A Still Pond and Gasping Koi: A Story of Life Below the Surface
In the tranquil gardens of Taipei, where the scent of orchids lingers and the air hums with the rhythm of cicadas, the pond in front of the botanic garden shimmered like glass. Beneath the surface, a mass of koi glided flashes of gold, amber, and crimson weaving between ripples and bubbles.
But there was something unusual. The koi, instead of cruising lazily through the water, clustered near the surface, mouths gaping repeatedly in what looked like gasps for air.
This wasn’t just feeding behavior. It was a silent biological cry for help.
Koi, like all fish, rely on dissolved oxygen in the water to breathe. Their gills extract oxygen molecules from the water as it passes over delicate filaments. Under normal circumstances, koi are content near the bottom or mid-layers of a pond, surfacing only occasionally to feed.
However, when oxygen levels in the water drop—especially in warm, stagnant ponds with a high density of fish—koi are forced to the surface, where oxygen is slightly more abundant due to air-water exchange. This behavior is called surface gasping, and it’s a classic sign of hypoxia—low dissolved oxygen.
Several biological and environmental factors could be at play here:
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High stocking density: A large school of koi in a confined space consumes a significant amount of oxygen, especially at night when plants and algae also switch to respiration and compete for oxygen.
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Warm temperatures: Warmer water holds less dissolved oxygen. In summer, especially in tropical or subtropical climates like Taipei's, ponds heat up quickly, reducing available oxygen even further.
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Poor circulation: Without proper aeration or water movement, oxygen does not circulate well, and the lower layers of the pond can become hypoxic or even anoxic (completely lacking oxygen).
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Water quality issues: Accumulation of waste from the koi (ammonia, nitrates) and decaying plant material can lead to eutrophication—a nutrient overload that triggers algae blooms. These blooms, when they die off, consume massive amounts of oxygen during decomposition.
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