Freshwater fishes primarily act as hyperosmotic regulators.
Explanation: Freshwater fish live in an environment that is less concentrated than their bodily fluids. To maintain homeostasis, they need to regulate their internal salt concentrations despite the dilution effect of their surroundings. This is referred to as hyperosmotic regulation.
Essentially, freshwater fish have evolved mechanisms to prevent the excessive uptake of water and to retain salts that are vital for their physiological functions. They do this by continually excreting large amounts of dilute urine and actively absorbing ions through specialized cells in their gills. Hence, the correct answer is that freshwater fishes are hyperosmotic regulators.