The protection afforded to blue whales in New Zealand waters may need to be increased, now a study has found they may be more regular visitors to the South Taranaki Bight than previously thought.
The whales had been thought to pass through the Bight while migrating to and from summer feeding grounds in Antarctica.
But a study by the National Institute for Water and Atmospheric Research (NIWA) of whale sightings between 1979 and 1999, and two surveys in 2011, show the whales are a regular presence.
NIWA marine ecologist Dr Leigh Torres said the study linked the presence of the whales to the large amounts of plankton - perfect whale food - in the South Taranaki Bight.
"Conventional wisdom has been that blue whales only transit through New Zealand waters while migrating," Dr Torres said.
"But this new information suggests that this is not an accurate understanding of their ecology. Blue whales appear to be present in the South Taranaki Bight with some regularity and density, and based on their foraging patterns and the availability of their prey in the area, we think they are feeding here."
Blue whales, more than 20 metres in length and weighing more than 100 tonnes, were extensively hunted and remain only in small populations.
"Blue whales are huge and need to eat vast amounts of food, which are tiny plankton, to support their energy demands," Dr Torres said.
"But there are just four confirmed blue whale foraging grounds in the Southern Hemisphere outside of Antarctic waters. So, it's very important that we properly document and protect their foraging grounds."
Dr Torres said the blue whale is classified as a migrant species, meaning they are not afforded the same level of protection as other large whales in New Zealand coastal waters.
More work was needed to determine the scale and significance of the South Taranaki Bight foraging ground.
"The South Taranaki Bight is also the largest offshore natural gas and oil exploration area in New Zealand, with seven production platforms, considerable seafloor pipelines, and significant plans for expansion in the near future.
"Shipping traffic and seabed mining activities have been shown to impact blue whales directly - altering their behaviour and degrading their habitat through acoustic disturbance and ship strikes. We need to gain a better understanding of how and when blue whales forage here so that possible impacts can be avoided."