An ecologist is monitoring the population of endangered white-clawed crayfish ($Austropotamobiuspallipes$) in a chalk stream. In the first sampling session, she captures 120 crayfish, marks their carapaces with a non-toxic water-insoluble paint, and releases them back into the stream. One week later, she conducts a second sampling session at the same site and captures 90 crayfish, of which 18 are found to have the paint mark.

Using the Lincoln Index, what is the estimated population of crayfish in this section of the stream?