As the problem indicates that fishes are <em>below sea level, </em>you take into account that sea level is 0 feet, and from there, as they are below, you mark their positions as <em>negative integers.</em>
See picture below where fishes position is marked. You can see there that the fish that is in position-3 is the number nearest 0, so -3 is greater than -4.
The first fish has an integer of -4 and the other fish has an integer of -3. -3 is greater because -4 is one less of -3. If you put it on a line then -3 is closer to the 0 then -4.