If your fish are dying, you should look for signs of attack on the carcasses. Predators such as cats and dogs can attack your pond fish, pulling them out of the water and leaving them to die. These animals may have slashed the fish’s beak or claws or stabbed them. If you notice any of these signs, you should consult a fish-keeping professional.
Excessive Vegetation
Is excessive vegetation causing fish in my pond to die? In many cases, it is. Plants in ponds provide a variety of benefits to fish and pond ecosystems. These plants provide food and refuge for fish and are necessary for preventing wave erosion. Here are some tips to help you identify and treat invasive aquatic plants.
Vegetation may be a major cause of algae die-off in a pond. If you have a shallow pond with lots of aquatic plants, make sure to limit the amount of vegetation. Aim to have 60 to 80% of the pond covered with vegetation. It is not a problem to have some vegetation, but more than that may cause an unhealthy environment for fish.
Vegetation in a pond should be treated slowly. Too much treatment can create an overabundance of decomposed plant material that is a breeding ground for bacteria. These bacteria can deplete the oxygen in water very easily. Herbicide treatment should only be used on a third of the vegetation. Wait for at least two weeks between treatments. Also, if you have livestock pastured nearby, try fencing off the area.
Ammonia Buildup
Ammonia is a naturally occurring chemical in your pond. It is created from the breakdown of waste products such as plants, fish food, and excrement. The good news is that the bacteria in your pond should naturally break it down. This is known as the nitrogen cycle. It will help the remaining components in your pond break down the ammonia, and prevent it from becoming toxic.
If you suspect that your pond is becoming overly sanitized, you should do an ammonia test. Ammonia is toxic to fish, and you should never mix it with another liquid. However, it is necessary to know your pond’s pH level to determine the exact level of ammonia. The pH level should be above 7.0, but if it is higher, you should try adding some nitrates.
Ammonia buildup is a serious problem. When the temperature of your pond falls below 55 degrees, your fish will go into a semi-hibernation phase, and they won’t be able to digest leftover food. The result is that their waste sinks to the bottom of the pond. The water will become saline and will contain ammonia. Fortunately, the good bacteria in your pond will break down the ammonia to nitrites and nitrates, which are both harmful to fish.
Low Dissolved Oxygen
If you’ve noticed that the fish in your pond are gasping for air in the morning, it might be because the dissolved oxygen levels in your pond are too low. Fish do not operate photosynthesis at night, so the dissolved oxygen in the water is low during those times. If you notice your fish are not eating pellet feed, your pond is probably suffering from low dissolved oxygen.
When the dissolved oxygen level drops below the normal level, plankton blooms are most likely to respond. When there is a sudden change in temperature, oxygen production decreases. Aerobic bacteria and fungi consume the remaining dissolved oxygen, leaving little room for the live phytoplankton to mix and produce more oxygen. Observe the bloom density using Secchi disks. If the blooms are so dense that they block visibility for a minimum of 12 inches, then the dissolved oxygen levels in the pond may be too low.
Some ponds experience stratification, whereby warm water near the surface is less dense than the cooler water at the bottom. This stratification causes the water to become stagnant and depleted of oxygen. As the condition worsens, the top layer of water mixes with oxygen-deficient water, killing fish. The worst part of all is that it can be deadly for your fish. If this happens to your pond, there are several simple solutions to combat the situation.
Parasites
One of the most common reasons for fish to die in ponds is a lack of oxygen. Green water is often the culprit, and plant material and algae in the pond also deplete oxygen levels. Aerators and agitators are helpful in providing oxygen from the top to the bottom of the water column. To avoid the problem, clean your pond regularly and change the water filter at regular intervals.
Overtime, plants can die off and reduce oxygen levels. This happens when the plants on the bottom of the pond die off and there is no way to replace them. Oxygen levels drop sharply and fish begin to die. Overnight, fish kills can occur on calm cloudy days or in the middle of summer when ponds are most likely to be filled with water. As long as you do prevent the cause, your fish should remain healthy.
Phytoplankton
In addition to algae, a high phytoplankton level in a pond may also be a cause for the poor health of your fish. Phytoplankton, which are tiny organisms that live in water, are the base of the food chain, and the higher the phytoplankton, the higher the population of fish. Phytoplankton can become overly abundant in a pond due to the runoff of nutrients and fertilizers. The same conditions can cause extreme oxygen cycling and kill your fish.
Phytoplankton die-off is usually caused by nutrient-rich ponds, which produce dense blooms of microscopic algae. These blooms give water its deep green color. However, sudden phytoplankton die-offs can also occur after several days of cloudy, hot, windless conditions. This rapid die-off causes a rapid reduction in dissolved oxygen in the water, resulting in fish death.
Excessive amounts of nutrients in the water can also contribute to the bloom. In turn, this can cause fish deaths. A good way to minimize the negative effects of phytoplankton die-off is to cut off the fish from feeding. Fish that are less active may feed less, which contributes to the high concentration of ammonia and the excess protein in feces. You can also consider installing a pond aerator. Adding aerators to the water may help increase dissolved oxygen levels and reduce the risk of the fish kill. Moreover, the fish will also be happier and healthier with higher dissolved oxygen levels in their water, a factor that is not easily cured by the fish’s own immune system.
Phytoplankton Become Overabundant
Phytoplankton are plants that grow in pond water. The plants absorb nutrients and remove ammonia nitrogen from the water, a potentially toxic metabolite. Phytoplankton also has a number of indirect effects on the quality of the water. During high-level phytoplankton blooms, fish can become stressed or even die. To reduce this effect, it is best to reduce nutrient inputs from the watershed.
When plankton becomes overabundant, fish in the pond can die. The excess growth of plankton can deplete oxygen and cause fish to starve to death. Phytoplankton blooms may die off naturally on cloudy days or may even happen quickly during cold snaps. Dead algae cells can change the color of the water to a brick-red color. They also produce a foul odor. The dead algae cells will settle to the bottom of the pond, reducing oxygen levels and depriving fish of food.
When phytoplankton blooms occur, the pH of the water may increase unacceptably. Phytoplankton concentrations can cause large pH and NH3 concentrations in the pond. Additionally, an overabundance of phytoplankton can promote the growth of blue-green algae, a type of algae that many biologists call cyanobacteria. Since blue-green algae prefer a high pH level, they are often a majority of the algal community in aquaculture ponds.
Phytoplankton is Toxic
Although phytoplankton is beneficial to aquaculture production systems, if the populations are too high, phytoplankton can have negative effects. Many of these organisms are helpful because they filter excess nutrients, but others produce toxic byproducts. For example, some phytoplankton species have high concentrations of ammonia, a byproduct of protein breakdown. This compound can be toxic to fish, shrimp, and other aquacultured organisms.
During an algal bloom, the concentration of phytoplankton increases rapidly, making the water look red. This is because these organisms live only a few days and die when they do not have proper nutrient and oxygen levels. Consequently, successive blooms can look like a continuous population, but they are not. Phytoplankton can also kill fish.
Anabaena variabilis, a blue-green alga, was isolated from a pond containing channel catfish. The algae’s low light intensity resulted in the formation of gas vacuoles, which accumulated on the surface during calm, warm weather. When the pond was subjected to a light injury, the scum quickly accumulated, resulting in the mass death of phytoplankton. Once the dead algae began to decompose, dissolved oxygen was quickly depleted. Ammonia and pH levels increased and fish mortality was high.
