Sustainable fishing – maximum sustainable yield and the tragedy of the commons

What it shows

A logistic fish stock grows fastest at half its carrying capacity, so the largest catch it can supply year after year, the maximum sustainable yield, is rK/4 at N = K/2. When many fishers share the stock, each gains by adding boats while the cost of a smaller stock falls on everyone: the tragedy of the commons. This Schaefer surplus-production model lets you compare open access, a total allowable catch and a co-operative, and test mesh size, closed seasons and marine protected areas. Grand Banks cod, which collapsed in 1992, is a real example.

How to use

Choose Regime and Fishers, then set Your boats and press Run or +1 year. Under Total allowable catch, set Quota. Try Mesh, Closed season, Marine reserve, Cost per boat and Variability. The upper graph shows stock and catch against K/2 and MSY; the lower one shows the growth curve and today's catch line. Press Add to table to compare runs.

Parameters you can change

• Management regime Open access, Total allowable catch (TAC), Co-operative / single owner
• Number of fishers (including you) 2–10
• Your number of boats 0–20 boats
• Total allowable catch per year 0–30 thousand t/year
• Mesh size Small (catches many juveniles), Medium, Large (juveniles escape)
• Closed season (spawning) 0–6 months/year
• Marine protected area (no fishing) 0–50 % of the sea
• Intrinsic growth rate of the stock r 0.1–1 /year
• Cost per boat per year 50–500 thousand USD
• Environmental variability None, Moderate, High

Questions to explore

1. Under open access, why does the stock still fall when you cut your own boats?
2. Is a quota set exactly at MSY safe, especially when the nets catch many juveniles?
3. Which fleet size gives the highest total profit, and is it larger or smaller than the one for MSY?