The extraordinary abundance of puffer fish in the Arabian sea off Kerala since 2007 could be due owing to a decline in the population of top predators in the marine ecosystem, a study conducted by scientists at the Central Marine Fisheries Research Institute indicates.

The study, reported in the latest issue of Current Science, attributes the increasing biomass of puffer fish to the excessive fishing of top predators like the cobia (Rachycentron canadum) and different species of catfish in the ecosystem.

The study carried out by K.S. Mohamed and others says the increased biomass of puffer fishes from 2007 signalled the beginning of a ‘trophic cascade’ in the Arabian Sea, marked by reciprocal changes in the population of predator and prey species. It says the loss of predatory control had a top-down effect on the population of puffer fish. From 2007, there have been reports of puffer fish wreaking havoc off the coast of Kerala, damaging fishing nets and causing extensive loss to traditional fishermen. The problem is acute during the post-monsoon period. Puffer fish tear fishing nets to shreds. They also attack other catch, particularly squids and cuttlefish, adding to the fishermen’s loss.

‘Prickly ball’

Belonging to the family Tetraodontidae, puffer fish is a mid- level carnivore characterised by four sharp, plate- like teeth and a spiny, loose-skinned rib-less body that can take in water to become a prickly ball. Because of these features, it is shunned as prey by most predatory fishes. Puffer fish are toxic for humans because of the presence of Tetradotoxin (TTX), a neurotoxin causing asphyxiation and death, in their body.

An assessment of the fish catch statistics in Kerala by the research team showed that catches of Tetradontids were meagre during the 1970s, 80s and 90s but increased sharply from 2006. Among the puffer fishes caught, the smooth-backed blow fish (L.inermis) was the major species, followed by the fat puffer (Arothron), and the porcupine fish (Diodon).

Analysis of stomach content of the major predators in the Arabian Sea ecosystem showed that the puffer was the preferred prey for the cobia and different species of catfish, both characterised by extraordinarily wide mouths, enabling them to gulp down a bloated puffer with ease.

Among the puffer fish predators, the cobia or Kingfish had come under severe fishing pressure lately, registering a decline of 44 per cent in catch from 2007. This decline was found to be strongly coincidental with the increase in puffer fish catch from the same year. The high percentage of puffer fish in the diet of cobias (between 8% and 36%) supports the inference that it is the sudden decrease in cobia biomass that has played a major role in the increase in puffer fish catches after 2007.

The study assumes that the drastic depletion of catfish population has played only a minor role in puffer fish catch increase. Other predators of the puffer fish like sharks, kingseer, and skipjack tuna are not likely to have exercised any influence over puffer fish biomass in the coastal ecosystem. The study, however, failed to establish the cascading effect of the increase in puffer fish biomass on the population of prey species such as anchovies and squids.

The researchers have called for a close watch on biomass changes to discern trophic changes in the Arabian Sea.