To determine how many fish the fisherman released into the lake, we need another piece of information that relates to the population of the new species at a certain time. Let's say that after 5 weeks, the population of the new species has grown to 500 fish. Then we can set up an equation:
P(5) = 7b^5 = 500
Solving for b, we get:
b^5 = 500/7
b = (500/7)^(1/5)
b ≈ 1.828
Now we can find the initial population P(0):
P(0) = 7b^0 = 7(1) = 7
Therefore, the fisherman illegally introduced 7 fish into the lake.