As spearfishers, freedivers and coastal foragers, we often think about habitat, visibility, and seasons – but buoyancy biology rarely gets the attention it deserves. Whether a fish floats after being caught or sinks like a stone is not luck, it’s linked to one of the most important internal organs in the marine world: the swim bladder.
Some species have them, some don’t, and some have highly modified versions. These differences shape how they swim, hunt, escape, hold depth, and even how they behave after being speared. Understanding buoyancy can help you identify species more accurately, improve shot placement, read underwater movement, and recover fish safely and efficiently.
What Is a Swim Bladder?
A swim bladder (also known as a gas bladder or air bladder) is an internal, gas filled organ used to control buoyancy, allowing fish to maintain neutral buoyancy at different depths without constantly swimming. It works by adjusting gas pressure inside the organ to offset the fish’s weight versus water displacement, a bit like built-in trim weights for a diver.
Fish with a functioning swim bladder can hover, glide, and stay mid-water, while fish without one must keep swimming or rest on the seabed to avoid sinking.
UK Fish That HaveSwim Bladders (Buoyant / Neutral-Buoyant)
Many pelagic (open water dwelling) and schooling species have fully developed swim bladders. Common UK examples include:
- Bass
- Pollock & coalfish
- Wrasse species
- Bream species
- Cod, haddock & whiting family
Spearfishing relevance: These species often hover confidently, make controlled turns, and are more likely to stay mid-water or above structure.
UK Fish That Lack Swim Bladders (sink immediately)
Some species rely purely on muscle power, oil content, body shape or seabed behaviour rather than buoyancy control, meaning they sink rapidly when motion stops. Common UK examples include:
- Flatfish: plaice, flounder, dab, sole, turbot, brill
- Mackerel
- Sharks, rays & skates
Spearfishing relevance: These species are often bottom-hugging, camouflaged, ambush based, and require close visual scanning rather than mid-water hunting.
Pros & Cons: With vs Without a Swim Bladder
| Feature | With Swim Bladder | Without Swim Bladder |
| Buoyancy Control | Excellent | None or very limited |
| Typical Habitat | Mid-water / kelp line | Seabed or open water hunters |
| Energy Use | Efficient cruising | Higher constant muscle effort |
| Predator Evasion | Agile gliding & turning | Speed bursts & camouflage |
| Shot Reaction (Spearfishing) | Higher chance to float or suspend | Drops fast – risk of loss |
| Pressure Vulnerability | Can suffer barotrauma | More pressure-tolerant |
Why It Matters to Spearfishers
Understanding buoyancy means you can predict fish behaviour and plan your dive:
- Know where to search – mid water vs sand/reef
- Choose correct approach – glide vs scan
- Expect post shot behaviour
- Swim bladder species may float or suspend
- Non-swim bladder species often sink instantly, so secure quickly
- Swim bladder species may float or suspend
- Improve safety – Understanding buoyancy helps prevent risky or unnecessary deep dives to recover fish.
Final Thought
Buoyancy biology isn’t trivia, it’s underwater intelligence. Recognising whether a species relies on a swim bladder can help you locate, identify, target and recover fish more safely and cleanly, while also appreciating how different species are adapted to their niche in UK waters.