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#92486 11/27/06 08:22 PM
Joined: Apr 2002
Posts: 15,183
Likes: 507
Field Correspondent
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Field Correspondent
Joined: Apr 2002
Posts: 15,183
Likes: 507

This is the introduction from an informative description about fish kills and its causes. The link provided below will download the entire article as a pdf file for you.
“A fish kill is an event in which dead or dying fish are observed in a
lake or waterbody. Some fish kill events involve small numbers
of fish and others may involve hundreds, or even thousands of fish.
There are many factors of natural and human-induced causes that can contribute to a fish kill. When fish are dying it is because they are under stress, which is directly or indirectly causing their death.” The information provided by the following link discusses many causes of fish kills during the open water season.


Some basic information about fish deaths during ice cover is presented below.

A fish kill during ice cover can occur in northern states. Winterkill was aptly described in a Pond Boss magazine article by professor Mark Cornwell. I reproduce most of it here for your education. “Winterkills result from a couple of problems coming together at once. Ice forms on the pond and essentially caps it from interacting with the atmosphere. Oxygen entering the water from the atmospheric diffusion is stopped. Snow accumulates on the ice and essentially draws a shade over the pond, which reduces or eliminates photosynthesis. Once the shade is pulled by snow on the ice, respiration exceeds photosynthesis. In the darkened situation, the existing DO in the pond is all the oxygen that aquatic life has for respiration until ice is off. Take a winter’s worth of deep snow on ice and add to it a productive, fertilized or shallow pond and you have big trouble. Will drilling holes in the ice help? No, if you are doing it to save your fish. But, yes, it will get you out of the house and give you exercise. The pond in winter is inversely stratified where the warmest and most dense water is on the bottom (that’s why ice floats). This bottom water will not circulate to the top to receive any atmospheric oxygen due to a few freshly cut holes. Solution? SHOVEL OR PLOW SNOW OFF TO LET IN THE SUN! This will allow the photosynthetic plant pathway to start again and add oxygen to all the depths where the light can penetrate into the pond water. Be sure to practice good ice safety and common sense whenever you are on ice of a pond or lake. If you decide to aerate, do it carefully and in shallow water. Don’t aerate deep. You do not want to chill the pond water very much below 39F by circulating all water upward to an extremely cold atmosphere.” Water colder than 39F for extended periods stresses and weakens warmwater fish which can also lead to fish deaths.

Adding a list of threads on fish diseases/stressors to Bill's post.

You may want to look at these 2 threads. The first takes a questioner through a winter BG dye off to assess what and why they were dying. The second is a list of diseases/stressors. The third is an excellent thread on stressors and fungus on fish.




Fish Kills -- how much fish bio-mass can a pond support -- by Bill Cody


Maximum gallons of pond would be 5,000. Shelving and internal structures take up volume and makes the pond closer to 4000 gal maximum and probably less.

You stated above: "but this time we counted the amount-it turns out we still have 178 fish.... but then there are 50 less". This calculates to now about 128 fish. If you have 4000 gal, each fish has 31 gallons of water. This is not alarming if the fish are small such as 2"-6", but if the fish are larger then it becomes a big concern. Example: 10 6" koi are the equivalent biomass (waste producers) as one 12" koi. One 20" koi is eqivalent to 5 12" koi. 100 1" fish are the biological equivalent of 1 6" koi or goldfish. Readers should get the idea. I think 128 fish larger than 6" with some being 12" or longer, is too crowded for a healthy fish enviornment unless you are providing intense aquaculture conditions. Aquaculture conditions require daily monitoring of vital chemistries and iintense mechanical and biological filtration.

Many backyard ornamental pond people have the "stamp collector" mentality when it comes to fish. This view holds that the value of the hobby is directly related to the number of fish owned rather than the quality of the aquatic habitat provided. How many dogs and cats do you have to own before you consider them worthwhile pets? Is 128 too many? High fish loads means high bacterial loads. High numbers of bacteria puts a greater burden on fish immune systems and this increases the chances of bacterial infections.

One water quality issue not previously discussed are DOCs - dissolved organic compounds - that are a host of carbon based substances that exist in pond water. The main ones of concern are those dissolved substances derived from metabolic and biological byproducts such as fats, proteins, amino acids and so on. These substances have an insidious impact on fish health and are linked to disease causing bacteria in pond water. Ornamental ponds with dense fish loads are notorious for having unusually high DOCs. Best way to reduce DOCs is maintain low fish loads and occassional water changes.

I think all your problems with fish deaths will relate back to too many fish for the water quality within the pond.

Most importantly when one is dealing with a backyard ornamental pond, it is not so much the number of fish per gallons but one has to carefully consider fish biomass per gallons. Can you provide an estimate to us of what sizes of fish make up the 128 remaining fish in your pond?
I suspect that your pond does not have enough plant life to properly biologically filter the water to maintain its quality. Each 12" fish (abt 12oz) should have 112 ounces (7 lbs) of living plants (emergent and or submerged) in the pond or in an external filter for proper biologicial filtration.

Lastly it is important to realize that as fish grow, one should not add fish but remove fish, from the ornmental pond to maintain proper fish load balance. Example, you might have 36 4" koi in a pond today. But when they reach 6", the pond is overloaded and 27 fish will have to go. This assumes that the pond is properly maintained and receiving adequate aeration, and mechanical, chemical and biological filtration. To determine the maximum safe fish load for a backyard pond divide the gallons by 100. The answer is the number of 12" koi the pond should support. Remember, when the fish grow, one 24" koi is equivalent to 12 12" koi. I have tables and charts that provide fish biomass equivalents.

One last thought, I think is was a mistake to put all your fish back into a newly cleaned pond. The entire old fish biomass will create way too much ammonia and other waste products for the cleaned ecological system and the biological filtration to "handle" or process and your water quality will suffer until the fish biomass is reduced to safe levels (deaths) and/or the biological filtration re-establishes itself to the point it can handle the fish bioload.

[ August 01, 2007, 11:38 PM: Message edited by: Bill Cody ]

I will try to put together a chart here for estimating fish bioload in ornamental ponds. This word processor is not real user friendly when it comes to making tables from data.
1. Divide the pond gallons by 100. This is the fish load limit (measured in 12" fish).
2. In the table find the appropriate size of fish you have in the pond & note the corresponding equivalence #.
3. Multiply the fish load limit (step 1) by the equivalance number (step 2).

This is the approx number of fish of given size you can keep safely in the ornamental pond. So if your pond will hold 30 12" fish, then it will hold 270 6" fish (30x9). It is ALWAYS better to error on the side of too few fish, because the fish you have will hopefully grow if you do not kill them first. Fish that are not growing at a normal rate are probably under some sort of stress.

Fish Lenght Equivalent----Number of fish
(Inches fish Load)---------to one 12" fish

One 12" fish and two 6" fish do not represent the same bioload in a pond. NOTE - expect your pond water to become green right after the biological filter is cleaned. It is not because nitrifying bacteria were washed away, but because the heterotrophic bacteria populations were dislodged. Lots of rapidly reproducing heterotrophic bacteria have been shown in studies to consume lots of phosphorus; more phosphours than the phytoplankton (green water). Phosphorus "feeds" the green water. Ornamental ponds often get real clear when the biofilter becomes clogged. As the biofilters become clogged with heterotrphic bacteria and other organisms the water will tend to clear up as the highly efficient heterotrophic community consumes phosphorus. This is why it is very beneficial to oversize biofilters (2X-3X) so the filters allow for substantial organic clogging by heterotrophs yet still allowing adequate or good filter flow rates.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management
Bill Cody #137120 10/26/08 08:48 AM
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