Some deep-sea fish have swim bladders; others have reduced or no bladder at all. The answer varies by species and depth context, so neither “all deep-sea fish have one” nor “none do” is accurate.
What a swim bladder does
A swim bladder is a gas-filled organ that helps fish control buoyancy and stability in the water column. NOAA describes this function for fish generally; it does not mean that every deep-sea fish has the organ. NOAA: Swim bladder
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Which deep-sea fish have one?
The pattern differs among species. In a 1958 account, researchers E. J. Denton and N. B. Marshall noted that the upper reaches of the deep ocean include bathypelagic fish with capacious gas-filled swim bladders, while numerous species living within and below that region have bladders that are markedly regressed or absent. That description does not establish a universal depth cutoff or the proportion of all deep-sea species with bladders. Denton and Marshall, “The buoyancy of bathypelagic fishes without a gas-filled swimbladder”
How a bladder-bearing fish manages pressure
Pressure does not rule out a swim bladder in every deep-sea fish. Some bladder-bearing fish have a closed, or physoclistous, bladder rather than an open duct to the gut. Specialized anatomy, including a gas gland and rete mirabile, supports gas exchange and helps maintain a pressure difference in the bladder. This describes one bladder design, not all deep-sea fish. Springer: Swimbladder anatomy
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How bladderless fish stay buoyant
Fish without swim bladders can use other traits to reduce sinking or maintain position. A NOAA Central Library summary of a 1971 comparison by Blaxter, Wardle and Roberts associates bladderlessness in the mesopelagic fish studied with soft bodies and high water content; the authors considered skeletal reduction and high water content apparent buoyancy mechanisms. A specialist chapter on buoyancy at depth also discusses lipid accumulation and hydrodynamic lift. These are documented or discussed strategies, not a single recipe shared by every bladderless species.
Measurements from one mesopelagic comparison
In the specific comparison summarized by NOAA, fish without swim bladders had reported water content of 88–95%, compared with 70–83% for the species that had bladders. Reported hematocrits were 5–9% in the bladderless fish and 14–35% in the bladder-bearing fish. These ranges belong to that study’s comparison; they are not universal values for deep-sea fish or diagnostic rules for an individual fish. NOAA Central Library, Mesopelagic Fishes bibliography (2021)
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Further reading
For a specialist treatment, Elsevier lists Deep-Sea Fishes, Fish Physiology Volume 16, a 1997 reference volume that includes Bernd Pelster’s chapter “Buoyancy at Depth” and a section on swim-bladder function. Elsevier: Deep-Sea Fishes, Fish Physiology Volume 16
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