What is going on?

This is a graph of a series of dives performed by Cassie the seal. Follow the black line as it goes down then comes back up.

Her dives start off being about three minutes long. She has a one to two minute rest between dives. At about the 20 minute mark, she starts to dive more. She has virtually no rest between dives. Then at the 30 minute mark she performs a very unusual dive.

During the unusual dive, Cassie dives to about 80 metres in depth and comes back up after about three minutes. She must need air because she has had very little rest time for the past 15 minutes. But even so, she turns around and dives back down to 80 metres. After more than six minutes of holding her breath, one of the longest dives she ever performed, she gets back to the surface.

After the unusual dive, Cassie spent about 15 minutes near the surface. Then she starts to dive again.

What do you think has happened to Cassie?

Some theories from our Scientists are:

            • she saw a white pointer shark and was trying to hide from it;

            • she was being chased by a shark;

            • she was chasing some big fish;

            • she was chasing behind a trawl net and trying to get fish out of the net.