by Kristin Elliott, Aquasend, USA


Each year as spring approaches, it is important to be prepared for springtime diseases and dissolved oxygen loss on your aqua farm. That said, if you do see a few dead fish floating in your ponds, there is no cause to panic.

Fish deaths are the result of many different causes including changes in temperature and dissolved oxygen levels, stress, pollution, disease, toxic substances, and more. However, there are a few specific factors that you should keep your eye on come spring.

Specifically, abrupt temperature changes, decreased dissolved oxygen levels, and bacteria are the most prevalent factors leading to springtime fish deaths. Luckily, they can often be prevented or alleviated with a little preparation.

Why your fish are dying

Springtime fish kills are unfortunately fairly common and can be the result of several different challenges that fish can face coming out of wintertime. In many climates, spring can mean extreme weather conditions or sudden temperature increases, both of which are detrimental to the well-being of your fish.

These conditions can contribute to low dissolved oxygen levels along with additional sources of stress that make the fish more vulnerable to diseases.

Fish have decreased appetites during the winter. Their metabolisms naturally slow down, so they don't need to eat as much to stay healthy when there are fewer nutrients readily available due to a reduced number of important compounds being produced by phytoplankton and other microorganisms.

During this time, fish immune systems are suppressed, which is detrimental when temperatures start to increase. In springtime, when temperatures begin to rise again, bacteria that are present in fishponds, which are able to recover from winter hardships much faster than fish can, will attack the vulnerable fish, resulting in diseases and potentially mass fish deaths.

Another factor in disease susceptibility is stress. Stress in fish, like humans, can result in lower immune system functions. One major stress that occurs for fish in the springtime is reproduction.

Spring is the spawning season for most fish species. In order for fish to successfully reproduce after a harrowing winter, their bodies produce a hormone called cortisol, a stress management hormone that also exists in human bodies. While it alleviates the stress from winter, cortisol also suppresses the immune system.

Many aqua farms may also experience partial fish die-offs during the spring due to low dissolved oxygen levels. Algae and phytoplankton, the main oxygen suppliers in most ponds, produce oxygen through the consumption of sunlight.

Cloudy weather conditions, and ice covers in certain climates, limit the amount of sunlight that the phytoplankton are able to consume which will also decrease dissolved oxygen production.

When pond water temperature increases too quickly, cold-water algae often die off suddenly and are then consumed by bacteria that feed on the dissolved oxygen and multiply rapidly, which in turn causes lower amounts of available dissolved oxygen for the fish in the pond for several days.

The aforementioned contributors to springtime fish kills are able to be visually detected in some cases, if you are paying attention. However, taking preventative measures and constant monitoring of your pond's water quality are the best ways to prevent or limit the impact of fish kills.

If you do see a concerning number of dead fish in your pond, ask yourself the following questions to determine the potential cause of fish deaths on your aqua farm:

Only the larger or smaller fish are dying

If big fish are dying but small fish are not, the problem is probably low dissolved oxygen. Another sign of low dissolved oxygen is an unusual number of fish gulping near the water's surface or pond edges and slow movements.

The upper layers of the pond tend to have a greater dissolved oxygen content than deeper waters, so if you see a large number of fish spending a lot of time near the surface, or near a waterfall or other source of new water, you should check on the dissolved oxygen levels.

If smaller fish die before larger ones of the same species, there could be a toxic substance such as livestock waste runoff, pesticides, pollution, etc. involved. Small fish are more vulnerable to toxins than larger fish.

All fish are dying regardless of size

In this case, you are probably dealing with a disease or parasite outbreak. Fish can contract bacterial, viral and parasitic diseases, just like any terrestrial animal. However, there are only a handful of drugs and vaccines available to treat fish diseases because of environmental concerns.

Sanitising your farm equipment and maintaining high water quality in your ponds are the best ways of minimising risk of disease. Signs of an outbreak include unusual discoloration or sores, erratic swimming, bulging eyes, external parasites clinging to their bodies and a singular species experiencing the symptoms.

Once you know the cause, or have an educated guess, as to why your fish are dying you can easily and quickly find a solution.

How to prevent fish deaths

Although knowing what to do when one of the aforementioned issues occurs is important, putting preventative measures in place will ultimately save you time and money. If you aren't sure what to do, don't panic! Here are a couple of ways you can prevent fish deaths and keep your fish happy and healthy in the springtime:

Don't over-fertilise your ponds

An excessive amount of fertiliser can be toxic and potentially fatal for fish. Nitrogen and phosphorus are the two most important minerals for promoting phytoplankton growth, which in turn promotes fish growth.

However, if there is an excessive quantity of phytoplankton (which can be determined by the turbidity level of the pond) there may not be enough available dissolved oxygen or other nutrients for the fish. There are several safe, effective ways to fertilise your pond depending on the size or your pond and the type of fertiliser you plan on using.

Avoid using excessive herbicide on or near your ponds

Herbicide, while helpful for killing off invasive plants, can go too far if used incorrectly. This can result in aquatic plants that normally produce oxygen for the fish dying off. One best practice is to treat no more than one third of your pond with herbicide at a time.

Don't overstock your fish

Overstocking creates resource scarcity; if more fish are fighting for the same oxygen and food, more will be unable to intake a healthy amount. The ideal pond size to fish ratio depends on the species and size of fish you are raising. However, a general guideline is 6-10 gallons of water per every one pound of fish.

The largest risk of disease in fish, aside from suppressed immune systems from abrupt temperature shifts, is contracting a bacteria or parasite from another nearby fish, so you should only stock healthy fish and separate sick fish from your main stock.

Your fish feed supplier may be able to supply medicated fish feed that is designed to treat bacterial disease, which can also be useful in preventing bacterial disease outbreaks.

You should also make sure that your ponds are not at risk of receiving waste runoff from any livestock, crop fertiliser, pesticide or any other organics.

Use fish farm technology to your advantage

Making use of existing fish farm technology is a further method of avoiding mass die-offs. By installing real-time monitoring devices in your ponds, you will be notified immediately if a drop or change in temperature or dissolved oxygen levels is detected in your pond. This will help you to avoid potential fish kills from suffocation.

Considering an aerator for your farm is also good practice, this is because aerators create bubbles under the surface of the pond which allows more oxygen to be circulated and dissolved.

There are different types of aerators that are best for specific pond sizes; aerated well water systems (splashed or sprayed) are most effective for smaller ponds, for example.

Using some, or all, of the above suggestions, you can preemptively prepare your aqua farm to have the lowest risk of a spring kill. Taking preventative measures before you start experiencing fish deaths is the best way to prepare for springtime.

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