Why You Should Never Feed Carp Bread: The Hidden Story of “Osmosis”!

I’m Ken Kuwako, your Science Trainer. Every day is an experiment.

The Wonder of Science Awaits You at the Park Pond!

On a beautiful day, when you casually stop by a park pond, you’ll see schools of brightly colored koi swimming serenely. As soon as someone approaches, they gather as if drawn by a magnet. Their sight is certainly endearing, and it’s tempting to reach for a snack to feed them, isn’t it?

However, my eyes were immediately drawn to a very ordinary warning sign standing right nearby.

“Please Do Not Feed the Carp”――.

Why Is Feeding Prohibited? The Profound Science Hidden Behind the Sign

Please do not feed bread to the pond birds or carp, as it contains too much salt.

This kind of sign is common. But this particular one contained detailed, specialized scientific knowledge. The dense text, written out on labels, looked more like a research paper than a public notice. Honestly, it might be a bit too complex even for a child to read. But that’s precisely why I was deeply impressed by its “sincerity” and the “passion for science and love for living things” it conveyed.

The salt concentration inside a fish’s body is maintained at about 0.9 percent. For freshwater fish, the internal concentration is higher than the surrounding water, so water is constantly entering the body due to osmosis. Excessive salt intake raises the internal salt concentration. Freshwater fish hardly drink water, but water still enters through their gills. Continuous water influx can cause cells to rupture, so they constantly excrete excess water as urine. This continuous excretion puts a strain on organs like the kidneys. Excessive salt intake is not good for humans or carp.

The Secret of Osmotic Pressure Hidden in the Koi’s Body

For fish, especially freshwater fish like those in the pond, the “salt” found in human foods like bread can become a serious, life-threatening problem. As the sign mentioned, the salt concentration inside a fish’s body is maintained at about 0.9%, similar to humans. However, the freshwater they live in has a salt concentration that is close to zero. The key phenomenon here is “osmotic pressure,” a concept you learn in science class.

Osmotic pressure is the force that drives water molecules to move from an area of lower concentration to an area of higher concentration across a semipermeable membrane (like a cell wall). Following this principle, the water in the surrounding pond (low salt concentration) is constantly “rushing into” the freshwater fish’s body (high salt concentration).

If a freshwater fish consumes too much salt, its internal salt concentration rises. This intensifies the osmotic pressure, causing even more water to try to enter the body. If cells absorb too much water, they swell up and there is a high risk of them rupturing. This occurs because the balance of water inside the body is fundamentally disrupted by the excessive salt intake.

So, how do freshwater fish adjust their internal water levels? They rarely actively drink water like we do, but water still enters through their gills during respiration. To cope, they maintain the balance of salt and water by constantly excreting the excess water as urine. This is similar to how our own kidneys regulate water levels. However, this continuous excretion process puts a significant burden on their kidneys and other organs. While excessive salt intake is bad for humans, it is the same for carp—or rather, for freshwater fish, it is a critical issue that can determine life or death.

Freshwater vs. Saltwater Fish: Opposite Survival Strategies

What makes this even more interesting is that the way freshwater fish and saltwater fish deal with osmotic pressure is completely opposite. They have evolved surprisingly rational biological mechanisms to survive in their respective, opposing environments.

• Saltwater Fish: The ocean’s salt concentration (approx. 3.5%) is higher than their body fluid (approx. 1%), so osmotic pressure constantly causes water to escape from their body. To prevent dehydration, they actively drink seawater and excrete the excess salt through their gills, conserving water.
• Freshwater Fish: The pond’s salt concentration is lower than their body fluid (approx. 1%), so fresh water is constantly entering their body. To expel this excess water, they rarely drink water, take water in through their gills, and excrete large amounts of dilute urine to regulate their fluid balance.

It is truly amazing that such a profound, grand mystery of life and a rational world of science can unfold from a simple sign in a park. Exploring the wonders of science through everyday phenomena is the best way to learn!

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