What are the consequences of some harvest strategies?
Suppose that we harvest 4 million tons of fish out of the 10 available this year and leave 6 million tons. That means we leave 6 million as the now remaining population this year to reproduce for next year. We follow up from 6 on the horizontal axis to the blue curve and find that next year the population will be 12 million. Next year we could then harvest 6 million [the distance between the reproduction curve (blue) and the line of exact replacement (red)] and again leave 6 million. We could then continue that pattern indefinitely, for a sustainable catch of 6 million tons.
Suppose that we are greedier and take 8 million tons of fish this year out of the 10 million available, leaving 2 million tons. We are lucky—those 2 million will reproduce to 8 million. But we can’t take 8 million tons next year, or we would wipe out the population. However, if next year we take 6 million, we would again be leaving 2 million. We could then continue that pattern indefinitely, for a sustainable catch of 6 million tons.