PREVENTION CONSIDERATIONS FOR FISH PONDS
- 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.
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
- 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
- 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.
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
- 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
- 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
- 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
- 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
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.
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.
- 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.
- 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 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
- 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
Web site: http://www.mosquitoes.org