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STORY: After vanishing more than a century ago, giant blue-grey sei whales are being spotted again off Argentina’s Patagonian coast.

The species was relentlessly hunted in the 1920s and '30s - but in the last 50 years, global bans on commercial whaling have helped them bounce back.

Biologists call the comeback a global conservation success but urge countries to protect whale moratoriums.

Biologist Mariano Coscarella leads research project Proyecto Sei.

The team has fit satellite trackers on some sei whales to map their migration patterns.

Coscarella says it’s taken decades for numbers to recover to an amount where the whales can be sighted again – which only started in recent years.

“What you have to bear in mind with this species is that they are migrant species and swim for long journeys. So, even though the species is seen in (a country’s jurisdictional waters), it impacts the regions where the species is found as it moves around. In the case of the sei whale, which we’re studying, populations are starting to bounce back as the moratorium is still in force.”

But he adds, if countries start to withdraw from whale moratoriums, hunting may start again, and that could lead to a delay of their recovery.

“It is a message of hope in the sense that when we all reach an agreement and protect the species, and we understand exploitation can’t be infinite nor Olympic, we can be successful in the recovery of these species."

The Japanese government announced on May 9 that it would add large fin whales to its list of commercial whaling species, which includes the sei.

The news comes five years after Japan left the International Whaling Commission, a global body that regulates the commercial hunt of marine mammals.