Why would a saltwater fish die in freshwater?

Saltwater fish have evolved to maintain a delicate balance of salt and water within their bodies, known as osmoregulation. When placed in freshwater, saltwater fish encounter a hypotonic environment, meaning the concentration of dissolved substances (mostly salts) in the water is significantly lower than that within their bodies.

Diffusion and Water Uptake:

- Osmosis is the process by which water moves across a semipermeable membrane, such as the fish's gills, to equalize the concentration of dissolved substances on both sides of the membrane.

- In freshwater, the lower external salt concentration creates an osmotic imbalance. Water from the environment rapidly enters the fish's body through the gills due to the concentration gradient.

Ion Regulation Disturbance:

- This influx of water causes the fish's internal salt concentration to become diluted. The fish's cells become engorged with water, disrupting cellular functions, including proper nerve and muscle function.

- Additionally, the loss of essential ions, such as sodium, potassium, and chloride, disrupts the fish's ability to maintain its internal electrolyte balance.

Swelling and organ failure:

- As the fish continues to absorb water, its body swells. The rapid change in size and the internal disruptions can lead to organ damage, including the gills, kidneys, and liver. The fish may experience difficulty breathing, reduced mobility, and eventually death if the osmotic imbalance is not corrected.

Electrolyte Imbalance and Ion Toxicity:

- The dilution of essential ions also affects the fish's ability to maintain proper electrical gradients across cell membranes. This can disrupt nerve impulses and muscle contractions, leading to paralysis and respiratory distress.

- Moreover, the low concentration of certain ions in freshwater can be toxic to the fish. For instance, low levels of calcium can interfere with muscle function, while low magnesium can affect the heart and nervous system.

In summary, saltwater fish die in freshwater primarily due to the osmotic stress caused by the difference in salt concentrations between their internal environment and the surrounding water. The influx of water and loss of essential ions disrupt cellular functions, organ failure, and ultimately death if the fish cannot adapt to the freshwater environment.