Climate events linked to reproduction of one of the most endangered marine mammals

The highly endangered North Atlantic right whale population is facing a difficult journey to recovery. That recovery may become even more precarious if North Atlantic climate takes a turn for the worse, according to Cornell University ecologists.

Cornell scientists say that winter atmospheric conditions over the North Atlantic affect the abundance of zooplankton eaten by right whales, one of the most endangered species of marine mammal. New models developed by these scientists can be used to explain the relationships among climate changes, atmospheric temperatures and winds; patterns in ocean currents, water temperature and salinity; the food resources required by whales and other animals; and the reproductive success of right whales.

Details of the whale-climate studies are reported by Charles H. Greene and Andrew J. Pershing, of the Cornell Ocean Resources and Ecosystems Program, in an article entitled “Impact of Climate Variability on the Recovery of Endangered North Atlantic Right Whales” to appear in the December 2003 issue of Oceanography. Other authors of Oceanography paper are Robert D. Kenney of the University of Rhode Island and Jack W. Jossi of the National Marine Fisheries Service (NMFS). A second article, “Climate and the Conservation Biology of North Atlantic Right Whales: Being a Right Whale at the Wrong Time?” will be published in the February 2004 issue of the journal, Frontiers in Ecology and the Environment .

As Pershing explains: “Only about 300 North Atlantic right whales remain, and their reproductive health depends on finding enough food. Starting in late winter, right whales make their way to the Gulf of Maine, where they feed on high-density patches of copepods (free-floating crustaceans roughly the size of rice grains), and just like New England weather, the physical conditions in the Gulf of Maine can be highly variable.” An earlier study by Pershing and Greene found that temperatures in the deep waters of the Gulf of Maine are closely linked to changes in ocean currents driven by winter atmospheric conditions in the North Atlantic during the previous year. With an additional time lag, the abundance of the common copepod species, Calanus finmarchicus , tracks these ocean temperature fluctuations. From these relationships, Pershing and Greene were able to develop a computer model to predict the number of right whale births from the observed number of Calanus and then relate these predictions to changes in North Atlantic climate.

“An important indicator of winter atmospheric conditions over the North Atlantic is the North Atlantic Oscillation (NAO), which is related to the average position of the jet stream over the Atlantic,” Pershing explains. “When the NAO is in its ’positive’ state, conditions over the Labrador Sea in the northwest Atlantic are colder and stormier, while the northeast Atlantic and northern Europe experience warmer and milder weather. When the NAO is in its ’negative’ state, the conditions reverse. After a winter of positive NAO conditions, the deep waters in the Gulf of Maine typically become warmer and saltier — leading to higher abundances of zooplankton. After negative NAO conditions, the waters become colder and fresher — not as hospitable for the food that whales need to eat.”

For example, Pershing and Greene report right whales suffered through two years of physiological stress and poor reproduction in 1999 and 2000, a setback that can be traced to a dramatic negative “flip” in the NAO in 1996 and a subsequent decline in copepod abundance in 1998. Then a sudden return of the NAO to positive conditions from 1997 to 2000 led to a 10-fold boom in copepod populations. Consequently, the northern right whales produced more calves in 2001 than at any time since records began in 1980.

During the 1980s, the NAO was predominantly positive, leading to warm conditions in the Gulf of Maine as well as plenty of Calanus and stable feeding conditions for the whales. After 1990, the NAO became more variable, causing water temperatures and Calanus populations in the Gulf of Maine to fluctuate. At the same time, the number of right whale births became highly variable, ranging from 22 births in 1996 to only a single birth in 2000, according to aerial surveys over calving grounds and feeding areas. (To determine the density of zooplankton food resources such as copepods, Greene and Pershing have relied on Continuous Plankton Recorder data collected by NMFS for nearly half a century.)

The Cornell scientists are still pondering all of the interacting factors in whale reproduction. Right whale physiology requires at least three years between births (a full year of lactation after the previous birth, another year to eat heartily — if they can — and amass fat stores for the next pregnancy, then a yearlong gestation), Pershing observes. “We think we’re seeing a one- to two-year time lag until poor feeding conditions take effect, as well as a slightly longer time lag from when changes in the NAO occur to when they have their effect on copepod abundance,” Green says, “and that gives us some confidence in predicting whale births a year or more in advance.”

Longer-term predictions of the right whales’ fate would be easier in an era of unchanging climate — which is rarely the case, especially in recent years. “Rather, we could be heading into a period of increased climate variability as a result of continually rising greenhouse gas concentrations,” Greene says. And 1996, when the NAO flipped even as right whales were enjoying a banner birth year, “could have been an unusual event — or a sign of the larger swings in climate that we might expect in the future.”

Development of the computer model by the Cornell ecologists was funded by the Northeast Consortium’s Right Whale Program. Right whales were so named by 18th and 19th century whalers because they were the “correct species” to hunt for their monetary value, slow swimming speed, and tendency to float after being harpooned. Nearly exterminated by uncontrolled hunting, right whales have been protected by international treaties since 1946.