A square no-take zone sits at the centre of the grid, sized by the MPA coverage slider. Cells inside it feel zero fishing pressure and rebound toward a healthy target; cells outside feel the full fishing pressure. Cells just outside the border get a small spillover bonus that fades with distance — the same effect that lets real MPAs boost catches just beyond their edge.
target = clamp(0.15 + recovery*0.55*protected + spillover - pressure*0.45*(1-protected), 0.02, 1.4)
v += (target - v) * min(1, dt*1.6)
- Ocean grid size — cells tracked for fish biomass.
- MPA coverage — fraction of the grid area enclosed by the central no-take square.
- Fishing pressure — harvest rate applied to unprotected cells.
- Biomass recovery rate — how fast fish populations rebound inside the protected zone.
Well-enforced MPAs like New Zealand's Goat Island reserve show fish biomass 5-10x higher than adjacent unprotected waters within a decade of protection.