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How do fish float?

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Quick answer

Fish float mainly with a swim bladder, an internal gas-filled sac. By adding or removing gas, a fish changes its overall density until it matches the water, letting it hover at any depth without effort. Some fish gulp air to fill it, others release gas from their blood, and sharks use oily livers instead.

Most bony fish float at a chosen depth using a swim bladder, a gas-filled sac whose volume they adjust so their overall density matches the surrounding water. Fish without a swim bladder, such as sharks, rely instead on oily livers and the lift from swimming to avoid sinking.

Step by step

  1. A fish's muscles and bones are slightly denser than water, so without help it would gradually sink.
  2. Gas enters the swim bladder, either gulped at the surface or released from the blood by a gas gland.
  3. As the bladder inflates, the fish's average density falls until it matches the surrounding water.
  4. At that point upward buoyant force equals the fish's weight, so it hovers without swimming.
  5. To rise or sink, the fish adds gas to or removes gas from the bladder, resetting its buoyancy.

Key things to understand

  1. Fish tissue and bone are slightly denser than water, so a fish with no buoyancy aid would slowly sink.
  2. The swim bladder is a flexible gas sac in the body cavity that lowers the fish's average density.
  3. Adding gas expands the bladder and makes the fish rise, while removing gas lets it sink.
  4. Some fish gulp air at the surface through a duct, while others secrete gas from the blood using a special gland.
  5. Sharks lack a swim bladder and depend on a large, oil-rich liver plus lift from their fins while swimming.

Labelled diagram

  1. Swim bladder

    A gas-filled sac below the spine that lowers the fish's density.

  2. Gas gland

    Releases gas from blood into the bladder so the fish can rise.

  3. Oval

    A patch of blood vessels that reabsorbs gas so the fish can sink.

  4. Buoyant force

    Upward push of displaced water balancing the fish's weight at neutral buoyancy.

How a swim bladder controls a fish's depth

Real-world examples

  • Goldfish and common carp can gulp air at the surface because their swim bladder connects to the gut.
  • Rohu and catla in Indian fish ponds use swim bladders to hold position at different water depths.
  • Great white sharks have no swim bladder and rely on an oily liver and constant swimming to stay up.
  • Fancy goldfish with swim bladder problems may float awkwardly on their side near the tank surface.

Common mistake

✗ Myth: Every fish floats using an air-filled swim bladder.

✓ Fact: Sharks, rays and many bottom-dwelling or deep-sea fish have no swim bladder. They stay up using oily livers, fatty tissues, fin lift or simply rest on the seabed.

People also ask

How do fish control their buoyancy?
Most bony fish adjust the amount of gas in their swim bladder. More gas lowers their density so they rise, and less gas lets them sink. Some fish gulp or burp air through a duct to the gut, while others move gas in and out of the blood using a gas gland.
Why do fish not sink?
Fish tissue is slightly heavier than water, but a gas-filled swim bladder balances that extra weight. When the fish's average density equals the water's, buoyancy holds it steady. Fish without swim bladders, like sharks, rely on light oily livers and the lift created by swimming.
What does a swim bladder do?
A swim bladder is a gas-filled organ that lets a fish stay at a chosen depth without constant swimming. By changing the gas volume, the fish fine-tunes its buoyancy. In some species the bladder also helps with hearing or producing sounds, such as drumming noises made by croakers.
Why do dead fish float upside down?
After death, bacteria break down tissues and produce gas inside the gut and body cavity. The belly region holds most of this gas, and the heavier backbone sits above it unsteadily, so the fish rolls over and floats belly up.
What is swim bladder disorder in aquarium fish?
It describes fish that float sideways, sink or cannot hold their depth because the swim bladder is not working properly. Causes include constipation, infection and body shape, and it is especially common in round-bodied fancy goldfish.
Why must some sharks keep swimming?
Sharks have no swim bladder, so they are slightly denser than seawater even with oily livers. Forward motion over their pectoral fins and tail generates lift that stops them sinking, and some species also need swimming to push water over their gills.
Why do deep-sea fish die when brought up quickly?
As pressure drops, gas in a swim bladder expands. Fish hauled up fast cannot release gas quickly enough, so the bladder swells and can push organs out of place, an injury called barotrauma.

Part of the guide: Nature & Earth: Living Things and Rocks →