Recirculating aquaculture system – does your choice of fish feed affect your plants?

Some background.

Our system comprises of Six 30x10m Tunnels of which Five are deep water culture (DWC) and One is a Nutrient Film Technique (NFT) system and no stone based methodologies.
We have 10 Dams in a separate fish house, each with different biomasses of Rainbow trout which averages 146kg per Dam or 14.6kg per 1000 L.

These dams are in pairs with a swirl filter, a mechanical filter and a Bio filter per pair.  The water percentage filtration vs dams is 63%, meaning that our dams are each 10 000 L usable space thus 20 000 L per set.  The filtration size per set is 12 600 L.

Each set basically feeds tunnels as follows: 

1) Dam one, two, three and four feeds tunnel Two and Three. This is a bit more intricate as dams three and four are also our quarantine dams.
2) Dams Five and Six feeds Tunnel One which is the NFT tunnel.
3) Dams seven, eight, nine and ten feeds tunnels four and five and also feeds tunnel six which is a decoupled tunnel.
4) All the tunnels are eventually interconnected via ONE sump from where the water is returned to the fish.

Aquaponics Operation

So where are we going with this?

Over the past few months we started testing some fish feeds and for the sake of this discussion we shall not divulge the manufactures, rather refer to them as Product A to C. 

For many years, or at least since our system has become a reasonably large system, we used Product A with confidence and, except for times when we had drastic problems with our fish like losing 1000kg to a parasite and had to treat the fish with antibiotics, we have never added any other supplements except for Iron. That, however, is a different discussion.

During May 2021 we decided to move to a new feed (Product B) which worked well and our plants were happy and being an Aquaponics Business we were not too worried about fish growth because the revenue is from plants and we were selling fish adhockery as and when there was a demand.

We found no noticeable difference in plant growth and as the demand for fish increased we started to pay more attention to our FCR or growth rate per month.  Although we have been weighing our fish monthly it did not really matter and in March 2022 really put the effort into calculating our monthly growth. We kept an eye on this and found that by the end of May 2022 we had just about no fish growth.

Disappointed at this we then weaned our fish off Product B and back to Product A. We do this slowly and systematically, changing the feed from the existing to the new. Our growth rate increased and we had happy plants, with nutrient levels around the 50mg/l mark to which we were accustomed.

Well as luck has it our fingerling supplier introduced us to a new Fish Feed supplier that he had started using a few months prior. He was very happy with his FCR and even found that he was getting 1:1,1 with better water clarity and happy fish.

So the next step in our lives on the farm started and we decided to run a trail keeping some of our dams on Product A and wearing some of the dams onto Product C. Our decision was to feed some fish of the same size and biomass on the new Feed (Product C) and to keep others on Product A. As this worked out and not purposefully (it was a fish growth test after all) the groups were per set of dams, meaning that a set feeding and filtration system and a tunnel or tunnels were on the same Product.

As “advertised” there was an “increased” in fish growth compared to that of Product A and we found that the growth was 245g from 8 of June to the 15 of August 22. The growth rate was thus 3,65 grams per day over 67 days.  Whereas the growth on Product A was 68g over the same period, giving us 1,01 grams per day.

In operation

What happened next.

What happened next (or systematically) was a shock and an education in itself. The plants that were receiving water from the new feed (Product C) were increasingly deteriorating with nutrient levels dropping to below 10. Making things worse was the fact that we use one sump where all the water is mixed from all the tunnels before returning to the Fish.

This then had an effect on all the tunnels except for the quarantine dams where the water recirculates from the dams to the filtration via the plants and immediately returns to the fish.  When in quarantine we use the one plant raft as a sump ensuring that the water does not mix with the rest of the system.

By “luck” the fish in quarantine were fed with Product A and the nutrient levels remained around 50mg/L what we were accustomed to.

Note the Picture, Tunnel Four and Six although standing side by side on the left, are both between 0 and 10mg/L with the Quarantine tunnel on the right at 50mg/L

We did not know all of this at the time and started searching for the answer until Victor who runs the fish house and does the measurements came up with this theory. “Well done Victor”.

As of now it is still a problem we have weaned all of our fish off of Product C and back to Product A. This has taken its toll and we were forced to remove 30 000 plants that were not happy, allowing Aphids to flourish, no matter what we sprayed. We did not have space for new plants that are ordered upfront and a decision to clean up was a difficult one because of market demand.

So where are we now and what have we learned?

Our Nutrient levels are increasing by the day /week and after two months of battles and attempting to sell of what we thought we could, returns flooded in. We made the decision to clean out the system removing all plants that don’t seem happy or even have a mark on them and replant with new seedlings. 

Our nutrient levels are at between 25 and 50 so we hope to get back to “normal” soon.
What can be taken from this experience with daily / weekly measurements (not a scientific test as we did not intend to do one) is that fish feed has a major impact on an Aquaponics system and if I were to advise on an Aquaculture system I would most certainly advise Product C and for an Aquaponics System Product A with B as an alternative if fish growth is not of major importance.

Gives a new perspective to: “Look after your Fish and your plants will be happy”.

Sample

Article by: https://www.aquaafrica.co.za/