A population surge in pufferfish in the Arabian Sea has been blamed on overfishing of the top predators.
The puffers are reported to be causing huge problems off the coast of Kerala by tearing fishing nets to shreds and attacking the catch inside them, particularly squid and cuttlefish. The problem is particularly bad in the post-monsoon period (October-January).
The species which has become most abundant in the region is the Smooth-backed blowfish (Lagocephalus inermis), but puffers from the genus Arothron and Diodon have also increased in number.
A study by scientists at the Central Marine Fisheries Research Institute, attributes the population boom to the loss of predators such as the Kingfish (Rachycentron canadum), due to overfishing. Puffers are the preferred prey of this species, which can reach a length of 2m and can swallow even an inflated specimen without any problems.
Unfortunately the Kingfish is considered a delicacy and carries a high price, making it a major target for fishermen. There has been a decline of 44% in Kingfish catches from 2007, coinciding with a steep increase in pufferfish catches - from very few before 2006 to almost 2000 tonnes by 2011.
The number of Arius catfish - also a pufferfish predator - has also declined, and although its consumption rates for puffers are much lower than that of the Kingfish, there were many more of them out there. However, since 1985 it has had the status of a collapsed stock. Scientists think the depletion in numbers of these catfish would have played only a minor role in the increase of puffers, as it happened much earlier.
Scientists were unable to ascertain whether the increase in pufferfish numbers is also due to a 70% decline of shark numbers, as they had no direct evidence of these predators consuming puffers in the Arabian Sea.
The report published in Current Science says that there are signs of the beginning of a trophic cascade in the Arabian Sea resulting in increased biomass of pufferfishes from 2007. A trophic cascade is triggered by the addition or removal of top predators in an ecosystem, which then causes reciprocal changes in the relative populations of predator and prey through a food chain, often resulting in dramatic changes in the structure of the ecosystem.
For more information see the report in Current Science: Puffer fish menace in Kerala: a case of decline in predatory control in the southeastern Arabian Sea.
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