Can be done with variables. The 3rd operand in the variables event is 'Random'. Set that to 1 to 100 and then with a Conditional Branch you can say...
@>Control Variables: [0001: Success Rate] = Random No. (1...100)
@>Conditional Branch: Variable [0001: Success Rate] >= 50
@> Catch fish
@> Else
@> Do not catch fish