By Paul Rincon
BBC News Online science staff
US scientists say monitoring the North Atlantic climate can predict the birth rate of an endangered species of whale.
There may be only 300 individuals left
There are thought to be just 300 right whales left in the North-West Atlantic.
Atmospheric conditions above the ocean can affect zooplankton concentrations on which the whales depend for food, impacting their reproductive success.
The team developed a mathematical model to describe the relationship which it will report in the journal Frontiers In Ecology And The Environment.
In late winter, North Atlantic right whales (Eubalaena glacialis) make their way to the Gulf of Maine, where they feed on its high concentrations of copepods - crustaceans about the size of rice grains.
The concentrations of one abundant copepod species, Calanus finmarchicus, are linked to a pattern of atmospheric pressure called the North Atlantic Oscillation (NAO).
About 15 metres long
100 tonnes in weight
3 years between births
The NAO is said to be in either a positive or a negative state.
When the NAO has been in the positive state, the deep waters of the Gulf of Maine become warmer and saltier.
These conditions favour zooplankton, typically leading to higher abundances of these creatures.
Following a period of negative NAO conditions, the waters become colder and fresher - conditions that are less hospitable to zooplankton, leading to a fall in their numbers.
It takes about two years for these changes in the NAO to affect zooplankton concentrations.
The researchers found that flips in NAO and consequent fluctuations in abundances of zooplankton affected the birth rate of right whales.
Andrew Pershing of Cornell University developed the "transitional probability" model.
"We can explain about 65% of the variability of right whale calving rates using this model," said colleague Professor Charles Greene, of Cornell University, and lead author on forthcoming research papers on the subject.
A two-year period of physiological stress and poor reproduction in the whales between 1999 and 2000 can be traced to a dramatic negative flip in 1996 and a decline in copepod abundance in 1998.
The NAO has been in a predominantly positive phase since the 1970s. But this may be changing.
"There are tantalising indications that we may be shifting towards a more negative phase [in the NAO]," said Professor Greene.
At the end of 2002, there was a large negative shift in the NAO. The team have not yet analysed this data to see how it affected the zooplankton.
"But if there's anything to this hypothesis, we would expect right whale calving to be adversely affected," said co-author Jack W Jossi of the US National Marine Fisheries Service (NMFS).
On the brink
Mr Jossi added that he expected to see adverse effects on zooplankton concentrations by winter 2004.
"A female right whale needs to put on a lot of fat to give birth. The bigger you are as a whale, the better you can get through pregnancy," said Dr Robert D Kenney, of the University of Rhode Island.
Dr Kenney said that drops in the concentrations of the zooplankton that right whales feed on may affect fertility in several ways.
The whales are frequently involved in collisions with ships
Firstly, the whales may need to achieve a certain level of fat in order to physiologically support a pregnancy.
Secondly, when starved of food, they may not able to produce enough milk to support their calves - and the youngsters die.
Thirdly, lack of food may cause an increase in miscarriages. Females have a minimum three-year period between pregnancies.
The most recent figures show that the population growth rate is -2.4% per annum. Other researchers have predicted the extinction of the species in 200 years based on this current trend.
But Professor Greene said extinction could occur in a much shorter time if conditions worsen.
The whales have high mortality rates as well as their decreasing birth rates. They frequently collide with ships and become entangled in fishing gear.