This species is not known to carry any diseases, but can become quite a nuisance.
Culiseta inornata
(The Winter marsh mosquito) This mosquito is generally found during the winter months and will take advantage of any clean water source available. The adult is moderately aggressive and is a large mosquito.
Culex tarsalis
(The Western Encephalitis mosquito) This species utilizes fish ponds with clean water primarily in the spring and summer. This species is not very aggressive in feeding on humans, but is a known transmitter of encephalitis.
Culex stigmatasoma
(The foul water mosquito) This mosquito is occasionally found in badly neglected ponds. It feeds primarily on birds.
Culex pipiens
(The house mosquito) This mosquito is occasionally found in badly neglected ponds. This mosquito will enter homes and aggressively attack indoors at night.
Ochlerotatus sierrensis
(The Western treehole mosquito) This mosquito is found in badly neglected fish ponds that are annually filled by rain water and generally have become covered by plant growth. This species transmits dog heartworm and is quite aggressive in feeding on humans.

A FISH POND CAN BE AN ENJOYABLE ADDITION TO YOUR LANDSCAPING AND WITH PROPER CONSTRUCTION AND CARE CAN BE KEPT MOSQUITO-FREE.




ALAMEDA COUNTY MOSQUITO ABATEMENT DISTRICT
23187 Connecticut Street Hayward, CA 94545
Phone: (510) 783-7744
Fax: (510) 783-3903
E-mail: acmad@mosquitoes.org
Web site: http://www.mosquitoes.org