A biologist wants to calculate the number of fish in a lake. On May 1 she catches a random sample of 60 fish, tags them, and releases them. On September 1 she catches a random sample of 70 fish and finds that 3 of them are tagged. To calculate the number of fish in the lake on May 1, she assumes that 25 \% of these fish are no longer in the lake on September 1 (because of death and emigrations), that 40 \% of the fish present on September 1 were not in the lake on May 1 (because of births and immigrations) and that the numbers of untagged fish and tagged fish in the September 1 sample are representative of the total population. What does the biologist calculate for the number of fish in the lake on May 1 ?