Afish controls the concentration of salt in its body by actively excreting salt across its gills, drinking, and producing very small amounts of concentrated urine.

The answer is hyper-tonic.

Explanation:

The hyper-tonic fishes are found in freshwater. In these fishes, their blood conveys a lower water focus than the encompassing freshwater. With the death of freshwater by means of the mouth over the layers of the gill, the particles of water diffuse from the freshwater into the blood as a natural side effect.  These fishes need to create an extremely tremendous amount of pee to offset this abundant utilization of water. These fishes therefore deliver little measures of concentrated pee.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.