This is a problem which requires the use of the differential and therefore belongs on the calculus thread. That said, here is how to solve it.

First we set up an equation that describes the relationship between the constants and variables:

then we differentiate that equation to put it in terms of rates:

That equation describes the general situation. In your example, the specific situation is when the "fish is being reeled in at a rate of 30cm/s" and "there is 8m of line out." So:

and

I will let you take it from there.