A unique species of killer whale is disappearing from the Ross Sea, and New Zealand's Antarctic deep-sea fishing may be partly to blame.

A group of Italian and American researchers who visit Antarctica yearly have noticed a large drop in the number of killer whales, or orca, since 2002.

Sightings dropped from a high of 120 orca at one time in 2002-03 to 18 at one time last summer, and the number of days on which orca were spotted fell by about half.

The researchers, who published their work in the journal Aquatic Mammals, believe the change is linked to the rise of a multimillion trade in Antarctic toothfish, led by New Zealand and Korea.

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