Aquaponics No Ka 'Oi

Adventures in Aquaponics on Maui, Hawaii.

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Something is Eating My Roots – Part 2

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And it’s not a tilapia this time.  This new problem had been consuming us for the past few months.  It started slowly, almost unnoticeably, but instinctively, you knew something was wrong.  Not all of the plants were affected at first, but eventually the symptoms spread throughout most of the system.  The last time this happened, the problem was pronounced and the solution was quick.  It was fun chasing that rouge tilapia up and down the trough to finally net it and return it to the tank.  This time the problem was more sinister…

Symptoms.  It started with the taro, and slowly moved on to other plants including the peppers and lettuce.  The leaves started wrinkling and wilting, eventually dying.  New leaves did not thrive.  The plants were stunted, and the color was not a vibrant green anymore.  New starts had a higher mortality rate, and so did existing plants.  The fish and other marine life were not affected.

Possible Causes and Discovery.  Investigated several possibilities including disease, nutrition, and water quality.  However, over a period of time, noticed that the roots were diminishing.  It started with the fine, hair-like water roots disappearing, followed over time by the thicker parent roots.  Then some sick plants were pulled out of the rafts with no roots at all.  This became more pronounced over time.  Another discovery that was not immediately recognized was that all of the snails and gammarus were gone from the troughs.

Procedure.  Trough by trough, removed all of the rafts and used a home-made seine net to screen and corral the critters in the trough to one end for examination.  Found only guppies and prawns, which were thought to be safe for plant roots.  The prawns, Macrobrachium rosenbergii, are considered to be detritivores, only eating decaying organic matter, and are reportedly grown successfully under raft systems at other aquaponics sites.  The “guppies” are probably a mixed breed of several small freshwater fish possibly including guppies, mosquito fish, neons, medaka, etc., and are not considered significant threats to plant roots, as they too have been successfully used in other aquaponics sites to control mosquitoes.

Testing.  Filled a separate 6 gallon aquarium with guppies and a small foam raft carrying one net pot with a lettuce start with fresh roots for observation.  After 5 days, the roots were gone.  The guppies had eaten all of the roots.  In a separate 10 gallon aquarium, placed a 4 inch long male prawn with a floating lettuce start with fresh roots.  After 7 days, the roots were still there.

False Conclusion.  Moved all of the guppies from the first trough, T1, to a holding tank as a test.  Blocked the input and output of T1 with screen filters to isolate it from any marine critters other than the existing prawns.  With no guppies in the troughs, added some lettuce starts with new roots to a raft in T1.  The next day all of the roots were gone.  T1 contained no guppies.  The only marine life in T1 were the prawns.

Action.  Moved all of the prawns out of T1 to a 300 gallon green tank filled with screens to give them more surface area for livable space.  About 50 prawns were found in T1, varying in size from 2” to 10” (including claws).  Selected 10 golden guppies to add back to T1, and tested with some lettuce starts with new roots.  After 7 days, all of the roots were doing well.  During this time, moved all of the prawns out of T2-T4 to the 300 gallon green tank.  Found about 200-300 prawns total.  Also added 10 selected guppies back to each of the remaining troughs.

Final Conclusion and Comments.  Three weeks after removing all of the prawns, the roots are coming back and some of the plants are recovering.  Not all of the plants are able to recover.  It seems that when the plants are traumatized over a long period of time, some of them just slowly die.  The new starts are doing well, and their roots are thriving, so the next crop generation should be much better.

Other aquaponics sites report success raising prawns under the floating rafts, so this may be an isolated incident.  The prawns may indeed be detritivores, but they also exhibited carnivorous behavior at this site.  When guppies were added to the 10 gallon aquarium with the prawn, the 4” male prawn reached up and grabbed a guppy in his claws and ate it alive to the horror of my daughter.  Males have also been observed eating the carcass of a female on occasion.  And of course, the males are very territorial and are constantly fighting, ripping off and eating the claw of another male (which grow back after molting).  Their jaws must be very strong as they eat their own shells after molting.

But the question remains as to how the prawns began to eat the plant roots.  It may be that the population in each trough may have been too high.  The PLs were bought last November and were only ¼” long when they arrived.  The minimum purchase quantity was 600 so they were distributed at between 100 to 150 per trough.  Problems with the plants were first noticed about two months after that.  Another possibly related factor to this is that gammarus and snails have virtually disappeared from the troughs.  Before the prawns were added, plant roots were infested with gammarus and snails, which may have attracted the prawns to the roots and affected their feeding behavior and habits.