20The trout population in a large lake has been threatened by the accidental introduction of parasitic sea lampreys through man-made channels from the ocean. The lampreys attach themselves to the sides of trout, eventually killing many of the trout. Lampreys leave wounds on the skin of the trout. To control the lamprey population, costly chemicals, traps, and barriers have been used to prevent sea lampreys from reproducing. The graph below shows the effect of this lamprey control program in reducing the number of wounds on the native trout population.