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Swim bladders, buoyancy and barotrauma

Explainer · Last reviewed 2026-08-10

A swim bladder is a gas-filled organ that expands as pressure falls, and a fish hauled up quickly from depth cannot deflate it fast enough to swim back down.

Most bony fish control buoyancy with a gas-filled swim bladder. At depth it is compressed; on ascent it expands. A fish rising under its own power adjusts gradually, and species vary in how quickly they can do it.

A fish hauled up on a line has no such option. The gas expands faster than the fish can absorb or vent it, and the consequences are mechanical: the bladder over-inflates, the stomach may be pushed out through the mouth, the eyes may protrude, and the fish is left positively buoyant and unable to submerge.

This is barotrauma, and its practical significance is that a released fish floating at the surface will not survive. It will drift, be taken by a bird, or die of exposure. Releasing it has achieved nothing.

The effective remedy is a descending device: a weighted release that carries the fish back down to a depth where the gas recompresses.

Which fish it affects and how badly

Physoclistous fish — those whose swim bladder has no duct to the gut — cannot burp out expanding gas and are the badly affected group. Most of the reef and bottom species in this catalogue are in it: rockfish, groupers, snappers, sea bass, hapuku.

Physostomous fish retain a duct connecting bladder to gut and can vent gas directly. Salmonids, pike, carp and their relatives are in this group and cope substantially better.

Some fish have no swim bladder at all — sharks, rays and most flatfish, along with fast pelagic species like mackerel and tuna — and barotrauma in the swim-bladder sense does not apply to them.

Depth matters non-linearly. The proportional pressure change is greatest in the shallowest water, so a fish brought from twenty metres to the surface experiences a larger relative change than one brought from sixty to forty.

Scientific evidence

Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.

  • FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species records of swim bladder type, depth range and habitat that determine barotrauma susceptibility.States this directlyPassage read and verified
  • The Fishes of the British Isles and North-West Europe — Macmillan, 1969 (Reference work)Relied on for: Anatomical descriptions of swim bladder structure across fish families.States this directlyCitation checked — the work exists and is described correctly

Descending devices and the case against venting by eye

A descending device is a weighted clip or inverted hook that takes the fish down and releases it at depth, allowing the gas to recompress. The research on this is reasonably clear and it consistently favours descending over venting for survival.

Venting — puncturing the swim bladder to release gas — is still practised, and done correctly by somebody trained it can work. Done by guesswork it punctures the wrong organ, and the anatomy is not obvious from outside. FishingHQ's position is to prefer a descending device, which requires no anatomical judgement.

The decision worth making before fishing deep water is what happens to released fish. If no descending device is aboard and the fish must go back, the honest conclusion is that fishing that depth for a species that will be released is not a workable plan.

Authoritative guidance

Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.

  • Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Catch-and-release research covering post-release survival, injury mechanisms and mitigation.States this directlyCitation checked — the work exists and is described correctly
  • NOAA Ocean Service tides and water levels educational tutorial — National Oceanic and Atmospheric Administration, United States (Government fisheries body)Relied on for: Government marine education material on pressure at depth.Background and mechanism onlyPassage read and verified

Where to go next

Fish this applies to

What these connections rest on (1)

Practitioner consensus — 1 connection

Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.

  • Black seabream

    A record joined to the mechanism that explains it, which is what makes a species page transferable rather than a list of facts.

    • Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK and European sea angling practice as reported by practitioners, including ground, seasonal timing, bait and method convention.States this directlyCitation checked — the work exists and is described correctly
    • General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated descriptions of species behaviour and regional angling practice, recorded as consensus rather than as scholarship.Background and mechanism onlyCitation checked — the work exists and is described correctly

Where it matters

What these connections rest on (1)

Scientific evidence — 1 connection

Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.

  • Deep-drop fishing

    Deep-drop fishing makes barotrauma unavoidable and release conditional.

    • FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species and habitat data underlying this connection. Deep-drop fishing makes barotrauma unavoidable and release conditional.States this directlyPassage read and verified

Related reading

What these connections rest on (3)

Scientific evidence — 3 connections

Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.

  • Depth as an environmental variable

    Barotrauma is a direct consequence of pressure change with depth.

    • FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species and habitat data underlying this connection. Barotrauma is a direct consequence of pressure change with depth.States this directlyPassage read and verified

  • What actually happens to a released fish

    Release survival for deep-water species depends on recompression.

    • FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species and habitat data underlying this connection. Release survival for deep-water species depends on recompression.States this directlyPassage read and verified

  • Wreck fishing

    Barotrauma in fish raised from depth is established physiology.

    • Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Treatment of depth, pressure injury and post-release survival in recreational fisheries.States this directlyCitation checked — the work exists and is described correctly

Sources for this page

  • FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Species records of swim bladder type, depth range and habitat that determine barotrauma susceptibility.States this directlyPassage read and verified

How FishingHQ labels evidence is explained in the evidence policy.

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