As a general rule, small ponds intensively managed for catfish are the most susceptible to die-off problems. Other common scenarios for summer die-off problems are ponds with large quantities of aquatic vegetation, ponds that are heavily or frequently fed with commercial fish diets, ponds that were stocked heavily or excessively and biomass now exceeds carrying capacity, or ponds that experience phytoplankton die-offs caused by a multitude of different reasons. How do you determine the cause of a fish die-off?
In most cases, asking the right questions will lead you to the cause or causes. Here are the questions I ask and the assessments made based on answers received to help a frantic pond owner:
1) When did the fish start dying and for how long have they been dying? The reason for this question is to determine if there is acute (very rapid) or chronic (slow and prolonged) mortality. The rate of fish mortality helps provide clues as to the cause. Oxygen depletions are typically acute mortality events in which the fish die quickly, within a few to several hours, and then the mortality ends. Chronic mortality spanning several days or even weeks is typically associated with disease or parasite issues where portions of the fish population die over prolonged periods. Exposure to lethal concentrations of pesticides or herbicides can cause either acute or chronic mortality, dependent upon the dose of the chemical the fish were exposed to, although mortality tends to be more acute as toxic pesticides tend to dilute and degrade quickly in the aquatic environment by simple dilution, oxidation, microbial deterioration, or UV exposure.
2) How many fish have died and what size are they? Agents often receive calls about a ‘fish kill’ in a pond, and after a lengthy discussion, discover that it was a single or a few large fish or perhaps 3-15 small fish. A few dead fish does not equate to a fish kill. Fish kills result when a major chemical or environmental event has occurred in a pond, that results in mass mortalities (10% or more of the entire fish population) of a single or multiple species.
a. Large fish – It is disheartening to lose a 5-10 pound largemouth bass or a 8-15 pound channel catfish, but believe it or not, some large fish do simply die of natural causes or old age in small impoundments. Larger fish tend to experience mortality during or immediately after the extremely stressful and body depleting spawning season. This means during the spring for largemouth bass and the late spring to mid-summer for channel catfish, it is not unusual to see a dead large fish or two every couple of days during or immediately after the spawning season. Large fish that are past their prime are also more vulnerable during the hot summer months, when their large body size and deteriorating body make them more vulnerable to low dissolved oxygen that may not be low enough to cause mortality in other fish in the pond.
b. Small fish – Small quantities (3-12 or more) of dead small fish, such as largemouth bass fry or fingerlings, small bluegill, redear sunfish, or other baitfish, often trigger pond owners to call about fish kills. Be sure to ask about environmental conditions prior to the discovery of dead fish, such as prolonged cloud cover and/or heavy rains, rapid or prolonged severe temperature changes, and shallow habitat cover (areas of heavy aquatic vegetation). Small fish are more susceptible to localized, rapidly changing environmental conditions than larger fish. The reason for this is that they are less mobile spatially in the pond environment and more restricted to shallow shoreline habitat. Their small size does not allow them to quickly navigate large distance, and typically restricts them to shallow or heavy cover environs, or else they become food for predatory species. Localized mortality of small fish tends to occur due to temperature or pH shock after a heavy rain event, rapid temperature changes in the shallows, or localized oxygen depletions, especially in areas of heavy aquatic vegetation. Heavy rains can cause large amounts of runoff that differs in pH and other chemical properties or temperature. The effects of these chemical changes are greatest in shallow shoreline environments where small fish cannot seek the deep or open water environments available to large fish. Rapid air temperature changes such as sudden and prolonged cold fronts or extreme heat spells also affect shoreline environments as shallow water is subject to the most extreme temperature change due to the interface between surface area and water depth. Small fish cannot escape to the more stable deep water environs that larger fish can. Small fish are also often associated with heavy cover for protection, and the foremost cover in ponds is aquatic vegetation. Oxygen concentrations are not always uniform in a pond, and areas of heavy aquatic vegetation can produce localized oxygen depletions at night in heavily vegetated areas of a pond. Small fish require less total oxygen than large fish, but they are much more sensitive to oxygen depletions which can result in localized mortalities.
3) How many different species are dying? What you are trying to determine with this question is that if more than one species of fish are dying, you probably are faced with a water quality problem (i.e., oxygen, ammonia, nitrites). If only one species of several species present are dying, it may be a disease/parasite problem, but not always because different species may have different tolerance levels for water quality parameters. If only one species of fish is in the pond and a die-off is occurring, you need more information.
4) Have any pesticides or herbicides been used recently that were introduced into the pond? This could include pasture insecticides washing into ponds or even cattle that were treated with an insecticide standing in the pond to escape the summer heat. Determine if any herbicides, aquatic or terrestrial, have been recently (within 4 days) applied within 250 feet of the pond or has there been any herbicides applied between 250 and 500 feet of the pond prior to (within 7 days) a rain event. Many terrestrial herbicide formulations are toxic to aquatic life including fish. Even non-toxic or low toxicity herbicides washing into a pond may lead to untimely vegetation die-offs. If herbicide is suspected in a fish die-off, be sure to inquire about aquatic vegetation die-offs as well to determine if oxygen depletion from plant decomposition is the culprit.
5) How big is the pond? The pond owner usually thinks the pond is 2x to 3x larger than it really is! Walk them through the process of estimating surface area in acres by dividing square footage by 43,560. Many pond owners find it difficult to estimate area in acreage or square footage, but satellite imagery, on-line tools, and GIS programs, such as Google Earth™, can be used to easily and accurately determine the square footage of a pond remotely by either the owner or Extension agent using the distance, mapping, or area tools. Remember, excessive depth does not make up for lack of surface area when it comes to fish production! Excessive water depth itself can periodically be the cause of fish die-offs in the spring or fall due to rapid destratification or turnover events leading to mixing of low dissolved oxygen waters. However, these turnover events require specific weather conditions, such as heavy cold rain or major cold front, in order to occur.
6) How many pounds of fish are present? Once you know the surface acreage, try to determine the pounds of fish present by asking: (1) for an estimate of fish stocked (minus those caught out) and (2) the average weight of the fish present. Be sure to ask if the owner notices or has caught any small fish and the number of those small fish in order to include an estimation of fish weight for offspring if the pond is more than a few years past stocking. This will help you estimate the total poundage of fish present.
STOP RIGHT HERE — if the total estimated pounds of fish exceed 1,000 pounds per surface acre (that’s only 100 pounds in a 0.10 acre pond), you are probably dealing with oxygen depletion. This accounts for about 85% of all fish die-offs in Texas farm ponds.
