It is known that the discharge from a certain plant has a probability of 0.001 of containing a fish killing pollutant. An instrument used to monitor the discharge will indicate the presence of the pollutant with probability 0.999 if the pollutant is present and with probability 0.01 if the pollutant is not present. If the instrument indicates the presence of the pollutant, what is the probability that the pollutant is really present?