A humpback whale and her calf in Tongan waters. Tonga's involvement with the Memorandum of Understanding is sought as it is considered the crossroads for oceanic humpback movements. Photo / Whale Watch Vava'u, Tonga

A humpback whale and her calf in Tongan waters. Tonga's involvement with the Memorandum of Understanding is sought as it is considered the crossroads for oceanic humpback movements. Photo / Whale Watch Vava'u, Tonga

The sight of a humpback whale breaching, only to crash beneath the surface ranks right up there among nature's greatest hits. Many humans identify with their winter lifestyle - wallowing in the tropics throughout their mating and breeding season, spouting forth.

Yet when, last year, the conservation status of these ungainly giants was downgraded from "threatened" to "least concern", New Zealand scientists were not among the cheerleaders.

They knew that the humpbacks which pass through our waters, a distinct sub-population, were far from thriving. Summoning up 20 years of data gathering and research, scientists succeeded in having oceanic humpbacks placed on the endangered list by the International Union for the Conservation of Nature (IUCN).

This week, as the Government was tossing out a bill to improve protection for marine mammals, representatives from a dozen Pacific countries met in Auckland to progress an agreement on the conservation of cetaceans (whales, dolphins and porpoises) in Pacific waters. The Memorandum of Understanding on Pacific cetaceans, agreed in 2006, is under the United Nations-backed Convention on Migratory Species.

The meeting, just the second of signatories to the Pacific MoU, adopted a fledgling recovery plan for Oceania humpbacks being developed by marine scientists. Blue and bottlenose dolphins are other cetaceans whose status in Pacific waters is causing concern, but the meeting's moves on the oceanic humpbacks highlights the scientific unease about their recovery.

New Zealand is in the middle of a migratory corridor for oceanic humpbacks which winter in the waters off New Caledonia, where they breed and rest before heading south to spend the summer gorging on Antarctic krill.

Once a common enough sight off our coast during their winter and summer migrations, humpback sightings remain rare more than 40 years after commercial whaling ceased.

That's not the case across the Tasman, where the genetically distinct east-Australian and west-Australian humpback populations are thriving.

The humpbacks of Oceania - several separate breeding stocks have been identified - are small populations, scattered over nine million sq km of ocean.

They are remarkably faithful - despite mingling with other humpbacks in Antarctic waters; most return to the same breeding grounds each year (although some island-hopping occurs).

Scientists call it maternal site fidelity - calves follow a migratory route learned from their mothers - which explains the development of distinct populations but at the same time makes them more vulnerable to threats.