Why Do Saltwater Fish Die in Freshwater?

Most freshwater fish are far easier to care for than most saltwater fish, as saltwater (or marine fish) have far higher demands in terms of water parameters than freshwater fish. Neither is able to survive in the other's environment. The reason saltwater fish cannot survive in freshwater environments is relatively complex, though at its most basic level it involves a process known as biological osmosis.

  1. Saltwater and Freshwater

    • All the world's oceans are saltwater environments.

      The most obvious different between saltwater and freshwater environments is in the name; saltwater contains far greater concentrations of salt than freshwater. Oceans, many bays, seas and capes are all saltwater environments while ponds, rivers, streams and lakes are usually freshwater environments. Saltwater environments also have higher pH levels (typically 8.3) than freshwater environments (typically around 7.0). Each environment has its own unique organisms adapted for it.

    Osmoregulation

    • Osmoregulation refers to the movement of water in and out of a fish's cells from an area of low concentration to an area of high concentration. The water in the cells of saltwater fish contains less salt than the water in their surrounding environment, so salt is constantly being expelled from the fish and back into the water.

    Desalinization

    • When the marine fish drinks, the water it ingests undergoes a process of desalinization, whereas most of the salt is removed and released back into its environment. The kidneys of saltwater fish are specialized for this task. However, the fish must constantly drink to keep up with its water loss, since saltwater flows from an area of low concentration to an area of high concentration.

    Cell Rupture

    • When a saltwater fish is placed in a freshwater environment, its cells still contain more saltwater than the surrounding water. This causes water molecules to overtake its cells. Saltwater fish, unlike freshwater fish, are built to get rid of excess salt and have no biological mechanism to remove excess water. Therefore the fish swells up and its cells fill with water until they burst and kill the fish.

    Diadromous

    • Diadromous are fish that are adapted to both freshwater and saltwater environments with biological mechanisms to remove both excess salt and excess water. Salmon are one of the more common examples. They live in primarily saltwater environments but often travel into freshwater environments to spawn.