How does density change with salinity?
As more salt is added to a solution, the density of the solution will increase.
Although both the mass and the volume of the solution will increase as more solute is added, the mass will increase much more significantly. This will cause the mass/volume ratio, the density, to increase.
Try putting an egg in a glass of water. It will sink, because the egg has a greater density than water. Add salt to the water and mix, and eventually the density of the solution will be greater than that of the egg and the egg will float.