Greenpeace raised the alarm as its flagship Rainbow Warrior arrived in Sri Lanka at the end of a two-month expedition in the Indian Ocean to monitor tuna fishing and poaching in the region.
"The monitoring of tuna fisheries must be strengthened," Greenpeace said in a statement, adding there was an "overfishing crisis in the Indian Ocean".
The Rainbow Warrior had set sail from Durban, South Africa, in early September and travelled to Mozambique, Mauritius, the Maldives and Sri Lanka.
"The lack of proper management and pirate fishing in the region is contributing to the demise of albacore and bigeye tuna and sharks that are targeted for the lucrative sashimi and shark fin trade," Greenpeace said.
It said local communities, who depend on fishing for their livelihoods, would be severely hit unless strong action is taken to curb overfishing in the region.