The science behind freshwater fish kills in Kerala

What causes freshwater fish kills?

Freshwater fish kills are common in Kerala. Reduced oxygen levels are the most common cause but this in turn is triggered by a variety of factors such as drought, rising temperatures, algal blooms and overpopulation. Disease-causing microorganisms and parasites can also lead to fish kills.

According to Shaji CP, a fish taxonomist from Malabar Natural History Society who has tracked several fish kills in the state, one important factor is rampant sand mining, which is causing rivers to dry up. This confines fish to smaller pools, rendering them more vulnerable to poor water quality.

Shaji found that in several cases, fish death was brought about by the use of fish poison for fishing. Release of effluents and dumping of wastes into the river is another common cause but Shaji pointed out that this is more so in the southern parts of Kerala, which is where most industrial areas are situated.

How do scientists investigate fish kills?

As soon as the Periyar fish kill was reported, researchers from the Kerala University of Fisheries and Ocean Studies (KUFOS) hurried to the scene to collect samples of water as well as live fish from the affected area. Anu Gopinath, a chemist at the university, was part of the expert committee constituted to investigate the incident. It was crucial that Gopinath and her team conduct this analysis on the day of the kill because, with time, the water composition was likely to change.

“The water samples were analysed for several chemicals including hydrogen sulphide, ammonia and toxic heavy metals,” she said. In such instances, even the colour of the water can say something. For example, too much green may be an indicator of toxic algal blooms.

What kind of tests are fish subjected to?

Apart from water quality, biologists from KUFOS conducted systematic analyses of live fish. One aspect that was immediately evident to Devika Pillai, Director of Research at KUFOS, was that fish across species were affected by this event. “This is an indication that certain changes in water quality have occurred,” she said. “If it was a pathological agent such as a viral infection, some species would have been more prone than others.”

Despite her hunch, they had to rule out the possibility of bacterial, fungal or parasitic infection. Pillai and her team further conducted an array of tests and post-mortem examinations on the sample fish such as gill smears, skin smears, and blood tests.

What did the scientists find out?

The preliminary report from KUFOS stated that dissolved oxygen levels in the affected area were indeed lower than ideal. Moreover, abnormally high levels of ammonia and sulphide were detected. The report also mentioned the dangers of opening regulators situated upstream at inappropriate times.

“Sudden mixing of upstream water and downstream water must have resulted in certain dynamic changes in the water quality,” the report said. Additionally, it highlighted the potential risk posed by the multiple poultry meal factories in the region.

Gopinath explained that factories are known to release their waste into the river when they hear that monsoon showers are expected, as they see this as an opportunity for the waste to be flushed away. However, if rains do not arrive as expected, this flushing does not happen and the water quality of the river is seriously impacted by the sudden release of toxic waste. It is not clear yet if this was what transpired in this particular case.

So, what if fish die?

If our freshwater bodies are indeed so polluted, it’s not just the fish that are in danger but all living beings dependent on them including humans.

Taxonomists like Shaji are pushing for more in-depth documentation of fish kills. His own studies alerted him to the fact that almost all fish kills in Kerala happen in May. This is the time that the fish are getting ready to breed, therefore the magnitude of loss is much greater. The implications are not just economic but also ecological.

“Fish are like the tigers of the aquatic world,” Shaji says. Occupying the top levels of the food chain, fishes regulate several ecological processes. Losing them, he warned, can disturb the whole ecosystem.

Nandita Jayaraj is the co-author of Lab Hopping: A Journey to Find India's Women in Science, which explores the gender gap in Indian science.