Monday, October 6, 2008

Hawaiian seals genetically poor, declining in population

Hawaiian monk seals have the lowest genetic diversity of any endangered species, according to a new study.

And although the endangered seals seem to have been able to recover somewhat from severe hunting pressure a century and a half ago, the lack of genetic variation worries scientists as the seals continue declining in populations.

(Image: Monk seal mug. Credit: NOAA.)

Great genetic diversity is considered one of the hallmarks of a successful species—it means that within the population, on a genetic level there's an ability to adapt, whether to climate changes, diseases or other issues.

It means, for example, that within a genetically diverse larger population, there are at least some individuals with resistance to specific viral diseases, some resistant to some kinds of fungus attack, some capable of handling warmer climate conditions, and so forth. Thus no one threat will wipe out the entire species.

“Genetically depauperate species may have a reduced ability to mount an effective defense against pathogens or to adapt to environmental changes, thereby increasing the risk of extinction,” says the new study, “Extremely Low Genetic Diversity in the Endangered Hawaiian Monk Seal (Monachus schauinslandi).”

It was published in the Journal of Heredity by lead author University of Hawai'i graduate student Jennifer Schultz, along with Jason Baker, Robert Toonen and Brian Bowen, who represent the University of Hawai'i Department of Zoology and Hawai'i Institute of Marine Biology, and NOAA's Pacific Islands Fisheries Science Center.

The paper suggests that Hawaiian monk seals may always have been somewhat genetically short-changed, but that it's likely severe hunting pressure drove it into unprecedented conditions for mammals.

The seals populated the entire Hawaiian archipelago before the first humans arrived. Early Polynesians are believed to have killed off the seals in the main Hawaiian Islands fairly early. European sailors nearly wiped them out in the Northwestern Hawaiian Islands—they're easy to catch while hauled out to rest on the beach.

A single ship's voyage through the northwestern islands in 1859 took at least 1,500 seals. That one ship's take is more than entire population today. The 1890s populations apparently had disappeared entirely from the islands of Laysan and Midway. It may be that the species was only saved because some of the population is nearly always at sea.

The paper suggests the number of seals possibly dropped as low as two dozen—and that the 1,200 or so seals now living all descend from them.

Hawaiian monk seals have been so well studied that close to nine out of ten adult seals have been genetically sampled. The finding of the latest study on seals is this: “The Hawaiian monk seal sets a new standard for low genetic diversity in endangered species.”

Despite that, the seal population rebounded to close to 3,000 animals in the middle of the 20th century. And since then, despite amazing efforts by federal and state agencies, the population has been slowly declining. Divers have been removing trapped seals from drifting nets, shooing sharks away from their birthing beaches and erecting fences to protect seals from the loving human masses. In spite of that, the population continues to drop at a rate of about 4 percent annually.

“Although genetic factors may not be driving the current population trend, we cannot ignore their potential impact on future population persistence because species with higher genetic diversity have experienced compromised fertility, reduced reproductive rates, high juvenile mortality, and disease epidemics,” the paper says.

So what is causing the seal numbers to decline? The 2007 NOAA recovery plan for the seals suggests these are the major factors:

“• Very low survival of juveniles and sub-adults due to starvation (believed to be

principally related to food limitation) has persisted for many years across much of the


“• Entanglement of seals in marine debris has and continues to result in significant levels of

seal mortality

“• Predation of juvenile seals by Galapagos sharks has significantly increased

“• Human interactions in the Main Hawaiian Islands (MHI) including recreational fishery

interactions, mother-pup disturbance on popular beaches, and exposure to disease

“• Hawaiian monk seal haul-out and pupping beaches are being lost to erosion in the

Northwest Hawaiian Islands (NWHI), and monk seal prey resources in the NWHI may

have been reduced as a result of climate cycles and other factors

“• Potential disease outbreaks could have a devastating effect due to small population size

and limited geographic range” --

©2008 Jan TenBruggencate

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