By Leslie Ter Morshuizen

Fish health management is a primary focus area for any fish farm manager. Certain water quality parameters can change rapidly, most especially the DO (dissolved oxygen) concentration,

with potentially fatal consequences to our fish. It is thus necessary for us to have a regular program according to which we manage the water quality and other health factors on our farms to identify when things are heading in the wrong direction before a crises occurs.

A large proportion of the health related issues we face pertain to `environmental conditions’ such as water quality, stocking density (too high or too low) and feed quality. These factors can compromise the growth and survival of our fish without a parasite or pathogen being present, and fortunately can often be solved simply by correcting the stressor.

At times we may find that a pathogen has entered our farm and is challenging the well-being of our stock. Under these circumstances it is vital to 1st identify the organism with a high degree of certainty prior to attempting to eradicate it. A good quality microscope that can magnify to 400x and appropriate text books (such as Edward Noga: Fish Disease, Diagnosis and Treatment) are essential in identifying the enemy. Once you have determined the identity of the intruder, you need to obtain 3 Ds from the text in order to know how to eradicate the pest.

3 Ds: Drug, Dosage & Duration

The disease textbook will advise you on treatment options that you can follow. Occasionally a rise in temperature or similar change is sufficient to kill the pathogen, but more often one needs to rely on the use of chemicals to poison the pest to death. It is in this regard that the 3Ds refer: Drug, Dosage & Duration.

We need to know what drug (chemical) to use. Usually there are options to select from and the choice should be based on the balance of: effectiveness, availability, cost, fish sensitivity, human toxicity, environmental impact, withdrawal period and the limitations of the drug.

Having selected the drug we need to know what dosage is required for it to be effective, but prior to it killing our fish! In order to apply the correct dosage we need to accurately calculate the volume of the system, inclusive of filters and sumps. There is a school of thought supporting the exclusion of the biofilter from treatment to prevent the desirable bacteria from being negatively affected by the cure. This is illogical to me as the pathogen (with few exceptions) can exist in the biofilter until completion of treatment after which it again infects the fish. Having determined the volume of the system we calculate the quantity of drug required to achieve the target dosage. This is weighed out accurately, dissolved in water (or alcohol) in a bucket and placed into the sump to be distributed throughout the system by the pumps.

The final D is the duration for which the treatment must last to be effective. Water changes should not occur during this time to avoid dilution of the dosage. If some water replacement is unavoidable, be sure that the new water is treated at the same dosage with the drug to maintain the overall dosage.

Prevention is always better than cure, so ensure that the regular water quality checks are being conducted and that new fish are properly quarantined to limit the risk of pathogens gaining access to your fish farm and thereby avoiding crisis management.


Reproduced here with permission from his September 2015 eNewsletter [This email address is being protected from spambots. You need JavaScript enabled to view it.],