How Do Fish Breathe? Gills, Oxygen in Water, and Why Summer Fish Go Deep

Fish breathe with gills: water goes in through the mouth, passes over thin red folds packed with blood, and leaves under the gill covers, and on the way the oxygen crosses into the blood. That is the whole trick, and it runs on very little: a liter of water holds one twenty-sixth of the oxygen in a liter of air. Below: how a gill does it, which fish break the rule, how much oxygen water holds at each temperature, and what that means for a bass in a livewell in August.

A liter of water holds about 8 cm³ of oxygen; a liter of air holds 210. A fish lives on one twenty-sixth of what you breathe, and its gills still pull more than 80% of it out of the water.

How do gills work

Lift the gill cover on a bass and you see four red, feathery arches. Each arch carries rows of filaments, and each filament carries hundreds of tiny folds called lamellae. They are red because they are full of blood, flowing a hair’s width from the water. Oxygen crosses that thin wall into the blood; carbon dioxide crosses the other way. The blood then carries the oxygen around the body, the job your lungs do with every breath.

The clever part is direction. Water flows across each lamella one way, and blood flows through it the other way. Scientists call this countercurrent exchange. Because the blood keeps meeting water that holds more oxygen than it does, it keeps loading up along the whole fold. FISHBIO puts the result at more than 80% of the oxygen removed from the water passing through. It has to be that good: water carries about 8 cm³ of oxygen per liter against 210 in air, so a fish moves a lot of water for a little oxygen. That is why the gill covers of a resting bass never stop.

Two ways to move the water

Most fish pump. The mouth opens and the throat widens, drawing water in; the mouth closes and the gill covers flare, pushing it out over the gills. Biologists call it buccal pumping, and it is why a bass can hover motionless under a dock and keep breathing. Washington State University’s Dr. Universe column shows the same rhythm in a goldfish bowl.

Fast swimmers found a shortcut: swim with the mouth open and the water rams itself through the gills. Tunas are described as obligate ram ventilators; they cannot pump, so they never stop. Most sharks ram ventilate while cruising, and some can switch to pumping and rest on the bottom, like the draughtsboard shark in Do fish sleep?. About 20 of the 500-plus shark species cannot, the great white and mako among them. For those few, “a shark must keep swimming or die” is true.

Do fish have lungs? Can fish breathe air?

No lungs, with one famous exception. The lungfish of Africa, South America and Australia carry true lungs alongside their gills. When its pond dries, an African lungfish digs into the mud, seals itself in a cocoon and breathes air for up to two years. A few others cheat without lungs: mudskippers hold water in their gill chambers and take oxygen through their skin for about three and a half days on land, and the mangrove rivulus, a small killifish, lasts about two months in damp leaf litter.

A bass, a bluegill, a trout or a catfish can do none of that. Out of water the lamellae collapse and stick together like wet tissue paper, and oxygen uptake stops within seconds. That fact sits behind the 10-second rule further down, and behind Do fish drown?. Gills have a second job as well, letting water in and salt through, which is the story of Do fish drink water?.

How much oxygen does water hold?

The unit is milligrams per liter (mg/L), which in fresh water equals parts per million (ppm). Cold water holds more, warm water holds less. The saturation figures in the table come from the USGS DOTABLES equation for fresh water at sea level: the most oxygen water at that temperature can hold when fully aerated. A real lake usually runs below that line, and lowest of all just before dawn, when plants have spent the night using oxygen instead of making it (Water on the Web).

The fish thresholds come from Fondriest, citing EPA and state water-quality criteria. Warmwater fish, meaning bass, bluegill, crappie and catfish, want more than 5 mg/L, avoid water below 3 and die below 2. Trout and salmon want 6.5 and need at least 4; the USGS calls anything under 2 hypoxic. Carp survive below 1, and northern pike have been reported alive for days at 0.1.

Why fish go deep, or to the creek mouth, in a heat wave

Heat squeezes a fish from both sides. Warm water holds less oxygen, and a warm fish burns more of it, because its body runs faster. The University of Miami said it plainly in August 2023, when the Gulf of Mexico hit 88 °F (31 °C): a fish can, in effect, drown. Summer fish kills work the same way, often on a still morning after a hot spell, when the plants that made oxygen by day have used it up overnight.

So fish go where the oxygen is: a creek mouth or spring bringing in cooler water, a wind-blown bank where oxygen is mixed in from above, or deeper, cooler water. Up to a point: Water on the Web notes that in a stratified lake the bottom layer can run out of oxygen by late summer, so fish stack just above it. The water temperature charts show where each species sits in that stack through the year.

The angler’s part: a livewell that keeps fish alive

Tournament research funded by B.A.S.S., summarized in AFTCO’s Bass Care 101, gives three numbers. Keep the livewell at or below 84 °F (29 °C). Never change its temperature by more than 7 °F (4 °C) at once. Keep dissolved oxygen at or above 5.5 ppm. And the finding Bassmaster keeps repeating: a bass that fought for one minute survives the day in the well far better than one that fought for three.

1. Fill the well before your first cast and start the aerator, so the first fish lands in water that is already moving.
2. Run the aerator constantly, not on a timer. At 80 °F (27 °C) saturation is only 8.0 mg/L, and a bass in a closed box pulls that down fast.
3. Check the temperature. If the lake is above 84 °F (29 °C), add ice in bags or frozen bottles, and drop the well no more than 7 °F (4 °C). A fish dumped from 90 °F into 75 °F is shocked, not saved.
4. Land the fish fast. Heavier line, a firmer rod and a net save a minute of fight, the minute the research measured.
5. Keep air time under 10 seconds. Cook and colleagues (2015) reviewed the studies and found harm rising with every second out of water. Lift the fish only when the camera is ready.
6. Do not overcrowd. Five bass share the oxygen of one. If the well is warm and full, release the smallest fish where you caught them.
7. Release into oxygen. Let a tired fish go in moving or shaded water, facing into the current, and never drag it backward: water flowing the wrong way over the gills does nothing. The full routine is in catch and release.

The facts worth repeating

• A liter of water holds about 8 cm³ of oxygen; a liter of air holds 210. One twenty-sixth.
• A gill removes more than 80% of the oxygen from the water, because blood and water flow in opposite directions.
• 32 °F water holds 14.6 mg/L of oxygen; 80 °F water holds 8.0; 100 °F water holds 6.6. Bass want more than 5, trout 6.5; below 2, fish die.
• Livewell: at or below 84 °F, no jump over 7 °F, oxygen at or above 5.5 ppm, one-minute fights.

More answers with a number in each, from whether fish sleep to whether they feel pain, are in fish facts.