Final answer:
A freshwater fish typically has a urine U/P ratio of less than 1 because they live in hypotonic environments and need to excrete the excess water they absorb through osmosis, resulting in dilute urine.
Step-by-step explanation:
A freshwater fish typically has a urine with a U/P (urine-to-plasma) ratio of less than 1. This is because freshwater organisms are surrounded by water that is hypotonic to their body fluids, leading to a net influx of water into the body due to osmosis. To maintain osmotic balance, freshwater fish must excrete this excess water, which in turn dilutes their urine, making it have a lower concentration of solutes in comparison to their blood plasma. Hence, the urine's specific gravity would be less than that of blood.
Freshwater fish actively take in salt through their gills and excrete dilute urine to rid themselves of the excess water. Specific gravity is a measure of the quantity of solutes per unit volume of a solution; laboratories can directly measure urine osmolarity, which is more accurate. However, the specific gravity gives us a general idea that in freshwater fish, diluted urine is produced to handle the excess water due to the hypotonic environmental conditions. It's worth noting that changes in specific gravity can reflect the presence of medical conditions in humans, but in the case of freshwater fish, it's a normal physiological response to their environment.