Fish Kill

No evidence of dead fish remained when we arrived at Dogwood the week after we discovered all of the dead fish floating on the water. The buzzards, vultures, owls and other scavengers did their job and cleaned up the mess, including the bones. The turtles seemed happy enough swimming around and the usual frogs serenaded each other with their croaking Friday night.

Here’s more information about what we now know has an official name: “Fish Kill.” It’s always nice to have an official name to put on a problem because it means someone else has dealt with it before and may have a solution. Fish kill is a common enough problem that there are lots postings on the internet about it and how to prevent it.

Fish kills are due to what we suspected right away: low oxygen concentration dissolved in the water. Incidents usually occur suddenly, like it did at Dogwood Ranch, resulting in fish suffocation during the early morning hours. The reasons for this take us back to chemistry class.

Warm water does not hold as much oxygen as cool water. Fish metabolism increases in warmer water so they require more oxygen as water temperature rises. Oxygen levels in water are lowest at dawn because aquatic plants do not photosynthesize at night, so less oxygen is released into the water at night. Fish swimming near the surface of the water or near the shore right before daybreak and gulping for air is a good sign of oxygen depletion water.

There are several causes of oxygen depletion in nutrient rich ponds: (1) the die-off of a large algae bloom; (2) decay of weeds in the water after treatment with an herbicide; (3) turnover of oxygen-poor bottom waters following a thunderstorm; and (4) runoff of livestock waste and other organics after a heavy rain. Since we know we did not use any herbicides near the pond and certainly not in it, 2 is probably not what happened. According to the rain gage, Dogwood did get a drop of rain for nine days prior to the fish kill so we do not believe 3 or 4 were the cause.

We believe that 1 is the most likely cause of the fish kill. The pond is nutrient rich due to the run-off of manure from the pasture and the cows generously dropping manure in and around the pond. During the day, algae produce and release oxygen as a by-product of photosynthesis. This is a source of oxygen in fish ponds like at Dogwood. At night, plants consume oxygen but algae blooms usually produce much more oxygen during the day than is consumed at night. But, under certain circumstances, there is reduced oxygen produced during the day while the oxygen consumption at night remains the same. If the nutrients algae use (like nitrogen from cow manure) is used up without influx of fresh nutrients, the algae bloom decreases, a natural oxygen source is temporarily lost leading to low oxygen levels in the water and the fish suffocate and die.

According to The FishSite at http://www.thefishsite.com/articles/289/oxygen-depletion-and-other-types-of-fish-kills: “Extremely calm days may also reduce photosynthesis and oxygen production, even under sunny conditions, by preventing phytoplankton [algae] in the middle layers of the pond from mixing near the brighter surface. In summer, oxygen problems may arise because of a simple physical property of water. The warmer the water, the less dissolved oxygen it can hold. When a dense bloom produces a surplus of oxygen on a summer afternoon, the oxygen will not stay in solution and escapes into the atmosphere. During the night, the bloom attempts to take more oxygen out of the water than what remains from daytime photosynthesis. When this occurs, dissolved oxygen levels approach zero. Fish begin to suffocate in the pond, and aeration must be applied to meet the demand for oxygen and prevent fish losses.” Similar conditions seem to be what caused the fish kill on Dogwood Ranch.

Since a week later there was absolutely no evidence of the fish kill remaining at Dogwood, perhaps fish kills occurred in the pond before but were not noticed due to when they occurred. For instance, if the fish kill occurred on a Monday morning, it is possible the scavengers cleaned it up before our arrival the following Friday night or Saturday morning. Moreover, in the summer, Dogwood can be unbearably hot and humid, so we do not go out there every weekend June through August. If fish kills happened before and we just might not have know it.

To prevent another one, we ordered a diffuser and hope to install it in the pond this weekend. A diffuser is a device that sits on the bottom of the pond. It is connected to a tube that runs to bank and is connected to an electric air compressor. The compressor, which we will keep in the well house plugged-in to electric power, will push air through to the tube to the diffuser at the bottom of the pond. The air pumped into the diffuser will blow out hundreds of tiny holes sending the resulting tiny bubbles floating up through the water column. The bubble will add oxygen to the water and help circulate the water at the bottom of the pond to the top. A pond diffuser is like the device used in a fish tank for the same reason, it’s just a lot bigger. We chose a diffuser over an aerator fountain for two reasons. One, the fountains type of aerator that you may see in ponds in city parks or retention ponds in neighborhoods or developments only aerate the top few feet of water column If the pond is deep, 12 feet or more, that still leaves most of the water at risk for oxygen depletion. Second, I think a fountain, although attractive in a city park or in front of an office building, would just not look right spraying up in a pattern at Dogwood. To me it would spoil the natural look and sounds of the pond.

We hope the diffuser will help prevent fish kills in the future and overall increase the health of the pond, cows or no cows! The next challenge is hooking-up the diffuser and sinking it to deepest part of the pond possible, without getting wet ourselves.