What do you do as the owner of a fully stocked fish farm when the electricity goes down for 20 hours, as happened to us recently? With several tons of tilapia at high densities in 31°C water, a backup plan is essential. Re-circulating aquaculture systems usually depend on mains electricity for pumping, aeration and filtration. These life-support systems are almost always required on a 24-hour basis for all fish farms, except small backyard systems, which may run on solar or battery power.
The fish themselves will warn you when the electricity fails. Critically low levels of dissolved oxygen in the water will have them gasping just below the surface about 10 minutes after a power cut. And fish in this situation could have as little as 10 minutes to live, if well-oxygenated water is not made available quickly. Fast action is required, especially if the stocking density is high (>20kg/m³ or one fish/10l water), or if the water temperature is elevated (>26°C). Water at 32°C holds only a fraction of the oxygen that it does at 25°C.
A portable 2,5KW generator will run a 0,75KW air-blower and a similar-sized pump. These standby ‘gennies’ cost about R3 000 and should run for 10 hours on a tankful of petrol. They are well worth the investment if you have a smallish system. The downside is that they are usually not equipped with an auto-start function – someone needs to be there to start the machine up and check that it has not ‘tripped’.
For larger systems, a diesel-powered alternator with auto-start is essential. The golden rule during any power outage is to avoid feeding the fish during and shortly after the power cut. Extra waste produced by feeding and excretion results in a further drop in the water’s oxygen level and water quality. If clean water is readily available, this can be reticulated through the system to:
- reduce the temperature;
- allow more oxygen into the fish tanks; and
- dilute the metabolic wastes in the tanks.
Obviously, flooding the system with cold, chlorinated municipal water is not an option! Bio-filters are living organisms. The filter medium is a colony of aerobic (oxygen-loving) and anaerobic bacteria. During a power cut, the bacteria may die in as little as half an hour – and on start-up, toxic hydrogen sulphide gas is released into the system. The smell of ‘rotting eggs’ is a clue to the presence of this gas.
Substantial water changes are needed to dilute this toxin and avoid poisoning the fish. New bacteria may take several weeks to establish themselves (just as in a new filter), and feeding must be reduced during this period. Skittish fish that are shy of one’s presence are a sure sign of excessive ammonia in the water due to an inactive filter. Again, you need to change the water and reduce feeding.
One of the best ways to reduce the risks associated with power cuts is to understock fish tanks in order to allow yourself a ‘margin for disaster’.To proudly claim that your tanks hold 45kg/m³ of fish is fine – but better to have 25kg/ m³ alive and growing fast than a tank full of floaters!
Nicholas James is an ichthyologist and hatchery owner.
This article was originally posted on 26 June 2015 in Farmer’s Weekly.