Every year gray whales migrate from the Bering Sea in Alaska to the Baja Peninsula in Mexico.
Eleven gray whales have died in the San Francisco Bay in the last three weeks．No one knows why the whales are dying.
Last year 270 whales died along the whales'migration route．Many people think starvation is the cause．This year the dead whales seem to have more blubber(fat)on them.
Twenty years ago,the gray whale was listed as an endangered species.
Some scientists think that the larger number of whales makes it hard to find enough food．More whales create more pressure on the food supply,a supply that some scientists say may have dwindled as a result of the warm waters of El Nino.
Most of the whales have been dead for many days before they are found and studied．This makes it hard to find the reason for death.
65 whales have been seen in the San Francisco Bay Area this year compared to 17 last year．The whales could be dying from many diseases,but it may be over-population．
Why might the increasing number of whales cause problems with their health?
由倒数第三段可知。
