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On the Edge of Extinction
From: Woods Hole Oceanographic Institution
| By:
Tracey Crago |
EDITOR'S INTRODUCTION |
The North Atlantic right whale is on the edge of extinction. According to models created by Woods Hole Oceanographic Institution (WHOI) population biologists Hal Caswell and his graduate student, Masami Fujiwara, under current conditions, the last members of this species will die by around 2202. They believe this fate can be avoided. Their models show that if the deaths of two females can be prevented each year, the population growth rate will rise above the critical level. WHOI communicator Tracey Crago describes the research of Caswell and Fujiwara and explains the implications of this work for the future of the North Atlantic right whale. |
 hey have been the subject of lawsuits, scientific research, public outcry and fisheries regulations, yet the plight of the North Atlantic right whale, Eubalaena glacialis, remains grim. If conditions remain as they are today, the northern right whale will become extinct in approximately 200 years, say WHOI population biologist Hal Caswell and his graduate student, Masami Fujiwara. But that doesn't have to be the end of the story: according to their survival probability model, extinction can be avoided. |
 The model and its take-home messages are big news for anyone following the right whale saga for the last two decades. To avoid extinction, the model shows that, at the 2002 population level (approximately 300), two female deaths must be prevented each year. That would increase the population growth rate to one, the critical level separating growth from extinction. The number of female deaths that must be prevented is directly proportional to the population level. |
"Our analysis indicates that the early-1980s population growth rate was greater than one, or increasing, though slightly," says Fujiwara. "Since that time, the population growth rate has declined, and populations started declining in the early 1990s. That means that to avoid extinction, we don't have to return to a situation [like that of] pre-whaling years; we just need to find out what happened between 1980 and now that is causing the whales to die," he explains. |
The two leading causes of death in northern right whales are ship collisions and fishing gear entanglements. These findings have resulted in a flurry of research and technological developments, along with fisheries management restrictions and shipping regulations. And legal battles. |
 The Massachusetts Division of Marine Fisheries (MDMF) settled a lawsuit waged by Richard "Max" Strahan, who accused MDMF of driving the northern right whale to extinction. In an April 12, 2001 Cape Cod Times article, Dan McKiernan, a MDMF lobster scientist, credited Strahan's lawsuit with accelerating the rate at which the state and federal government were working to reduce the impact of fishing gear and ship strikes on right whales. "Since 1996," the article states, "the state and federal governments have embarked on a multi-faceted program to save right whales. A critical habitat area was established in Cape Cod Bay. Survey flights now crisscross the bay pinpointing concentrations of right whales and warning fishermen to move their gear. New regulations required fishermen in areas where whales are present or migrating to alter their gear". |
MIT Sea Grant has supported the development of a high-tech buoy that can detect the distinctive right whale vocalizations at a range of about five miles. The device can be used to supplement current visual observation techniques, and may prove especially useful in poor visual conditions such as fog and darkness. With all of the attention on the northern right whale, the most endangered species of large whales, scientists wanted to determine whether or not the population had any hope of recovery. Caswell, Fujiwara and Solange Brault of the University of Massachusetts, Boston, conducted an initial study, published in 1999. The model employed rigorous statistical estimates of survival probability and found that, not only was right whale survival probability going down over time, but the population was declining as well. |
A 2000-2002 study, supported in part by WHOI Sea Grant, attempted to go a step further, by looking at the survival probability differences among sexes and life stages. To do this, Fujiwara and Caswell considered the differences between males and females, and classified the population by stages: calf, immature, mature and mature mother with calf. Next, they looked at the probability of each individual in the study making a transition from stage to stage, including death as a stage. As with the previous study, Fujiwara and Caswell used data gathered by the New England Aquarium (NEAq) as part of the North Atlantic Right Whale Consortium, to look at individual sighting history data. Their data includes individuals sighted between 1980 and 1997, for a total of about 370 individuals. Sightings are gathered primarily in Massachusetts and Cape Cod Bays, Bay of Fundy, Great South Channel, Brown's Bank and off the coast of Florida and Georgia. |
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| Using the bumps as identifiers, researchers have been able to photograph and catalog most of the approximately 300 Northern right whales in existence, registering them in a database kept by the New England Aquarium. | |
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The model is based on the mark-recapture statistical method, part of which was newly developed by Fujiwara with WHOI Sea Grant support. These methods can be applied to other populations for which individual sighting data are available. Fujiwara and Caswell use what are called "maximum likelihood" methods to estimate survival, transition probabilities, sighting rates and the response of these variables to environmental factors. This method, says Fujiwara, "lets the data speak for itself, rather than making us guess or make assumptions." |
The new study has yielded some specific details about the population. One important finding, says Fujiwara, is that "the survival probability of females who reproduce is declining." Reports of surging birthrates during the December 2000-March 2001 season could eventually factor into the model, first as females with calves, then the calves on their own. |
As for the application of their model to management efforts, Fujiwara acknowledges that there are many efforts underway to reduce ship collisions and entanglements but it will be important "to assess the effect of those management actions." This will require continued measurement of the population growth rate, which, he admits, is not an easy task. Any models that must contend with small population sizes--and thus a limited sample size-- require very sophisticated analysis, says Fujiwara. "We are dealing with very slight changes in survival probabilities and a small number of individuals," he says. |
As such, it means that 'not everyone' can do this type of model. Fujiwara is now among a relatively small circle of population biologists with the necessary tools, resources and know-how to perform such sophisticated models. In fact, it was his interest in mathematical population models, quantitative biology and statistics that brought him to the MIT/WHOI Joint Program in Oceanography in 1997, and to Caswell's lab in particular. Fujiwara graduated in 2002, and is now conducting postdoctoral research at the University of California, Santa Barbara, in the Ecology, Evolution, and Marine Biology Department. |
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