Southern right whales, the slow-swimming gentle ocean giants that quickly became the main target of whalers, are making a steady comeback. Their New Zealand population crashed from about 30,000 animals to fewer than 40 mature females by the 1920s (mirroring a global collapse), but a 2000-strong, genetically distinct NZ population of tohorā is now roaming the Southern Ocean again.
Their remarkable recovery is driven by amazing mums, says University of Auckland biologist Emma Carroll, who leads a tohorā research project. “They have one calf about every three years. The babies grow about a metre per month and the mum loses a quarter of her body size in the first few months of nursing her young.”
Today, the greatest threats to tohorā are entanglement in fishing gear, noise pollution and ship strike, as well as the impacts of climate change on the abundance and location of prey.
Carroll and her colleagues spent three winters attaching satellite tags to whales in their main breeding grounds around the subantarctic Maungahuka Auckland Islands to track where they go to feed during summer.
Now, they have compared the locations of 29 tagged whales with those of vessels during the peak breeding period between June and October and year round. The overlap, particularly with fishing vessels, is high, Carroll says, even inside marine areas designated for protection. “These areas, which are important habitat for the whales but overlap with human activities, need to be considered as the whale population, as well as vessel fleets and offshore industrial activities, increase.”
One of the added long-lasting consequences of whaling is that the NZ tohorā population retreated to the subantarctic region during the winter breeding season. Whales of all ages favour the Auckland Islands for socialising, but some have ventured as far north as mainland New Zealand as the population slowly recolonises former wintering habitats.
Around the Auckland Islands, tohorā and other marine mammals are protected in an internationally recognised sanctuary. Their migration routes to feeding grounds also pass through other types of marine protected areas, including no-take reserves prohibiting any resource extraction and other regions where only some fishing methods are prohibited.
Carroll says evaluating the effectiveness of marine protected areas is challenging because marine mammals, and tohorā in particular, have large home ranges that can span entire ocean basins. The research aimed to assess how well these areas, with their set boundaries, protect a migrating species from ship traffic and other potential threats. The results show that only a small proportion ‒ about 10% ‒ of tohorā habitat is protected, and that even these areas are frequently used by ships.
As the population continues to recover, more whales are likely to spread beyond their winter breeding area to places where they could be more at risk, says research team member Leena Riekkola. The study confirmed that the Auckland and Campbell Island groups are the most important regions for socialising and breeding, but it also identified two previously unknown and currently unprotected areas southern right whales use for foraging or resting, she says.
Whales’ legal protection from human activities is insufficient, says Carroll. With the projected expansion of global fishing fleets, the overlap between shipping routes and migration paths of recovering tohorā populations will likely increase.
The team hopes the research will help inform the design of future marine protected areas as part of a global effort to protect 30% of the world’s ocean by 2030. “If protected areas are poorly designed or managed, they may not achieve their intended conservation outcomes.”
In parts of the ocean where whales and vessels can’t be fully separated, a more effective approach could be to add measures such as temporary closures and speed restrictions, which have helped Bryde’s whales avoid ship strike in Auckland’s Hauraki Gulf.
