Tilapia fry mortalities

synaptoman

I have a problem at my Tilapia Hatchery. I wonder if someone could give me some advise. Recently (last month or so) my fry mortalities have sky rocketed. Prior to that I lost very few. They seem to get
listless at about 1 week old and then lose equilibrium over a day or two (swim around funnily, flashing etc.) and later sink to the bottom where they die. I use water from a rainwater tank (which has got a bit low recently) but use the same water for my brood stock and they are unaffected. I do at least 25% water changes per day in my fry tanks. I suspected the feed and stopped feeding from this batch and
used a new bag but still no difference.

All my water readings are well within specs and I oxygenate well.
They are stocked very lightly.

Temp : 28 C
pH : 7
Ammonia : 0.1
Nitrites : 0
Nitrates : 0

Any ideas what the problem could be?

Attached an image of a dead fish. It's about 3 weeks old, 18-20mm long. If you look carefully you can see a hair-like protuberance sticking out of it's body just behind the gills. It's about 4mm long.

RupertofOZ

Could it be a "gill fluke" (Dactylogyrus )... a trematode... (parasitic worm)... http://en.wikipedia.org/wiki/Trematoda

Common aquaculture pest that is especially present in crowded fingerling stock environments...

thorn

Aside from the normal water test kit readings, there is also a dissolved oxygen level. Maybe that has something to do with it? I've never been able to test this myself because the electronic testors are $140+ USD. Do you have aeration going with the fry? Is their tank shaded to prevent mid-day higher water temps?

synaptoman

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Do you have aeration going with the fry? Is their tank shaded to prevent mid-day higher water temps?

Plenty of aeration. also very low stocking densities. shaded and heating to 28 deg (thermostat controlled).

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Could it be a "gill fluke" (Dactylogyrus )

That's what I'm thinking. I don't have a microscope and my eyes aren't that great but it appears to have a different coloured pin head on the top but that is about as much as I can see

RupertofOZ

In http://www.fishdoc.co.uk/disease/flukes.htm it is wrote:

Damage and diagnosis

The fish response to the irritation caused by flukes is similar to any other irritant, whether it is parasites or chemical. Initially there will be rubbing and flashing. As the disease reaches a more advanced stage the fish will become lethargic - which is the point when some people think the problem has gone away. At a very advanced stage the fish will isolate itself and spend long periods laying on the bottom with its fins clamped to its body. Other signs may be skin cloudiness resulting from excess mucus production, skin hyperplasia, or focal reddening. A definite diagnosis can only be made via a skin scrape or gill biopsy. In large numbers, flukes will kill fish either directly, or indirectly through secondary infections.

In http://ezinearticles.com/?Skin-and-Gill ... h&id=78794 it is wrote:

Gill fluke is a worm that specifically attacks the gill membranes, causing them to turn red and acquire a coating of slime that makes it difficult to breathe. Fish will hang at the water's surface, gasp, and lose weight rapidly. The same tank treatments as skin fluke can be used, but with gill fluke, removing to a hospital tank and adding short baths in either formalin, salt or ammonium hydroxide to the regimen will help kill what is on the fish, and you can then treat their environment.

http://www.geocities.com/steevward/dactyls.html

http://www.koiandponds.com/disease-gillflukes.htm

synaptoman

Seems logical but these "hairs" are huge and visible to the human eye. I'd say 4mm whereas everything I've read about gill flukes is that they are microscopic or at least under a mm. If you look at the image it's almost as long as the body is wide !!

thorn

Maybe try completely covering the tank (allow oxygenation though) to eliminate all light for a few days to eliminate the oddball possibility of it being an algae. (how long does it take of no light to actually kill algae?)

hydrophilia

Just as a test, perhaps you could put some fry in 8ppt or 10ppt salt water. Tilapia can handle it fine and it may kill whatever it is....

badflash

I agree with the salt idea. You can go as high as 15 ppt. Once they get to be fingerlings you can bring them back down to fresh fairly quickly.

TCLynx

The other possibility might not be infectious but something to do with the water. What type of rain water tank is it? If the rain is being collected from a roof, what is the roofing?

I have a feeling though that the culprit might be some infection has gotten into the system. If that is the case, salt is probably your best next step if you haven't already tried it.

synaptoman

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What type of rain water tank is it?

Plastic

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what is the roofing

Ordinary roof tile (cement)

We use the water from the roof for the garden as well as the brood fish and they're fine so I don't think that it's the water.

thorn

You could try switching to well/tap water to see if the situation improves.

synaptoman

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You could try switching to well/tap water to see if the situation improves.

I think I'll try this for a while. I am just trying to find out if our water is treated with Chloramine first before I do.

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Tilapia fry mortalities

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