How do you find out how many seals live in the Thames Estuary?
Wildlife experts have been counting how many harbour seals live in the River Thames.
The Zoological Society of London does the survey every year and it's found that the number there has doubled this time.
But how do you count seals? They used a plane to count them from the air of course!
If you like seals, try these:
Seals who can sing 'Twinkle Twinkle Little Star'