Why do salt water fish die in fresh water?

Saltwater fish are adapted to live in a high-salinity environment, where the concentration of dissolved salts (primarily sodium chloride) in the water is much higher than that of their body fluids. This difference in salt concentration creates an osmotic gradient, which drives water movement across the fish's gills and other membranes.

In freshwater, the salt concentration is much lower, so the osmotic gradient is reversed. Water moves from the fish's body into the surrounding water, causing the fish to swell and eventually leading to cell damage and organ failure.

To counteract this, saltwater fish have evolved several adaptations that help them maintain their internal salt balance:

1. Specialized Gills: Saltwater fish have specialized gill structures that are adapted for efficient ion exchange. These gills contain densely packed lamellae (thin, plate-like structures) that increase the surface area for water and ion exchange.

2. Active Salt Transport: Saltwater fish actively transport ions, such as sodium and chloride, from the surrounding water into their body fluids. This transport process requires energy and is mediated by specialized ion transport proteins in the gills and other tissues.

3. Reduced Water Permeability: Saltwater fish have reduced water permeability across their gills and other membranes, which helps minimize water loss to the surrounding environment.

When a saltwater fish is placed in freshwater, these adaptations are insufficient to maintain the necessary osmotic balance. Water enters the fish's body, causing dilution of its body fluids and disruption of cellular functions. The fish's kidneys and other organs may also fail to function properly due to the altered ionic environment.

As a result, saltwater fish are unable to survive for an extended period in freshwater due to the osmotic stress and ionic imbalance caused by the lower salt concentration.