No. Fish do not urinate through their gills. Their kidneys make urine, which leaves through the urinary system. Gills do release much of the nitrogen waste—especially ammonia—for many fish, but that process is not urination.
How fish get rid of nitrogen waste
As fish use protein and other nutrients, their bodies produce nitrogenous waste. In many fish, much of it is ammonia, which can pass from the blood across the gills into the surrounding water. The gills are therefore a major excretion surface as well as a breathing organ; ammonia crossing them is not urine flowing through them.
The proportion handled by gills versus kidneys and other routes varies with species and conditions. The reviewed sources do not establish one percentage that applies to all fish.
What the kidneys do
Fish kidneys produce urine and help regulate water, ions, and acid-base balance. They also contribute to nitrogen-waste excretion, with the extent of that role differing among fish and circumstances. Gills and kidneys work together to maintain the animal’s internal balance; neither organ has just one job.
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How freshwater and marine fish differ
The water around a fish affects how much water and salt move across its body. Freshwater and marine teleosts face different osmotic challenges, so their urine patterns generally differ:
| Habitat | Osmotic challenge | Typical urine pattern | Role of gills |
|---|---|---|---|
| Freshwater | Water tends to enter the fish’s body from the less concentrated surroundings. | They generally produce more dilute urine to remove excess water. | Help regulate ions as well as exchange gases and excrete nitrogen waste. |
| Marine | Water tends to leave the fish’s body toward the saltier surroundings. | They generally conserve water and have lower urine flow. | Help regulate ions as well as exchange gases and excrete nitrogen waste. |
These are broad patterns, not rules that apply identically to every fish species. The Virginia Cooperative Extension overview of fish biology and the University of Tennessee fish physiology resource describe these habitat-related differences.
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Why the gills are easy to mistake for a urinary route
Both gills and kidneys help remove waste and regulate the fish’s internal environment. Because ammonia can leave across gill surfaces, it may look as though the fish is urinating there. In fact, the gills release dissolved waste into the water, while urine is made by the kidneys and released through the urinary system. The review of fish gill function in Physiological Reviews and a review of ammonia production and excretion in fish explain the gills’ central role in nitrogen-waste removal. The University of Florida IFAS guide to ammonia in aquatic systems likewise describes gills as the primary route for most fish ammonia, with smaller amounts leaving in urine or across other tissues.
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