MIAMI—Chinese squid vessels were documented using wide nets to illegally catch already overfished tuna as part of a surge in unregulated activity in the Indian Ocean, according to a new report by a Norway-based watchdog group that highlights growing concerns about the lack of international cooperation to protect marine species on the high seas. The report, published Wednesday by Trygg Mat Tracking, found that the number of squid vessels in the high seas of the Indian Ocean—where fishing of the species is not regulated—has exploded six-fold since 2016. The vast majority of the vessels sailing in the high seas off the coast of Oman and Yemen were flagged to China, whose overseas fleet, the world’s largest, has been dogged by accusations of illegal, unreported, and unregulated fishing around the world. Behind the surge is a lack of oversight and decades of overfishing that has pushed China’s overseas fleet—officially capped at …