Because it is so dark they can't see
it helps them locate a direct food source (the fish).
Dog
Dolphins chase fish, get their sense of direction, and maneuver the ocean through echolocation. Basically, like bats, they send out waves that bounce back and help them navigate, locate prey, and avoid obstacles. They can even use this to locate fish hiding in the sand.
To get energy rather than using sunlight because in the abyssal zone there is no sunlight to uses photosynthesis so there are chemosynthesis.
The cockroach nymph uses it feelers to find its way in the dark. It also uses them to locate food sources.
Bottle nosed dolphins hunt individually and as teams. They locate fish by using echo-location, and are fast and agile swimmers, so they can pursue the fish and and catch it in their teeth.
Echoes are used by dolphins to hunt for their prey in the deeps of the ocean to locate food. They make sort of clicking noises which bounce off the fish to let them know of how far away they are and how big the fish is.
To find food, whales (and dolphins) use echolocation, which uses sound waves to locate krill or fish. they filter krill out of the water
the dark
because their are scare in dark
They are attracted by the carbon dioxide you exhale.