On an island, there are three kinds of seabirds: puffins, eels and Arctic terns  Among them, eel is the main food for Arctic terns and puffins  In the decade from 1980 to 1990, the eel population dropped from 1 million to 500000
Strangely, in this decade, the number of puffins has dropped from 1 million to 600000, while the number of Arctic terns has dropped from 1 million to about 250000
Puffins eat less eels and eat stony fish or other fish instead, but Arctic terns do not do so