Explanation

If you are a seal, and you sense the presence of the shark, you definitely do not want to stay in the water column where you are more visible! You want to try to hide, to scan the water column, to understand if the shark is getting closer to be able to escape, to have a chance to survive. To do so, of course, you want to spend less time as possible at the surface, cause it's where you are most visible. At the same time, you do not want to travel too slowly in the water column, you want to get to your safe spot as soon as possible, therefore you increase both your descent and ascent rate. Once again, you want to stay at the bottom of the water column, cause it's where you have more visibility of your surrounding, and can easily escape!