Swimming-with-the-whales tourism in northern Tonga is disturbing the natural behaviour of humpbacks, who are there to give birth, bond and nurse, new research shows.
The study by New Zealand's AUT University found tourism intensification was forcing mothers away from their calves as they dive to avoid tour boats.
The boats are also providing an unnatural attraction for whale infants.
The study found mothers uninterrupted by tourist boats spend more time feeding, teaching and bonding with their young.
AUT's Mark Orams, who was involved in the research and with cetacean study in Vava'u since the mid 1990's, said the industry had grown significantly since that time.
"What's unique about the Tongan situation is that, unlike most other whale watching destinations, they actually allow people to get in the water and snorkel with the whales, Professor Orams said.
"That is often a really wonderful experience for the tourist but does have the potential to disturb what is a really important part of the life cycle of Humpback whales."