Freshwater fish do not drink water but constantly urinate to maintain osmotic regulation.
Saltwater fish actively drink water to stay hydrated and have adapted mechanisms to prevent excess water loss.
Deep dives
Fish Hydration in Freshwater
Freshwater fish do not actively drink water like humans do. Drinking water would dilute their blood and tissues due to the lower salt content in fresh water. Instead, they constantly urinate to keep their internal salt concentrations stable. By conserving salts and minerals and excreting watery urine, freshwater fish prevent their bodies from becoming too diluted and maintain osmotic regulation.
Fish Hydration in Saltwater
Saltwater fish, on the other hand, actively drink water through their mouths to stay hydrated in the salty ocean environment. To maintain electrolyte balance, their kidneys remove excess salt while conserving water. Additionally, the cells in their gills pump salt into the water. Saltwater fish have adapted mechanisms for this osmoregulation process to prevent excess water loss and to avoid becoming too salty.
Fish Hydration in Brackish Water
Some fish can thrive in both freshwater and saltwater environments. For example, when salmon migrate from the ocean to freshwater rivers to spawn, they go through an intermediate stage called a staging area. In this brackish water, which is a mix of freshwater and saltwater, the salmon regulate their osmotic balance before completing their migration. Climate change and melting sea ice can disrupt these staging areas, affecting the salmon's ability to adapt effectively.