Feeding is Fundamental to a Successful Recirculating Aquaculture System (RAS)

By Chris Manley

A lot of factors come into play when operating a recirculating aquaculture system (RAS), but none are as important as feeding.

Not only is feeding fish stocks one the largest operating expenses when a RAS is up and running, but accurate feeding calculations are vital when planning a new system – even more so than with a flow-through aquaculture system.

What makes feeding in a recirculating aquaculture system unique? Simple: because it’s a closed-loop system, everything you put into it is going to stay there unless you take steps to remove it. Feed goes into one end of the fish and comes out the other as feces. In addition, increased activity during feeding elevates the respiration of the fish, increasing CO2 and reducing O2. All that is going to affect water quality, so you need to design the system to either remove those elements or convert them into something that won’t harm the fish.

Factoring in Feeding

When you think about a recirculating aquaculture system, it’s really processing the feed more than it’s processing fish. Feeding is the thing you need to get a handle on at the outset because it drives all the other calculations and determines your overall water quality and system performance.

For instance, we know that a kilogram of feed will generally:

  • Create 250 grams of solid waste
  • Consume one kilogram of oxygen
  • Create 1.2 kilograms of CO2

There are other factors that affect these rule-of-thumb numbers, such as the size of the fish and water temperature. But as long as we know the production goals for a system (aka the target biomass), the feed type and the feed rate, we can determine the proper design for the system to maintain optimal water quality.

There are additional elements to consider as well, such as the amount of protein in the feed, which influences ammonia levels in the water. Protein levels are usually related to the life stage of the fish you’re raising as younger fish require more protein for optimum growth.

So if you’re going to create a successful system for RAS fish farming, all of this needs to get factored in upfront when planning the system.

Best Feed for RAS Systems

The quality of fish feed in general has improved enormously over the last 10-15 years, with manufacturers developing new formulations that improve the stability of the feed, minimize fines and target formulations to particular species. In addition, many of them have created feeds specifically for RAS systems.

What should you look for in a RAS-friendly feed? First off, one that’s easy for fish to digest. That translates into less solid waste for your system to process.  

No matter what, however, there’s going to be some waste. So it’s important to use a feed that creates a stable feces that won’t leech ingredients into the water or break down into fine particles. That makes it easier it easier for the filters to remove the waste so it’s less taxing on mechanical components. 

You also want a feed with stable pellets that produce minimal oil residue, which can create a waxy film on equipment that will require more frequent cleaning maintenance.

The final factor to consider is the density of the pellet. In general, look for pellets that maximize time in the water column to extend the feeding window. This can be affected by water flow rates.  

While it’s true that feeds designed for RAS systems are more expensive than traditional feeds because of the reasons outlined above, over time they’re worth it because they optimize efficiency and growth rates and help minimize maintenance on equipment.

Aquaculture Feed Storage

One thing that’s often an afterthought when planning a RAS facility is including an area dedicated to feed storage. That’s a big mistake.

Fish feed is perishable, and if not stored properly it’s susceptible to bacteria and mold growth. In addition, oxidation can reduce the lipid and vitamin levels in the feed. Without a proper place to store feed, you run the risk of paying top dollar for a quality product and then having it deteriorate or spoil.

Better to incorporate a storage area that offers the ability to control temperature and humidity levels and provides adequate ventilation. You also need enough space to account for logistics issues – whether it’s extra room to store reserves in case of an emergency or a layout that lets you ensure you’re always using the oldest feed first. 

Feeding Schedules

Establishing a consistent feeding pattern is essential to optimizing production. If possible, a 24-hour feeding schedule with fish being fed around the clock at two-hour intervals is ideal. There are two reasons for this.

First off, it’s better for the system because it limits bursts of activity where the system has to work extra hard to process waste and maintain water quality. Regular feedings every few hours help “flatten the curve,” so there are fewer spikes in oxygen consumption, CO2 release and ammonia production. 

Secondly, it’s better for the fish. Fish learn quickly, and if they discover they’re only getting fed a few times a day, they’ll get aggressive at feeding time to make sure they get their fair share – or more. Short term that can lead to stress and injury. Long term it can lead to fish that are different sizes because not all of them are eating enough. So the more aggressive fish grow properly while the meeker fish don’t.

Feeding regularly throughout the day makes the fish more relaxed and allows them to reach satiation. One of the best way to do that cost effectively is with automated feeders, which do a better job at consistently distributing feed, reduce labor costs and track data that will help you better manage your feeding.

Want to learn more? Check out our webinar on RAS feeding strategies to learn everything you need to know about this topic. Or contact us today to find out how Innovasea’s experts can help with your RAS system.

About the Author

Chris Manley is a RAS designer at Innovasea, where his primary focus is on designing, modeling and managing recirculating aquaculture projects. He has spent more than a decade improving marine hatchery husbandry protocols and developing novel marine species to be successfully cultured in recirculating aquaculture systems. Chris holds a bachelor’s degree in marine science from Coastal Carolina University and a master’s in coastal sciences (specializing in aquaculture) from the University of Southern Mississippi Gulf Coast Research Laboratory.


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