With a population that hovers around 1,100, most of the world's silvery, seven-foot-long (two-meter-long) monk seals live on the uninhabited northwest Hawaiian Islands, in the Papahānaumokuākea Marine National Monument.

They've lived there for as long as Western history records, said Charles Littnan, lead scientist with the U.S. National Oceanic and Atmospheric Administration's Hawaiian monk seal research program in Honolulu.

But their population has been in decline for the past 30 to 40 years. Scientists are still trying to figure out why.