Ships are warned of nearby whales Picture by New England Aquarium

The results could have important implications for the survival of the species.

The researchers found that the age profile of observed females had changed dramatically in the last 20 years.

In the past, females aged up to 50 years old were sighted; now, few live beyond the age of 15.

Right whales need to be at least 10 years old before they can reproduce, and only have a calf every three to five years. This means an animal dying at the age of 15 will only have had one calf - not enough to sustain the population.

Easy catch

The scientists say that if northern right whales are to survive, more effort needs to go into conservation measures, such as preventing ship collisions, rescuing whales from fishing nets, and even designing nets from which the whales can more easily break free.

Preventing the deaths of just a few females each year will allow the animals to breed more than once and hence slowly build up the population, they say.

Masami Fujiwara told the BBC that the "mandatory ship reporting system" introduced in 1999 was having a positive effect. After whales are sighted, warnings of their presence are sent to large ships in the area.

Whalers drove the animals to the brink of extinction because they were the right whales to kill.

Their slow speed in the water, their migration close to the coast, and the fact that they stayed afloat after being killed made Eubalaena glacialis an easy catch.