Leslie Ter Morshuizen on Mistakes made by new Fish Farmers

There are certain errors that are commonly made by new entrants into the aquaculture industry and I thought it would be useful to highlight these, as a guide to new entrants so that they can avoid these errors, and to remind more experienced farmers to duck these mistakes.

Stocking too many fish

Surely the most common error made by new (and less new) fish farmers is to stock too many fish into the tank, cage or pond.  We want to grow as many fish as possible and wish to maximise return on our investment, so we push the boundaries a little.  The result is as predictable as night following day; oxygen levels are compromised resulting in reduced growth, a poor FCR (feed conversion ratio) and high N-compounds as the filters struggle.

Every cage, pond or tank has an optimal density and if you stock more fish than the optimal density dictates, the growth of the fish will be compromised.  Some people logic that, by stocking more than the optimal number of fish, although the individual fish mass is decreased, the biomass will increase (due to the greater number of fish to be harvested).  The opposite has in fact been shown to be the case: the total biomass decreases if the fish are overstocked.  Furthermore, larger fish often command a higher selling price than smaller fish, again favouring the production of fewer, larger fish.  A final thought is that if a problem occurs, such as a very hot day impacting ponds or a power failure in a RAS (recirculating aquaculture system), the tank with the higher density of fish will have more demand for oxygen than the tank at a more moderate density.

The lesson: know what the appropriate density is for the infrastructure you farm and avoid the temptation to stock beyond that point.

Too many tanks; too few filters

RAS farmers starting out often place too much emphasis on creating lots of tank space to house the fish with the water management, and consequently the filtration system, only receiving a scant afterthought.  Adequate mechanical filtration is essential to remove the suspended and settleable solids from the flowing water, and the biological filter must be both large enough and efficient enough to convert all the ammonia to nitrite, and all the nitrite to nitrate.  Failure on either side will result in poor water quality, reduced growth, poor FCR and increased disease susceptibility.

The lesson: have the RAS professionally designed to ensure that the filtration system can maintain the target water quality even when the system is fully stocked.

Too little water flow

Another common mistake in RAS’ is to have an exchange rate that is too slow.  Two related concepts are both important here: circulation and exchange rate.  Circulation refers to the velocity of the water movement within the tank, generally referring to a circular tank and the water moving around inside the tank.  This is important as it keeps the solid wastes in suspension until they exit via the central bottom drain, thereby ensuring that the tank always remains relatively clean.  Larger fish tolerate faster circulation, leading to more efficient cleaning, than smaller fish of the same species.

Exchange rate relates the volume of the tank to the volume of water flowing into the tank per hour and is an important calculation as this dilutes the dissolved organics by replacing the water regularly.  We have seen that an exchange rate of 1 tank volume per hour is a good basic guide but this needs to be adapted to many variables including the density of fish, species and fish age.

Both circulation and exchange rate are usually achieved simultaneously by pumping water into the fish tank at an angle that creates a circular flow within the tank, carrying the solid and dissolved organic wastes out through the center bottom drain to the filters.

The lesson: know the correct rates for your system and species, and scale the pumping system accordingly.

Too little aeration

This goes hand-in-hand with the points above; if there are too many fish in the tank relative to the rate of oxygen replacement oxygen levels will drop, consequently reducing the growth rate of the fish, efficiency of feed usage and increasing vulnerability to disease.  In addition, if there is inadequate oxygen (≤4mg/ℓ) then the biofilter bacteria will also fail to efficiently convert ammonia to nitrate.

The lesson: Ensure that there is adequate aeration for the fish and biofilter to function optimally.

Using poor genetics

Tilapia are extremely easy to fish to breed and tilapia farmers are consequently most at fault here.  The temptation to grab the first fish you find when your infrastructure is ready to receive fish is great because you want to get farming.  However, the most available fish are often inbred and of limited economic value.  These fish usually have a slower growth rate and poorer yield (ratio of the fillet to body) than commercial strains.

The maths is simple to do: compare how many fish you can produce from your ponds in 5 years if you use a strain that takes 5 months to reach market mass vs a strain that takes 7 months.

The lesson: Be patient and find the best quality fingerlings you can, both in terms of growth rate and yield.

Not managing the livestock

All too often we find newcomers plan to place fish in their systems in Month 1, leave them there until harvest size and then harvest them for the first time.  This strategy is inefficient for several reasons, including, failing to provide an opportunity to remove slow-growing fish, underutilises infrastructure in the early months, fish may not feed well initially as low-density results in vulnerability stress, and it does not group fish according to size so the harvest includes a range of sizes which creates marketing issues.

It is far better to stock the fingerlings into a small container for a limited period – typically 1 or 2 months.  They are then harvested, sorted according to size, the slow growers are discarded and the rest are restocked into a larger pond at a reduced number per cubic meter, and given a further period of 1 or 2 months before repeating – harvest, sort and restock.

The lesson: have a plan according to which the tanks, cages or ponds are stocked, harvested. size sorted and restocked, and then work the plan.

Feeding incorrectly

The feed is the fuel that enables the fish to grow.  Feed too little and the fish may have fantastic genetics and excellent water quality, but they will lack the fuel that drives their growth.  Equally, if you feed too much the fish cannot eat all that is provided, resulting in waste, dirtying of the water and compromising of growth.  It is therefore vital to feed the correct amount of feed.

Feed quality is also vital.  We have seen in our hatchery that good quality feed results in less size differentiation, much larger fish, greater survival and better water quality than in adjacent tanks being feed poorer quality feed.

The lesson: use high quality feeds only and apply appropriate techniques to ensure adequate feed for excellent growth.

These are some examples of errors commonly made by new fish farmers, yet each of them can easily be avoided.  I encourage you to have your system professionally designed and constructed, and then to manage the system in accordance with the BioPlan to ensure that you achieve the expected output.

Contact details:

Leslie Ter Morshuizen

Email: info@aquaafrica.co.za

Tel: +27 46 622 3690 

Website: www.aquaafrica.co.za

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