Chapter 5 · Try it yourself
Cod Quota
Cod paid for New England’s churches, ships and fortunes for three centuries, and the cape is named for it. Here you run a model cod fishery. Every year the fish grow a little; every year you take some. Take too much for too long and the fish that built New England stop showing up.
Pick a story
Run the fishery
8 units a year
Year 0
Cod in the sea
100% of an unfished stock
Caught last year
– units
New fish grown last year
– units
Years to rebuild with no fishing
0
Stock status: Healthy
The stock, year by year
- Cod in the sea
- Catch that year
- Carrying capacity (K)
- MSY level, half of K
- Overfished below this line
- Collapsed zone
Show the numbers as a table
| Year | Cod at start | New growth | Catch | Status |
|---|
Why half?
A nearly empty sea has too few parents to make many young. A full sea is crowded, so each fish grows and breeds less. The stock adds the most new fish each year when it sits at about half of its carrying capacity. That biggest yearly surplus is the maximum sustainable yield (MSY): in this model, 10 units a year in cool water, but only 6 once the water warms.
The trap: MSY is a knife edge. Catch even a little more than the surplus, or keep the old quota when the sea changes, and the stock slides every year, and the slide speeds up as it shrinks.
What really happened next
The model is kinder than the sea. Here, cod climb back quickly once fishing eases. Real cod mostly did not. Canada’s northern cod stayed closed for 32 years, until a small commercial fishery reopened in 2024. In US waters, NOAA Fisheries lists both of its long-managed cod stocks (Gulf of Maine and Georges Bank) as overfished (2021 assessments). Scientists have since found five separate cod populations in US waters, and managers are reworking the rules to match.
Why so slow? Very small stocks can struggle to find mates, predators and competitors move into the space cod left, fishing for other species keeps catching some cod, and warm water cuts the survival of young cod.
Simplified model, illustrative only: this is not a stock assessment. Logistic growth with r = 0.4 a year and carrying capacity K = 100 units in cool water; “warming water” lowers them to r = 0.3 and K = 80 (illustrative, in the direction Pershing and colleagues found for Gulf of Maine cod in 2015). “Overfished” uses the US rule of thumb, below half of the MSY level; “collapsed” means below 10% of K. The fleet can never take more than 80% of the fish in a year. The History story uses an invented fishing-pressure script shaped like New England’s: heavy foreign and domestic fishing in the 1960s, a dip when foreign fleets were pushed out in 1977, a bigger US fleet in the 1980s, and cuts after 1994. Its years and events are real; its numbers are not. Dates: US 200-nautical-mile zone in force March 1, 1977 (Magnuson Act of 1976); Canada’s northern cod moratorium July 2, 1992, lifted in 2024 (Fisheries and Oceans Canada); Georges Bank closed areas December 1994 (NOAA Fisheries); Gulf of Maine warming 2004–2013 faster than 99% of the global ocean (Pershing et al., 2015, Science). Stock status from NOAA Fisheries (2021 assessments); five biological cod populations from the 2023 Atlantic cod research track assessment (New England Fishery Management Council).