Pure H2O freezes at 0°C, but that point can be lowered by dissolving salt in the water. If the solution is further cooled to below the new freezing point, ice will start to form. As a result, the remaining saline solution becomes further concentrated, until the saturation point is reached. The actual temp required to freeze the water in the mixture before the salt settles out is dependent on the mixture ratio of the saline solution, eg: a 5% solution of NaCl melts at -3 C, a 10% solution melts at about -6.5 C.
The moral is: Dont go ice fishing with a Playboy playmate.