Step 1 :The question is asking for the number of prisoners in 1990. Since the variable $x$ represents the number of years since 1990, we can substitute $x=0$ into the polynomial to find the number of prisoners in 1990.
Step 2 :Substitute $x=0$ into the polynomial $-2.08x^{2} + 78.46x + 742$ to get the number of prisoners in 1990.
Step 3 :The prison population in 1990 was \(\boxed{742}\) thousand.