die off of zebrafish babies

Thomas Lauttenbach tom at marinebiotech.com
Fri Sep 29 15:27:20 EST 2000


After reading about the zebrafish fry (30 days old) dying at the NIH 
lab and considering the husbandry techniques outlined, we would 
like to offer a scenario for explaining and potentially solving the 
problem.  

Please refer to the "The Zebrafish Book", Westerfield, 1993. In this 
book a "Simple Method for Rearing Larvae" is outlined. The 
prefered technique outlines a procedure in which the zebrafish fry 
that are 15 to 20 days old are fed 3 to 4 times a day and are 
switched from an "off system baby-tube" to an "on system baby-
tube" and finally a regular "on system" grow-out tank. I think the 
key here is that the amount and frequency of the water changes 
must increase as the amount and frequency of the feedings 
increase. Water Quality, water quality, water quality.  

System water quality can change quickly when the system is a 
small 1 or 2 liter tank container. As the size of the fish larvae 
increase the tendency is to feed more. If the fry are only fed twice a 
day then the amount of feed given at any one feeding may have to 
be too high. As the amount of feed delivered to a tank increases 
over time so does the production of waste, microbial tank growth 
and availability of uneaten food (in this case AZ 100 is a powdered 
feed that stays in suspension) in the tank. A tank which is in 
continuous use will develop a microbial population which increases 
the bio-loading of the tank. The microbes/protozoans compete with 
the fish for food and oxygen etc. They also produce waste from 
eating the food intended for the fish. Water changes become more 
important in an old tank (2 to 4 weeks old) because of the 
microbes which increase the loading without being seen. Siphoning 
on a frequent schedule, adequate water changes and changing 
tanks every couple weeks will make a big difference.  

More frequent water changes will improve water quality but will also 
wash out suspended feeds. A side benefit to more frequent feeding 
to go along with the more frequent water changes is that the 
feedings can be lighter. Less feed in the water column at any given 
time will reduce the amount of feed that is diverted to the microbes 
in the tank. The fish should still be able to get the food they need 
but the microbes will be kept in check. Tetrahymena is normally 
free-living and usually a secondary invader in any disease situation. 
It can become a major problem in situations where there is over 
feeding and high organic matter in the fish holding system. In the 
end they turn on the fish.  

Check your water quality (NH3, pH, D.O.) in the fry tanks as the fry
grow and the inputs and outputs vary.

Hope this helps.

regards,

Tom Lauttenbach
Marine Biotech Inc.


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