Responding to Fish Kills

During the summer and early fall, it is not unusual for fish kills to occur in commercial ponds under certain circumstances. The cause of a fish kill can usually be determined by trained investigators if an investigation begins immediately. In some situations, fish kill investigations result in the filing of lawsuits, so investigations must be able to collect reliable evidence as quickly as possible and follow strict procedural guidelines for the evidence to stand up in court.

Many substances can kill fish directly if accidentally introduced into ponds, but most summer fish kills are caused by oxygen depletion from algae die-offs or weather-related “turnovers.” Algae blooms occur when excess nutrients such as nitrogen and phosphorus are present. In commercial ponds, high nutrient levels are an unavoidable result of daily feeding. In a typical catfish production pond, there may be as much as 2,000 to 3,000 pounds per acre of algae suspended in the water column in the summer months.

Algae blooms add oxygen to the water while the sun is shining, but they consume some of this oxygen at night. If a pond experiences several days of hot, calm, cloudy weather, the amount of oxygen present may not be sufficient to meet the night-time demand from the algae, fish, bacteria and other organisms. When water is very warm it holds less oxygen in solution, so a large portion of the oxygen produced in the daytime has already left the pond within a few hours after sunset, even on a sunny day.

Occasionally an algae bloom dies suddenly. Die-offs can occur naturally or can result from poisoning by certain herbicides. As a die-off progresses, millions of bacteria use the dead plant cells for food, and these bacteria consume large amounts of oxygen. Imagine dumping 2,000 – 3,000 pounds per acre of green grass clippings into your pond. This would have roughly the same effect as an algae die-off. Since the algae are no longer producing oxygen, oxygen levels can drop to zero even with the sun shining.

What To Look For

Some of the physical signs to look for when oxygen depletion is suspected are:

• Fish gasping, swimming to the surface,, acting sluggish
• Some fish species affected much earlier than others
• Larger fish tend to die earlier than smaller fish of the same species
• Kill occurs at night or in early morning
• Few microscopic animals in the water; lots of algae, mostly dying
• Oxygen very low (<2ppm)
• pH low to normal
• Water color brown, gray or black, occasional bad odor, often becoming clear later

Fish kills can occasionally be traced to inadvertent run-off of chemicals, improper application or drift of insecticides, or contamination by herbicides (which can kill the algae bloom and cause oxygen depletion). Some physical signs of fish kills associated with pesticide toxicity are:

• Convulsive, erratic swimming, sluggishness
• Some fish species affected much earlier than others
• Smaller fish die earlier than larger fish of the same species
• Kills may occur at any time
• Zooplankton and/or algae dead or absent
• Oxygen normal
• pH high or normal
• Water color and odor normal

Low dissolved oxygen levels and high temperature can magnify the effects of toxic substances on fish.

What To Do

If you observe a large number of dead or dying fish and suspect that fish have been exposed to some sort of pesticide or toxin, contact professionals knowledgeable in the correct procedures to follow. If the fish kill is suspected to have been caused by pesticides, the local office of the Louisiana Department of Agriculture and Forestry (LDAF) should be contacted immediately, Call the state office at (225) 925-3763. The official contact at the LDAF is:

Director, Pesticide and Environmental Programs
Louisiana Department of Agriculture and Forestry
5825 Florida Blvd.
P.O. Box 3596
Baton Rouge, LA  70821-3596
Phone: (225) 925-3763

A fish pond owner should immediately contact his or her county agent or an Extension specialist.

By law, the office of Pesticides and Environmental Programs, Louisiana Department of Agriculture and Forestry, must be notified within 15 days of the discovery of a fish kill if the kill is suspected to be the result of pesticides.

I.      As preventive measures, fish farmers and farm pond owners should take the following steps:

1. Get the name and phone number of all adjacent landowners and farmers so they can be contacted in case of suspected pesticides drift.
2. Advise all adjacent landowners of the location of you fish ponds.
3. Get the name and phone number of all aerial applicators who will be flying for adjacent landowners. Advise them of the location of all ponds on your property. Obtain a description of their plane(s) and “N” number(s). Also, give them your telephone number.

II.      If fish losses occur and pesticides are suspected as a cause, take the following actions IMMEDIATELY:

1. Advise the Office of Pesticide and Environmental Programs of the problem immediately (see above.)
2. Request assistance from the county agent or one of the Extension specialists in determining the type of information to collect until the investigator arrives. If the investigation indicated that a pesticide may be responsible for the losses, all adjacent landowners and applicators who may have been involved should be notified.
3. Advise all parties of the location where losses are occurring. If the fish kill occurs in commercial fish ponds, report the number, size and kind of fish stocked in each pond and the number of fish lost in each pond.
4. Make sure that official samples of fish and water for analysis are collected from each body of water involved AS SOON AS POSSIBLE. These samples should be taken by Louisiana Department of Agriculture and Forestry personnel or their designated representatives.
5. Other information that should be noted and given to all parties involved includes: 

               a.  Time of day the kill first started
               b.  Kinds and sizes of fish present, if known, and 
                    kinds and sizes of fish dying
               c.  An estimate of the number of fish killed
               d.  For fish production ponds, the number of fish in the
                    pond, when stocked, amount of food being fed and
                    type of food
               e.  Occurrence of fish kills in adjacent ponds, rivers or 
                    bayous and location in relation to water bodies 
                    where no kill has occurred
               f.   Location of farm lands in relation to the affected
                    area(s) and the type of crop(s) being grown
               g.  Location of any pesticide spraying, either by,
                    ground or air, in the area, and the type of pesticide
                    being used.
               h.  Identification number of any spray planes in the 
                    area or ferrying over affected ponds
               i.   Wind speed and direction

III.      Adjacent landowners and aerial or ground applicators who are advised that their operators may have been responsible for fish losses should take the following steps:

1. Verify by an on-site visit, as soon as possible, that a fish kill has occurred or is occurring
2. Cooperate as fully as possible with all parties in determining the cause of the fish losses

IV.      It is the responsibility of all landowners, aerial applicators and operators of ground rigs to ensure that there is no direct application of drift of material (regardless of what kind) into any pond.

Remember, many agricultural chemicals break down quickly, leaving little trace. Rapid response is critical in proving the cause of fish kills when there is reason to believe toxic substances are involved.