According to the ocean's system described, the tuna population eats mackerel, mackerel population eats shrimp, and shrimp population eats plankton. Thus, if humans kill the tuna, it will affect the complete food chain drastically. But the first effect would be seen on the direct prey whose population will increase significantly. This is because of no or less availability of the predator. Thus, according to the given conditions and possible statements, the mackerel population will increase after the tuna are killed by humans.
