Final answer:
Saltwater fish prefer a hypertonic environment, actively secreting salt to maintain osmotic balance. In contrast, freshwater fish live in a hypotonic environment, taking in salts to prevent water overload. Euryhaline organisms like salmon can survive in both types of environments due to their specialized osmoregulatory adaptations.
Step-by-step explanation:
Saltwater fish prefer a hypertonic environment, which means that the environment they live in has a higher concentration of salts compared to the fluids within their cells. To maintain osmotic homeostasis, saltwater fish secrete salt through their gills and produce highly concentrated urine. In contrast, freshwater fish live in a hypotonic environment where the outside water is less salty than their cell fluids, leading them to actively take in salts and excrete diluted urine. Marine animals like salmon that can tolerate both freshwater and saltwater are euryhaline organisms and have evolved specialized osmoregulatory mechanisms to adapt to these changing environments.