Harri Deacon of Balclutha Primary School asks :-

Why do sharks sink when they stop swimming?

Jean McKinnon, a marine zoologist at Otago University, responded.

Sharks like most fish, are heavier than seawater so they sink. The reason they don’t sink when they swim is because they have very well designed fins. The pectoral (side) fins are designed rather like airplane wings to give lift as the animal moves forward (sharks can’t swim backwards!). This is hard work so to help the shark save energy they have a couple of weight saving features. Firstly their skeleton is made of cartilage not bone. Cartilage (the stuff your ears and nose are made of) is much lighter than bone and secondly they have an enormous liver! The liver of a shark can make up over a third of the sharks body and it is filled with oil. Oil is lighter than seawater so it helps the shark stay up in the water.

Other fish are also heavier than water but they don’t sink when they stop swimming (unless they want to). This is because they have a special gas filled balloon in their body called a swim bladder. By controlling how much gas is in the swim bladder the fish can sink or float as required.

Some fish live on the bottom and are designed to do so, even some members of the shark family, like rays, skates and carpet sharks. They have their mouths on the bottom so they can scoop up their prey easily and they can pump water over their gills. Many other sharks, like the white pointer or mako must keep swimming with their mouths open to force water in and over the gills, so they never stop swimming.