How pollution and cages are killing fish in Lake Victoria

Dead fish in a cage, Lake Victoria. [File, Standard].

Upwelling could also have compounded the situation. Here, the bottom cold water mixes with the top warm oxygenated water. During this process, the particles of dead algae, disturbed mud, and sediments clogged fish gills and prevented them from breathing. Additionally, the death decomposing macrophytes mainly of the water hyacinth and hippo grass settled at the bottom of the lake may have been re-suspended and utilised any remaining DO for further decomposition.

Whatever has happened in the lake in the past few months where we saw massive fish kills, it is not different from the above scenario. In both instances, the challenge faced is due to high level of pollution exacerbated by increased anthropogenic activities in the catchments.

However, there is one anthropogenic activity that has been taken right into the centre of the lake and is behind the challenges we are witnessing, but is not given attention at all. This is the cage culture that is estimated to produce about 12,000 tonnes of fish per production cycle (of eight months).

In fact, its effects are dire compared to the benefits. The waste from food given to fish and fish defecation is enormous, adding a lot of nitrogen and phosphorous. This exacerbates eutrophication hence proliferation of algal blooms. Fish cannot consume all the given feeds with more than 75 per cent of it going into waste.

Further, less than 30 per cent are digested and absorbed by fish for metabolic activities with the rest being excreted into water. For every tonne of tilapia production in cage culture, 11,041kg of organic matter are discharged into the water. Out of this, 45kg of nitrogen and 14 kg of phosphorous are produced.

Dead fish in cages at Lake Victoria. [File, Standard]

In Lake Victoria, this translates into 12,487,560kg of organic matter. With this, fish kills are inevitable. No wonder, this occurred in cages only.

With this scenario in Lake Victoria ecosystem, the problem didn't begin yesterday, it has taken a while, only that it is becoming complex by each day. It may take many years to reverse the state of environment once the management interventions are undertaken because polluting an environment is faster than restoring it.

-Mr Kobingi is a PhD student of Limnology at Kisii University and a Research Scientist, KMFRI, Baringo Station. [email protected]