How a fish floats at any depth

In the picture

1. 1Swim bladder near the surface: low pressure, so the gas spreads out into a large pocket
2. 2Water pressure at depth: the deeper the fish, the harder the water pushes on every side
3. 3Squeezed swim bladder near the bottom: compressed gas takes less space, so the fish would sink
4. 4Gas gland: blood vessels release gas into the bladder to refill it to its normal volume

Examples

• A diver’s buoyancy jacket must be inflated with more air on the way down and vented on the way up, just like a swim bladder.
• A sealed, half-empty plastic bottle carried down to the bottom of a deep pool gets crushed smaller, then expands again as it is brought back up.

Quiz: How a fish floats at any depth (Age 17)

1. 1

   As a fish swims deeper, what happens to the gas in its swim bladder, and how does the fish respond?

   • The gas is squeezed into a smaller volume, so the fish adds gas to the bladder
   • The gas expands into a larger volume, so the fish releases gas from the bladder
   • The gas is squeezed into a smaller volume, so the fish releases gas from the bladder
2. 2

   Compared with its volume at the surface, roughly how much space does a pocket of gas take up at about 10 m depth?

   • About a tenth of its surface volume
   • About the same as its surface volume
   • About half of its surface volume
3. 3

   What does it mean for a fish to be neutrally buoyant?

   • Its average density equals the water’s, so it neither sinks nor rises without swimming
   • It is slightly less dense than the water, so it drifts gently upward
   • Its swim bladder is filled to the largest volume it can hold

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.