The South Taranaki Bight has New Zealand's only known blue whale foraging ground, Dr Leigh Torres says.

In 2015, the first year of a three-year survey, Dr Torres' group saw 33 blue whales. This year they saw 68 different individuals, in multiple sightings over nine days.

They saw five mother and calf pairs, and two male whales having a race - possibly to show off and attract mates.

The whales looked thinner than they should be, and Dr Torres doesn't know why. Many wore scars from predators or collisions with vessels.

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They will undoubtedly be affected if iron-sand mining goes ahead in the bight, Dr Torres told the Environmental Protection Authority hearing on seabed mining this week.

For whales, hearing is more important than sight. Sound from the iron-sand mining operation proposed by Trans-Tasman Resources could disrupt their actions or drive them away.

They feed on the marine crustaceans called krill, at depths of 70m-200m. Mining could increase the amount of fine sediment in the water and affect the presence of krill.

Mining would also increase the number of vessels in the bight. Vessel strike is another danger for the marine mammals.

Dr Torres' researchers have placed five hydrophones in the sea area between Farewell Spit and Cape Egmont. They record whale calls and help trace whale movements.

Recordings from January to July show a fairly constant whale presence, and Dr Torres expects to find the same for the rest of the year.

In January-February 2016 the sea was warmer than usual and more whales were sighted in the cooler water west of Farewell Spit. This year the water is cooler and the food supply is better in the central and eastern bight, Dr Torres said.

The closest whales to Whanganui were recorded 60km to 70km offshore. The closest sighting to the proposed iron-sand mining area was 20km from its edge.

The whales are pygmy blue whales. At 20m-21m they are slightly smaller than Antarctic blue whales - 28m.

Those offshore are related to Australian pygmy blue whales, but may be a separate and unique New Zealand population.

Blue whales were hunted almost to extinction in the 1800s. There are now estimated to be 10,000 to 25,000 worldwide. They are still listed as endangered.

There are lots more questions Dr Torres would like to answer about those in the bight, but getting more information depends on continued funding for her research.

She said the mitigation strategies in Trans-Tasman's mining proposal were incomplete and it would be naive to think 35 years of mining wouldn't affect the whale population.

It was also important to recognise the rights of the whales themselves "to live in their natural habitat without disturbance, to feed without compromise, and to communicate without disruption".

• To read about the research team's latest time in the South Taranaki Bight, go to Oregon State University's student blogs online.

• Watch Footage of a blue whale calf feeding from its mother, filmed last year by a drone.