basic needs of cultured fish in recirculating systems

Module 4 Introduction


In this module, you will complete readings and exercises that will allow you to understand the basic needs of cultured fish in recirculating systems, outline the nitrogen cycle, and explain its importance to basic aquaponics function. The fish farming component of the aquaponics system is important. Essentially, you need to provide an environment that is sustainable and non-threatening to the well-being of your fish. If you think about natural ponds, rivers, and lakes, Mother Nature provides filters and waste removal naturally. You need to mimic these same processes for your fish to thrive. At the end of this module, you will write a quiz.

Learning Outcomes

Upon completion of this module, you should be able to

  1. Identify various types of fish options for aquaponics
  2. Explain the relationship between fish feed, feeding, and growth
  3. Recognize the physical environment and habitat requirements of fish
  4. Analyze the effects of components of fish culture in raising fish in recirculation aquaculture systems (RAS)
  5. Recognize normal, species-specific fish behaviour
  6. Differentiate normal fish behaviour from abnormal, concerning behaviour
  7. Plan feed requirements and estimate fish growth rate

Activity: Fish Behaviour

Before you dive into the reading for this module, take a few minutes to review the following videos. As you watch them, pay attention to the behaviour of the fish. Take notes about your observations.

  1. What do you see the fish doing?
  2. What do you notice about the specific fishes’ behaviour?
  3. How do they feed?
  4. How do they interact? 
  5. How do they move (e.g., fast, slow, straight lines, up, down, diagonal)?
  6. Predict why they might be acting the way they are. Are they distressed or diseased perhaps?

Please save all your notes as you will return to them later in this module.

The Role of Fish in Aquaponics

A fish rearing tank in a DFT system at the Lethbridge College Aquaculture Centre
of Excellence.

Water Temperature and Growth

The fish tank requires oxygen replenishment and waste removal. The density in your aquaponics fish tank is likely going to be much higher than a natural pond, stream, or lake. As such, waste removal is of critical importance to the health of your fish. The water quality must be continually monitored and maintained, especially initially until you begin to understand your system and how it behaves. Your fish should be healthy and grow well if you create the proper environment for them. Maintaining the proper temperature for your fish is also critically important.

Fish are cold blooded, so their growth and longevity are governed by metabolic rate. The higher the water temperature the more food fish require, the faster they grow and the more oxygen they consume. Water temperature influences fish in many ways:

  • Growth rate
  • Longevity
  • Age of sexual maturity
  • Feeding rate
  • Feed conversion
  • Oxygen consumption and incidence of disease

If you look at the following graph, it lays out the simple aspect of how temperature impacts any species of fish, with an optimal range of temperatures for the particular fish species, temperatures outside of this range can cause stress or death. Know the optimal temperature range for your particular fish species and stay within it if you want to keep your fish growing, healthy.

In this next figure, you can see how the increase in temperature affects weight gain (growth). Basically at lower temperatures, metabolism is low and fish grow slowly. If you increase the water temperature they are able to grow faster, up until a certain point. In this particular example, this point occurs at around 20°C and represents the stress range for the fish. Fish lose weight because energy is spent dealing with the stress. What happens if we continue to increase the temperature? The line is a short one and represents fish mortality. This example is accurate for Rainbow Trout.

This cold versus warm water figure represents two different species of fish and associates the level of feeding over the range of temperature tolerance for each species. As you will notice, the feeding rate increases over the optimal range of temperature and declines outside of this. Practically, this translates to lowering or increasing your feeding rate respectively if temperature in your system fluctuates.

Feeding Versus Temperature

Increased water temperature leads to increased body temperature, which results in increased metabolism. This in turn leads to increased food consumption and increased oxygen consumption. The final result is increased growth.

Growth Characteristics of Fish

  • Continuous growth
  • Feed conversion ratio (FCR) – the amount of feed fed vs. weight gained (should be 1.5 or lower, the lower the better)
  • Weight gain decreases as fish ages because growth slows
  • Young fish have a much higher metabolism and growth rate and are fed correspondingly

In the following graph, two points are illustrated with respect to fish growth:

  1. Fish growth is continuous over its life span, as a fish never stops growing (indeterminate growth).
  2. Fish growth is not uniform, being rapid early in life and slowing with age.

Fish go through three growth phases:

  1. In the accelerating phase, there is a large increase in percent body weight per day.
  • Feed 25% bw/day
  1. The linear phase. There is a reduced feed rate and weight gain and feed rate might be reduced to 3-5% bw/day.
  2. In the plateau phase, growth has slowed even further. The onset of sexual maturity can further divert energy into gamete production.

If the plateau phase is initiated prior to the fish achieving market size, then you will see reduced growth efficiency. Poor fish genetics and poor general fish care can greatly reduce fish growth and increase economic loss. As the figure below demonstrates, good conditions of culture, or selecting all male tilapia can enhance fish growth and reduce time to market.

Feeding Fish: Energy and Nutrition

Before explaining why fish feed is important in aquaponics, it is imperative to start with this fundamental statement: How much produce you can grow will depend on how much feed you provide your fish per day, not how many fish you have. Essentially, you will work backwards, from how much grow area you have, what kind of plants you want to grow, to how much feed you need to provide adequate nutrients for that grow area and then figure how many fish that feed can support.

It is important to choose the proper feed for the fish you are raising. Providing a complete diet will make a big difference in not only fish growth and health, but in the water chemistry of your aquaponics system. It is important to remember that the type of fish diet you choose and the amount of it you decide to feed not only feeds your fish, but impacts your water chemistry and provides the nutrients from which your plants will grow. For instance, even though tilapia are omnivores and can eat almost anything, feeding them a balanced, high quality commercial diet is crucially important as it will provide the best nutrition for fish health maintenance and growth, and also maintain consistent, high quality nutrients. If you decide to change the diet, be prepared that your water quality and conductivity (nutrient concentration) may also change. Providing a cheap food for your fish may seem like it is cost-saving, but it will cause problems for you in aquaponics if you are not careful.

Here is an example of how giving your fish a cheap, readily available, alternate feed source can cause problems. Consider that the characteristics of a good diet include palatability (how much the fish like the taste), digestibility (how well the fish are able to break down and absorb the diet), integrity (how well the feed stays together in water) and nutrient content (how it provides the right balance of what fish need). When you provide a low quality diet, a number of things can happen:

  1. The fish may not eat it
  2. The fish may like it, but it is of low quality and not readily digested
  3. They may like it, it can be digested, but it does not provide the right balance of nutrients for proper growth

These issues, or any combination of issues, will result in some or all of the following:

  • Increased solids (TSS) in your aquaponics system
  • Reduced fish health and growth
  • Reduced feed conversion
  • Difficulties in balancing plant nutrients in your system
  • Inefficient bio-filtration
  • Erratic water chemistry
  • Reduced oxygen

Sticking to a reliable and reputable commercial diet can save you a lot of headaches and tribulations.

Feeding Fish, continued

Having said this, there is one aspect of a good commercial diet that aquaponics producers should be more aware of than your typical fish farmer. Because aquaponics is an ecosystem and we need to be concerned at all times about more than just the fish, what goes into the feed—the feed ingredients—needs to be scrutinized. At the Aquaculture Centre for Excellence at Lethbridge College, the researchers have had a number of incidents where a feed manufacturer had substituted its protein source in the fish diet with a poultry by-product meal. The concerns and problems caused by this are that poultry are treated with a number of antibiotics during production and the residual antibiotics in this dietary component is then introduced into the aquaponics system, causing problems in the maintenance of the system’s microbiology, which directly impacts the water quality, nutrient availability, and plant production. You have spent a long time developing the microbial culture of your system, you want to protect it!

The feed should provide the energy needed for normal functioning, activity, growth, and depending on your situation, potentially even reproduction. Energy is the ability to do work (calories). Obviously, a fish’s activity level increases its energy use. Maintenance energy must be satisfied before growth is achieved, and in fish, 70% of dietary energy is used for maintenance (some wasted as feces, urine). If we look at energy requirements in general, carnivorous fish require more energy than omnivorous fish, who require more energy than herbivorous fish. Younger fish also require more energy than older fish. Energy in the diet is provided by fats (fatty acids), proteins (amino acids), and carbohydrates (glucose). Protein is one of the most expensive components of the diet and so far fish meal provides the best balance of essential amino acids for fish. Protein requirements are influenced by species, age, size, water quality, daily feed allowance, and protein quality.

The size of the diet should also be proper for the size of the fish, and for aquaponics. This will typically be a floating (extruded) feed type. A floating diet allows you to better visualize the behaviour of fish feeding and determine if you are feeding enough, too much, or if a larger problem may be looming.

Proper Nutrition

  • High protein (25-50%)
  • Fat (5-15%)
  • Low carbohydrate
  • Minerals and vitamins
  • OneSize and Type of Feed
  • Feeding Practices
  • Feeding Rate and Frequency
  • Diet size = 1/40 the length of the fish
  • Wet, semi-moist, dry, compressed and extruded types
  • Carnivorous versus herbivorous fish diets

Other Useful Calculations

Other calculations that may become useful when planning your aquaponics fish production would be:

  1. How fast will my fish grow?

    Depending on your fish feed conversion ratio (FCR), which is something you have to determine through experience, your fish will grow at predicable rates if healthy and happy.

    You have 100 Tilapia, with a total weight of 7.8 kg. Fish have an FCR of 1.3 and you need to know if they will be of market size by a certain date. Practically this means: (1) weighing a sample of your fish to get average weight/fish (W0); (2) keeping track of how much feed you give your fish over time (Ft); and (3) re-weighing a sample of your fish to assess the average weight gain (W1). FCR = avg fish weight gained / avg amount of feed per fish fed, or FCR = (W1– W0) /(Ft / number of fish).
  2. How much food to order and how long it will last?

    You can predict this by using the tables above as a guide, taking in account the number of fish, the fish avg weight, species, water temperature and FCR. You can sample your fish periodically (monthly) to assess growth rate and average size to tailor your feed rate. Basically the feed used per month will equal the total weight of your fish x the daily feed rate as a decimal (i.e., 3% = 0.03) multiplied by the number of days, or Total Feed (kg) = Total Fish Weight (kg) x Feed Rate x Number of Days

Storing Food

Feed is best when stored for no longer than six months. When food is spoiled, fish experience lower growth rates, illness, or death. Food should be stored in a manner that keeps it cool, dry, and pest free, usually a freezer.


Raising plants and fish together is complex and can be challenging. According to Rakocy et al. (2006):

To recover the high capital cost and operating expenses of aquaponic systems and earn a profit, both the fish-rearing and the hydroponic vegetable components must be operated continuously near maximum production capacity. The maximum biomass of fish a system can support without restricting fish growth is called the critical standing crop. Operating a system near its critical standing crop uses space efficiently, maximizes production and reduces variation in the daily feed input to the system, an important factor in sizing the hydroponic component. There are three stocking methods that can maintain fish biomass near the critical standing crop: sequential rearing, stock splitting and multiple rearing units. (p. 2)

In sequential rearing, the fish rearing tank will contain fish of several age groups. With this method, when one cohort of fish is matured and at marketable size, it is removed and replaced with an equal amount of fingerlings. This method has three key issues:

  • The harvest can stress the remaining fish, which may trigger disease
  • Fish that have stunted growth are not captured. They stay in the system and waste space and feed
  • There is management uncertainty and unpredictable harvests; it is challenging to keep accurate stock records

Stock splitting “involves stocking very high densities of fingerlings and periodically splitting the population in half as the critical standing crop of the rearing tank is reached. This method avoids the carryover problem of stunted fish and improves stock inventory” (Rakocy et al., 2006, p. 3). There are problems with this method as well, particularly the stress moves can cause for the fish. This can be limited with the addition of a “swimway.” A swimway connects the rearing tanks using a hatch and movable screens. The fish are maneuvered from tank to tank. Dividing the fish population this way can be challenging however because the fish cannot be weighed or counted. Fish can also be moved from tank to tank by way of a fish pump and screens. 

Operating multiple rearing units involves moving an entire fish population to a larger rearing tank “when the critical crop of the initial rearing tank is reached” (Rakocy et al., 2006, p. 3). The fish can be herded from tank to tank via swimways. This system may be designed similar to a raceway with movable screens whereby as the fish grow, their compartment grows. As they move closer to the end of the raceway, they approach harvest size. A variation of this system is to use multiple tanks of the same size. Each tank will contain fish that are of the same age group. Various age groups exist across the tanks. This is not an efficient use of space, but it does mean the fish are not disturbed/moved.

Maintaining your system at maximum stocking densities does have its drawbacks. It can lead to reductions in growth rate, survival rate and feed conversion. If you have never raised fish before, the risk is higher than the potential reward, because unlike aquaculture, if you fail with your fish in aquaponics, your plant success will also follow suit. Though from an economics standpoint it might make sense to increase your fish stocking, we have found through our research that the potential plant growth coming from current published numbers (57 g of feed/m2 of plant growing area/day (Rakocy, 1989)) can be greatly reduced (e.g., 6–10 g feed per square meter grow bed per day). You may be able to grow all the plants you need with much less feed (fewer fish), essentially improving your economics and also reducing your risk to fish health.

Required Reading Masser, M. P., Rakocy, J., & Losordo, T. M. (1999). systems (Links to an external site.) On pages 6 to 8 of this document, read the section titled ” Fish production management.” The subsections included within are “Stocking,” “Feeding,” and “Feeding Skills.”

Fish Options for Aquaponics

Working with fish and plants together can be challenging and choosing which fish to work with is an important consideration. Identifying your market is one of the first deciding factors in choosing which fish species to culture. The markets can be for live food fish, processed fresh, frozen or value added, or maybe even for stocking, but you need to investigate this prior to deciding what fish to grow. Having a reliable source for your fish is also a consideration, which can be impacted by season and especially legal restrictions based on where you are located. If the fish you are considering is an exotic variety and capable of establishing in local waterbodies, you will likely contend with more costs, regulatory restrictions, and permitting.

Fish management begins before the chosen fish are introduced to the aquaponics system. Fingerlings (young fish) should be purchased from reputable producers—individuals who have knowledge of handling, transporting, and disease problems for the selected species. When an aquaponics system is started with poor quality fish, the chance of failure greatly increases. You will read more about fish health and behaviour later in this module, but for now let’s start with some common fish types used for aquaponics. 

There are many fish options for aquaponics: 

  • Tilapia
  • Catfish
  • Silver Perch
  • Jade Perch
  • Murray Cod
  • Barramundi
  • Rainbow Trout
  • Bluegill
  • Goldfish
  • Koi
  • Siamese Fighting Fish (Betta)
  • White Cloud Mountain Minnow

Fish Behaviour


As with humans, normal fish behaviour is strongly influenced by stressors or stressful situations and can be seen in responding behaviour. Fish stress management is a key component of effective fish rearing. Fish can become stressed from a variety of things, such as

  • Bright lighting
  • Loud noises or banging
  • Temperature changes
  • Water quality changes (e.g., tilapia will come to the surface for oxygen at 1 mg/liter DO)
  • Handling
  • Feeding
  • Infighting

Like humans, when fish are stressed, they have a much higher chance of becoming sick and contracting a disease. When fish are being kept in high densities, fish loss can be quick and extensive if problems are not detected and resolved quickly. A common bacterial disease known as bacterial gill disease can cause 80% mortality within 3 days of onset—scary to say the least. So the best chance you can have at helping your fish is going to come from your ability to detect a change in their behaviour before they start floating to the surface or sinking on the bottom. Being attentive to how the fish behave when you feed them is very important, as most often when fish are beginning to fight the first signs of an infection or disease they experience a reduced appetite. If you experience this, do not continue to push feed on the fish, as doing so makes the situation worse. It’s similar to feeding someone who has no appetite because they have contracted the flu; forcing them to eat would surely cause unfavorable results as well.

Required Reading In the following document, read the charts titled “Behavioral alterations observed in fish associated with different stress agents” and “Individual and group endpoints for movement analysis.” on pages 15–21. Kane, A. S., Salierno, J. D., & Brewer, S. K. (2005). Fish models in behavioral toxicology: Automated techniques, updates and perspectives

In the behavioral alterations table, you can see the general fish behaviors displayed when fish are exposed to specific disease causing organisms and even certain water quality issues. Do you see how many of them list inappetance or lethargy? You see how important it is for you to be cognizant of your fish’s behaviour during feeding. Impaired balance, reduced social interaction and pigmentation changes are also good things to look for.

The list of terms on page 21 “Methods in Aquatic Toxicology” of the reading provides you different ways of describing fish behaviors, basically what to look for in general. As an example, you may look at a tank of fish and your description of them might initially be rather rudimentary (e.g., They are sitting in the tank together and breathing, but not moving). This description does not provide any insight into where in the tank they are, if they are breathing normally, or anything about their orientation and physical appearance. 

What makes this more complex is that not all fish species behave the same, and what is considered normal for one fish species might be a behavior of concern for another. Using the example of a fish gulping air at the surface, a behavior referred to as piping, if you are raising goldfish this would constitute normal behavior, but in rainbow trout it would be a behavior of concern. The reason? Goldfish are able to utilize oxygen from the esophagus and so gulping surface air provides them oxygen, rainbow trout on the other hand do not have this capability and doing so actually collapses the gills making it more difficult to breath. Trout and many other fish display this piping behavior when oxygen concentrations in the water are too low and they are suffocating. Recognizing this behaviour can alert you early to alleviate or diagnose the problem before you actually have fish mortality. Also, it is important to know the behaviour and physical limitations of the species of fish you choose to use in your aquaponics system. Here is a list of other general fish behaviors to keep alert for when monitoring, feeding and caring for your fish.

According to Masser et al. (1999), signs fish may be stressed or afflicted with a disease include

  • Excitability or lethargy
  • Flashing or whirling
  • Skin or fin sores or discoloration
  • Staying at the surface
  • Erratic swimming
  • Reduction in feeding rate
  • Gulping at the surface – piping
  • Cessation of feeding

Problems with the Fish

Before we start talking about what can go wrong with your fish, let’s discuss some big steps you can take to avoid your fish getting sick.

  1. Handle your fish as little as possible. That mucous coating on your fish is a protective barrier to the risk of infection, so any handling with a net or hands will compromise that coat and make your fish more susceptible to getting sick.
  2. Avoid sudden changes in water quality. Many fish contract disease after they experience a sudden change in temperature or pH. When receiving fish into your system, make sure that the water the fish are coming in and the water in which they are going into are very close in temperature and pH. Slowly add your aquaponics system water into the bag or container of water the fish are in and acclimate the fish over time. One degree C and 0.5 pH per hour would be a safe acclimation rate as long as the water differences in these two parameters is not too great.
  3. When feeding fish, do not over feed. If fish are feeding great, but if they are not eating everything you give them, then cut back your feed rate. If they are feeding lethargically or appear uninterested, then stop feeding them. This is often a common sign that your fish are fighting something.
  4. Reduce any stress to your fish. Shade them from intense or direct light, avoid loud bangs and noises that can startle them, do not overcrowd them, keep fish of similar sizes together to avoid bullying, and maintain good water quality.
  5. Be picky about where you get your fish and water from. No matter how well you maintain your aquaponics system, if you bring in diseased or sick fish, or water of poor quality that is possibly contaminated, you risk your entire system getting sick.
Required Reading Masser, M. P., Rakocy, J., & Losordo, T. M. (1999).
  • On pages 8 to 12 of this document, read the section titled “Off-flavor” and “Diseases.”
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