the effect of lower temperature on growth performance of catfish

the effect of lower temperature on growth performance is well
established. At 25-28*C, catfish pertforms best both experimentally and
practically (other factors properly considered), with higher survival
rate at the upper limit for both juveniles and adult fish. Catfish can
however remain alife sometimes below 20*C but not growing to potential.
Yields have been confirmed to be almost double at 28*C what they are at
20*C or lower (which means you could harvest about 2 tons at optimal
temperature of 28*C instead of 1 ton at 20*C or lower), therefore it is
expedient to consider alternatives to reduce cold in your culture
system.
For all ponds dug below-ground, it is important to know the coldline
depth (frostline depth in countries that experience winter and
experience snow). This is the depth into the soil that cold gets to
during cold climates. Once you know this, it is then advised that pond
depths are made double the cold line depths. If you measure and find
the coldline depth is for instance 12 inches, then it is safe to make
your pond 24 inches in depth (your pond acts in response to the
surrounding soil temperature) . As a general rule, in tropical climates
such as ours in Nigeria, we always advise our farmers to dig ponds
which are at least 1.5 meters in depth even though the coldline depth
never reaches half of this, just to be on the safe side. Once the
surface is cold, fish swim deep to avoid the adjustment to temperature
changes. Moreover, not all fish tolerate wide temperature variations!
However, for all other ponds above-ground (regardless of if made of
cement, fiberglass, plastic, canvass etc), the effect of reduced
temperature are more drastic. Experts advise that above-ground ponds be
operated indoors under cold situations or to use temperature regulators
like heaters where practicable. However, where this is not practicable
(especially where cheap source of power is a challenge), is is better
to practise one form of temperature manipulation or the other to avoid
adverse cold. As an example, a pond shielded from direct effect of
harmattan wind would be better protected than one which is openly
exposed to the vagaries of the weather. Water change to culture system
could be reviewed and done in such a manner that water from a reservoir
is added to culture tanks and its temperature is at optimum.
At lower temperatures, metabolism in fish slows down leading to a drop
in respiration rate. None-the-less, nitrite still builds up slowly and
if not addressed properly, could ultimately lead to "brown blood
syndrome" due to acute oxygen derivation.
The first sign you would notice that decrease in temperature in
harmattan is affecting your fish is increase in mortality beyond what
you are used to. Close observation during fortnightly or monthly
grading sessions (if you do grade continuously) or by routine
observation would also reveal reduction in mean body weight gain
compared to that during non-harmattan period, otherwise you may not
need to worry. In the final analysis, there is a need to evaluate the
essential gains you stand to benefit from any increase in economic
activity as it adds value to your profitability or not. Afterall, fish
farming is everything about profit/loss accounts.
May Allah assist us in Nigeria.

starch digestibility 1

It is not necessarily a binder that will solve your problems. You can achieve a lot with proper processing conditions.
If you run a pellet mill, do you also have a steam boiler ?
Some guidelines:
- Try to grind as small as possible
- Use a die which is thick enough so that you can build more pressure. You will use more electricity and capacity can decrease, but your pellet will be harder. A ratio of >15 thickness/hole diameter should be OK for a feed with a fat content of <10 % ( for example a die with 4mm holes and a thickness of 60 mm). But it depends a lot on the formula if you need to increase. Generally, this means: more fat and moisture in the diet requires a thicker die, more fibers require less thickness.
- Try to heat the mash to 90°C before it enters the pellet mill. This can be done with steam injection in the preconditioner and/or by heating the preconditioner from the outside (steam jacket or electric pads)
- Try to keep the pellet at 80°C for about 20 minutes.

Of course there are a lot of variables like raw materials, formula, machinery, etc.

But I hope this info already helps.
Best regards,
Eric De Muylder
Aquafeed consultant
CreveTec
www.crevetec. be

starch digestibility

I forgot to mention that African catfish (just like ourselves) can only poorly digest raw starch.
When starch is cooked during the extrusion or pelleting proces the digestibility increases, which means that the fish will have a better feed utilisation and lower FCR.
Cooking of starch starts at about 60°C in the presence of some water, so for example in the preconditioner when steam is added or in the postconditioner if you have. The higher the temperature and the longer the period, the more % cooking will happen.
Starch sources are for example wheat, corn, sorghum, tapioca.
It is this cooking (or gelatinization) that makes your starch bind the pellet bind. It is exactly the same process as baking bread: you mix water with flour and put it in the oven and you have bread. If you put only water without heating or you only heat the flour without mixing water, nothing will happen.