Six Hypotheses
Hypothesis #1: The sea otters died of starvation.
What the Scientists Did:
They captured some sea otters and weighed and measured them.
They measured the time it took for otters to dive and return to the surface with an urchin or another type of food item. Then they compared their data with data that had been collected in 1970s and 1980s.
What They Found:
The otters were larger and heavier in the 1990s.
The otters took shorter dives in the 1990s.
Hypothesis #2: The sea otters died of a disease.
What the Scientists Did:
They looked for dead otters that had washed up on beaches so they could examine them.
What They Found:
They found very few dead bodies. If a disease had swept through the otter population, they would have expected more.
They found no evidence of disease in the dead otters that were examined.
Hypothesis #3: The sea otters died from toxic pollution.
What the Scientists Did:
They looked for sources of toxic pollution, including evidence of oil spills.
They looked for dead sea otters that had washed up on beaches so they could examine them.
They collected other animals from the sea otter food chain and tested them for pollutants such as heavy metals that would become concentrated at higher levels in the food chain.
What They Found:
They found very few, and small, sources of pollution in the Aleutian Islands and no evidence that an oil spill had occurred that had not been reported.
They found very few dead bodies. If pollution had killed a lot of otters, they would have expected more.
The bodies they did examine did not have high loads of pollution.
The animals lower in the food chain did not have high loads of heavy metals or other pollutants.
Hypothesis #4: The sea otters died as a result of human harvests of the otters themselves or from becoming entangled in fishing gear.
What the Scientists Did:
They looked at the records of subsistence harvests of sea otters by people who lived in Aleutian Island communities.
They looked at the records from people who fished around the Aleutian Islands to see how many sea otters had became entangled in their nets and died.
What They Found:
The harvest of sea otters by people in the communities was small, about 100 otters per year.
There were few reports of otters getting entangled in fishing nets or other gear.
Hypothesis #5: The sea otters did not die; they moved somewhere else.
What the Scientists Did:
After surveying all of the Aleutian Islands in 2000, they surveyed the Alaska Peninsula coast and all of the shoreline of Kodiak for sea otters.
What They Found:
Sea otter numbers had declined everywhere in this area.
Hypothesis #6: The sea otters were eaten by killer whales.
What the Scientists Did:
They asked the scientists who had been studying killer whales if they had observed killer whales attacking and eating sea otters.
They looked for other observations, by fisherman or people in local communities, of killer whales attacking and eating otters.
They looked for dead killer whales washed up on beaches so they could look at the contents of their stomach.
What They Found:
Killer whale scientists had observed only a few attacks over the many years that they had studied killer whales.
One attack was observed in the Aleutian Islands in 1992.
Nine more attacks were observed over the next several years.
One killer whale was found dead with the remains of five sea otters in its stomach along with nine harbor seals.
