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Tilapia, Phytoplankton and EMS

 

Matthew Briggs (koygung101@yahoo.co.uk): Hi Loc, I just read your paper in the Global Aquaculture Advocate about tilapia’s effect on early mortality syndrome (EMS).

 

[Shrimp News: Here’s the reference for the above article, which is available online for free at GAA’s website (link above): Tilapia Could Enhance Water Conditions, Help Control EMS in Shrimp Ponds.  Loc Tran (thuuloc@email.arizona.edu), Kevin Fitzsimmons and Donald Lightner.  Volume 17, Issue 1, Page 11, January/February 2014.]

 

I am confused about the big differences in results between treatment B and treatment C.

 

Treatment B was in tanks holding tilapia for 14 days prior to stocking shrimp.  Then the tilapia were removed and the shrimp were added and challenged with EMS.  They had a very high survival.

 

Treatment C was in tanks holding tilapia for 14 days prior to stocking shrimp, then the tilapia were moved to a cage within in the tank, and shrimp were added to the tank.  The shrimp were challenged with EMS and had very poor survival.

 

Was the presence or absence of tilapia during the challenge the only difference between these treatments?  If so, how can you explain the difference in survival based on the presence or absence of tilapia during the challenge?

 

Loc Tran (thuuloc@email.arizona.edu): Dear Matt, treatment B had an “enough” algae bloom, and treatment C has an “over blooming of algae” caused by an over populated tilapia.

 

No algae equals disaster; too much algae is not good; and “enough” algae is good.

 

My speculation is that too much algae may cause a deposition of dead algae on the bottom of the pond, supplying surface and nutrients for the EMS bacteria, which gets eaten by the shrimp, instigating the disease.

 

Matthew Briggs (koygung101@yahoo.co.uk): OK, thanks for the clarification.  Sounds like a reasonable hypothesis.  Good management rules again!

 

Gruenberg Daniel (seagardenfoods@mac.com): Hi Matt, I agree with Matt and Loc that good management will be the key to resolving the EMS problem.

 

But the use of tilapia to control algal blooms sounds like a “hack job” to me.  It would be interesting to see if diatom management would achieve the same results.

 

Loc, did you do any phytoplankton quantification by comparing blue-green algae, green algae and diatoms during this experiment?

 

We have seen a correlation with improved survivals in diatom-dominated ponds versus ponds with green and blue-green algae.  It is well known that tilapia like to consume blue-green algae.

 

Loc Tran (thuuloc@email.arizona.edu): Hi Daniel, the algae quantification was not included in the article, but I did that part, too.

 

In general, treatments with tilapia had green algae, diatoms and some blue-green algae.  Those without tilapia had green algae, blue-green algae and diatoms (from the most abundant to least abundant).

 

I have a feeling that diatoms and green algae are both good.  Diatoms may be a little bit better than green algae, but harder to maintain.

 

Gruenberg Daniel (seagardenfoods@mac.com): Loc, that confirms with my observations.  If you are interested in ways to learn how to maintain diatoms in shrimp ponds, I would be willing to show you how to do it.

 

Matthew Briggs (koygung101@yahoo.co.uk): Thanks Daniel, there is an interesting question here.  It seems there is agreement that diatoms are beneficial as the dominant algal species in shrimp culture ponds, but there is also a consensus that polyculture with tilapia is also beneficial.  Is it just that tilapia promote diatoms, or do tilapia have another effect that is complimentary or perhaps even more important?

 

I have heard the following theories on why tilapia are useful in shrimp polyculture:

 

1. They promote diatoms and consume harmful blue-greens.

2. They disturb and aerate bottom sediments, which prevents the accumulation of toxic gases.

3. They produce an antibacterial fish slime that kills harmful Vibrios.

4. They eat sick shrimp and thereby preventing horizontal transmission of the pathogens.

 

It would be interesting to study the above factors in the presence of EMS to see which of these effects really works.

 

From the article by Loc in the GAA Advocate, it would appear that the main effect the tilapia have against EMS comes from promoting a stable diatom bloom, but there may be more to it than that, and the tilapia may be adding something else?  If not, and if its just the effect on the bloom, there may be better ways of stimulating these diatom blooms without having to resort to the often difficult to manage co-culture with tilapia.

 

Hank Bauman (hank_bauman55@yahoo.com): I did a lot of bacterial plating in tilapia tanks that were intended to produce “green water” for Penaeus monodon tanks in the Philippines.  I also had five years experience, or so, using the same system for Macrobrachium rosenbergii hatcheries in Hawaii.  But with P. monodon, our hatcheries didn’t have enough fresh water to keep salinity below 22 parts per thousand, or so.  At higher salinities, the green algae wasn’t stable.  It would bloom up and crash.

 

However, further research showed that perhaps it wasn’t the green algae that was the real benefit.  Using TCBS (thiosulfate citrate bile sucrose agar) plating on a daily basis, we found that even clear water and the slime on the sides of the tilapia tanks were very high in yellow Vibrios.  Not sure of specific species, but the result was the same—unbelievably fast growth in our postlarvae.  We would get monodon postlarvae to 12.5 millimeters with five-tooth rostrums by PL-8.

 

The tilapia may create a bacterial profile—and maintain it very well—that outcompetes the Vibrio parahaemolyticus strain and holds its numbers in check.  The same system eliminated a luminous Vibrio from the ponds and seawater.

 

We intend to try it again because EMS is just north of us in Mexico.

 

Gruenberg Daniel (seagardenfoods@mac.com): Hi Matt and Loc, first some basic pond phytoplankton ecology that you really don’t find in the textbooks.

 

I have created a grading scale for pond plankton structure beginning with Grade-A, which is diatom dominated with a few green algae, no blue-green algae and usually accompanied by copepods as the dominant zooplankton.

 

On the other end of the scale, Grade-F is blue-green algae dominated with little or no zooplankton.  There is a continuum from A to F.

 

Tilapia seem to push the structure from Grade-D/F to Grade-B/C.

 

Secondly, I have observed a predominance of Grade-D/F structures in ponds since the EMS outbreak.  It makes me wonder if the toxin that V. parahaemolyticus release into the pond environment is affecting the plankton structure of the pond.

 

Thirdly, all of the above is nothing more than an observational correlation and not necessarily cause and effect.

 

The current challenge is to design prospective experiments to test different theories that Matt and Loc have brought up.

 

It is my gut feeling that the actual toxin “baggage” that the V. parahaemolyticus is carrying has some effects on the plankton structure.

 

We have observed—and it was corroborated by a different group in Mexico—that despite having some EMS, some ponds do better when the plankton includes more diatoms, perhaps an indication that the toxin was not concentrated enough to do its dirty work.

 

If my theory is correct, we can show a significant difference in EMS survival rates upon challenge by manipulating plankton structure via our micronutrient system, even without the tilapia.

 

It would be nice if we could compare micronutrients alone to tilapia alone, with no treatments as a control.

 

Gruenberg Daniel (seagardenfoods@mac.com): Hank, this is excellent information.  Tilapias are also know to be effective blue-green algae grazers so their consumption affects the phytoplankton structure of the pond.

 

It is quite well established in the literature that diatoms have a significant ameliorating affect on Vibrio counts under a broad range of conditions, although there have been some contradictory reports of increased Vibrios associated with diatoms.

 

In any case, I think we have opened up an interesting line of inquiry that definitely is worthy of further study.

 

Hank Bauman (hank_bauman55@yahoo.com): I’m no bacteriologist by a long shot, but the plating we did showed that not all Vibrios are bad.  The lower salinity green-water tilapia ponds had yellow Vibrios that were too numerous to count on TCBS.

 

The algae in tanks were 90% green.  Low salinity (13-18 ppt) produced a larger cell, like Chlorella.  At 30 ppt, I saw very small Nannochloropsis type cells.  But that was very unstable, and I couldn’t harvest 30% of the volume per day and maintain the cell count.

 

As far as how to manage it in ponds, I’m not sure what would work best.  In hatcheries you could culture the green water separately in tanks with about one kilogram of tilapia per cubic meter, feeding them very little, almost to starvation (1.5% by weight per day?).

 

One thing we found is that once you remove the water from the tilapia tank and put it into shrimp larvae or PL tanks, the yellow colonies start to turn green within one day.  After three days, or so, the greens dominate.

 

This would indicate that if you use a reservoir for tilapia to use in ponds, you must replace the water in your shrimp pond every two days or so.  That means a lot of pumping.

 

Or, as people have tried, put 2,500 to 3,000 tilapia fry per hectare in the shrimp ponds.  They will eat feed, but the feed conversion ratio stays below two in my experience.  Using this system, we got 34-gram monodon in 75 days in a pond in the Philippines.  But this was also at low density with only eight shrimp per square meter.

 

Tilapia are really intelligent [hmmm...].  I tried using a floating, cheap, low protein feed before feeding the expensive shrimp feed.  It worked for about five days.  Then the tilapia figured out that the floating feed was followed by the “good stuff”, and they stopped eating the floating feed.  Smart little buggers.

 

Jimmy Maneckshaw (j.n.maneckshaw@gmail.com): Guys in my ponds I’ve used jaggery (a crude sugar) at five kilograms per week to keep my blooms golden-yellow-brown.  Done it for two crops now with good, but not perfect results.

 

Ramon Macaraig (monmac52@yahoo.com): What production intensity level for shrimp are we talking about here?  Two to five metric tons per hectare per crop or ten metric tons per hectare per crop?

 

We are all worried about feed conversion ratios, aggravated by low survival rates caused by diseases like EMS and WSSV.  Are we not supposed to focus attention to the part of the daily feed that does not become shrimp biomass, the part that messes up the growing environment and invites growth of unwanted pathogens?  At a shrimp biomass of eight metric tons per hectare and a feeding rate of 1.8% a day, a rough calculation shows that 100 kg out of 144 kg of feed remains in the pond system and has to be managed daily.  If all of the wasted feed were mineralized, there could be a maximum 1.4 percent ammonia nitrogen in a 1.2 meter-deep pond everyday.  Can biofloc turn all this into bacterial protein as supplemental feeds?  Can we also do efficient mineralization without the need for anaerobic bacteria?  Can algae scoop up the mineralized nitrogen, carbon and phosphorus quickly enough to add dissolved oxygen during the day and open possibilities for polyculture?  What if we just drain out the feed residues?  How do we clean up that mess?

 

Only when we have a clear objective can we look into which algae, zooplankton, bacteria species, devices and stocking densities are most appropriate.

 

Jimmy Maneckshaw (j.n.maneckshaw@gmail.com): I stocked 50 PLs per square meter and with partial harvesting got ten tons per hectare after 240 days.  The final average body weight was 45 to 50 grams with a feed conversion rate of 1.5 and a survival rate of 85 plus percent.

 

Source: The Shrimp List (a mailing list for shrimp farmers).  Subject: EMS Webinar.  December 31, 2013, to January 1, 2014.

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