Storms, especially tornadoes, hurricanes, severe cold fronts, and floods, can devastate ponds used for holding or rearing fish, especially small ponds which can be drastically affected because of their small volumes (their physical characteristics and chemistry can change quickly). Not only can such storms dump huge amounts of organic debris (leaves, limbs, sewage, etc.) into ponds but they can rapidly change the pH, temperature, salinity, chemistry, etc. of ponds by the addition of large amounts of water to ponds. Hurricane winds can fill ponds with debris and hurricane water surges can flood ponds with salt water, sewage, and debris; heavy rains can flood ponds with water (sometimes containing organic debris, and/or sewage) of a different pH, temperature, and composition than that formerly in the pond; and severe cold fronts can cause a massive amount of leaves to fall in a short period and also drop the water temperature to critical levels for some fish. Thus some storms can give a double or triple whammy to ponds, adding debris plus sewage plus water of different physical and chemical properties.
A hurricane can give the most severe triple whammy to ponds near the ocean, causing surge salt water and debris (including sewage) plus wind-driven debris to almost literally "fill" a small pond with debris and sewage and salt water, a very drastic change in the pond's physical properties and chemistry. Such a change can be fatal to all, or nearly all, life in a pond unless quick action is taken. I have seen some animals that are mobile survive, e.g., alligators, turtles, and frogs. I would also suspect some lower life as bacteria and viruses might survive.
Severe weather can cause the loss of fish or a pond in the following ways (See figure):
---Debris (including sewage) (from freezes, storms, floods, surges, etc.) put into pond decomposes, dropping pH and causing the pond to become more acidic, and the decomposition processes can use up most, or all, of the pond dissolved oxygen, and thereby can kill fish from pH shock or oxygen depletion, or a combination of both factors, or stress and subsequent infection.
---Too much freshwater from rain/floods (1) can change temperature and pH and water chemistry so quickly that fish either cannot adjust to the changes or do not have time to physiologically adjust to the changes and they die from physiological stress and/or subsequent infection, OR (2) can cause an algae or submerged plant CRASH (after a bloom that results from the added nutrients (with new water) to pond and crash occurs after these nutrients are used up), OR (3) can cause a crash/die-off by the sudden new physical or chemical water properties not being tolerated by algae/plants/animals in pond. Decaying plants and animals can cause an oxygen depletion, affecting the fish that may have survived the water property changes.
---Salt water from surges can kill many fresh water pond species that cannot tolerate increased salinity, including pond plants; affected organisms then decompose and can cause oxygen depletion and pH drop, etc., which may affect any surviving pond life.
Even small to moderate amounts of debris or water inflow into ponds may cause fish stress; such ponds should be observed closely for any sign of fish stress. Excess debris should be removed from any pond as soon as possible to keep demand for oxygen due to decomposition at a minimum, and to keep pond pH from becoming too acidic. Some nutrients derived from "normal" pond debris decay are needed to keep the pond plants going.
One might be able to save some fish if they: (1) remove what fish they can to clean compatible water as soon as possible after the storm passes and treat them with medications that promote the formation of a protective mucus coating and with wide-spectrum antibiotics (used because bacteria involved are not usually identified), or (2) remove all the organic debris as soon as possible right after the storm, before the debris has much time to decay and release/form tannic, humic, fulvic, and hydrophilic acids, etc. which can turn the pond water too acidic for most fish, and before most of the oxygen in the pond is used up by decaying organics. If one waits until most of the oxygen has been used up, or the pH has dropped very much, then one should just write off the fish in the pond, and start all over. The affected fish are generally so highly stressed that if removed to another confinement they usually succumb, mostly due to bacterial or fungal infections.
In order to get the pond cleaned and suitable for new fish one must expend a LOT of labor and time in clearing the pond of debris and doing a total water change, and then conditioning the pond for addition of new fish.
To get the debris out of the pond you can either: (1) pump out the water, then pull, pitch, lift, etc. all the debris out, or (2) get the debris out of the pond with hooks, rakes, and nets until almost all is out, then pump the pond down, and remove the rest of the debris. I usually prefer the second method, as water helps buoy up the debris thereby making it somewhat easier in lifting it to top of the pond. But once out of the water the debris is made heavier by the added water weight; each should use whatever removal method is easier for them.
If sewage is suspected (or known) to be in the pond one should wear protective gear to lessen exposure to human viruses and bacteria. This gear would include water-repellant boots, rain gear, respirator, goggles, hat, gloves, etc. This gear should be rinsed in bleach water after use, and one should take a thorough decontamination shower after work on the pond. There are many viral and bacterial infections possible; bacterial eye infections received while working on sewage-flooded land or ponds can be quite painful and bothersome (personal experience from Hurricane Katrina). Hepatitis A and B shots (and any newer hepatitis shots) are also recommended prior to working in sewage-flooded areas (I had the series of shots prior to working in the really bad stuff after Katrina.).
If one's time is limited and a pond cannot be cleaned of debris for a while, certain things should be done to slow down the release of decomposition gases (which may offend some people if they come close to pond), and to stop mosquito breeding. To slow down decomposition gas release one can either keep the pond pumped down and free of water, or put a hole in the liner (which can be repaired later) so water goes into the ground (Salt water going into the ground can kill trees and have other effects.). Mosquitofish (Gambusia affinis) should be added to any debris-filled pond that contains water; this fish can live in ponds with a lot of decaying debris because it can get its oxygen near the surface. It is tolerant of salt water (up to about 40 ppt) and water conditions that would kill many other fish, and will keep mosquito larvae under control.
After the debris has been removed from the pond, rinse the pond out several times, pumping out the fouled rinse water each time. Then fill the pond with clean water from your source, and condition the water for addition of new fish.
Conditioning would consist of procedures to make sure the pond water is safe and ready for fish, including: (1) removal (aeration) or neutralization (using various chemicals) of chlorine or other harmful water toxins added to city water supplies, (2) addition of submerged plants to get oxygen production going in the pond, for hiding places, and for cover against predatory birds, (3) verifying the temperature, pH, etc. are compatible to values from the prior habitat of the new fish, (4) testing the water with a live fish to verify there are no killing toxins remaining in the water.
New fish should be treated for Gyrodactylus, etc. before addition to the pond.
After the new fish and plants are added the pond should be observed for a while to verify there are no signs of stress on the fish. These pond-cleaning procedures may have to be repeated over and over again after each passing storm if too much debris or water is added to the pond.
A partial (or temporary storm) cover or netting to exclude falling and wind-borne debris plus having the pond's edge higher than surrounding land to exclude "normal" rain water running into pond will lessen pond problems from too much debris and new water. Large floods and salt water surges are difficult to protect against. Bird netting (installed so leaves hitting it fall outside the edge of pond rather than collect on top of the netting) could be used to discourage predation by birds AND to exclude falling or wind-borne debris.
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