Shark habits remain a puzzle at ‘twilight zone’ depths past 1000m

Apex marine predators such as great white sharks regularly visit the midnight zone more than 1000m below the ocean’s surface – but it’s a secret society as scientists don’t know why they do it.

Great white sharks have been discovered diving deeper than 1100m below the surface of the ocean.

A study published in scientific journal PNAS showed large marine predators, such as great whites, tuna and swordfish, made regular visits to the depths of the ocean.

The movements of 12 different types of predatory fish were tracked as they travelled 200m to 1000m below the ocean’s surface to the mesopelagic zone, or twilight zone, and even further to the midnight zone, which is 1000m to 3000m below.

Researchers said they didn’t know why the sea creatures travelled so deep into the ocean. They said the twilight and midnight zones were much deeper than the zones where the marine predators usually fed.

Great white sharks were recorded diving as deep as 1128m, while whale sharks were recorded diving 1912m. Swordfish went as far as 2000m below the surface.

Woods Hole Oceanographic Institution assistant scientist and study lead Dr Camrin Braun told LiveScience magazine the deep ocean must be an important habitat for ocean predators.

“It’s clear there are good reasons for these animals to dive deep, otherwise why would they all do it?” Dr Braun said.

The researchers’ findings showed there could be a link between the dives and the deep scattering layer (DSL), which is an ocean layer densely packed with small fish and other creatures.

The study suggested that large marine predators fed on the creatures in the DSL. But the reason some predators travelled even deeper than the DSL remained a mystery.

“There’s good evidence for some species or situations in which diving deep is clearly for foraging,” Dr Braun said.

“However, we also find several cases where we can pretty definitively say the use of the deep ocean is not for feeding — or if it is, it represents a totally different kind of predator-prey interaction.”

Dr Braun said the evidence suggested the twilight zone had more creatures than scientists first thought and that its role for predators needed to be better understood before humans were allowed to fish in it.

GLOSSARY

• marine predators: sea animals that hunt and eat other animals

• oceanographic: scientific study of oceans

• habitat: place where animals live

• densely: thickly or heavily

• foraging: looking for food

• definitively: say with certainty

• predator-prey interaction: the way animals hunt other animals

BASIC SHORT ANSWER 1

Fill in the blanks with NO MORE THAN 3 WORDS AND/OR A NUMBER

1. Scientists refers the place where great white sharks go to as a ...

2. There are huge fishes studied in a research including great white sharks, ...

3.How many species of predatory fish are studied in the research?

4.The deepest area is called ...

5.What type of fish can travel down to 1912 meters below the ocean surface?

6.The large marine predators dive deeper because there are plenty of small fish there as well as ...

7.The reason for dives deeper than the DSL zone of large marine predators is still a ...

8.It is proved that some marine predators are diving deep for ...

9.The other finding suggest another reason for diving deep in the ocean is to show various kinds of ...

10.Scientists should study more about the twilight zone before giving permission to ... to catch fish here.