Exercise 3 solution

Each year the fishery takes 30,000 tonnes while the stock grows by only 10,000, so the stock falls by 20,000 tonnes a year: from 100,000 to about 80,000, 60,000, 40,000 and so on, reaching a very low level in under five years. As it falls, growth usually falls too (fewer spawning fish), so the decline can speed up and the stock may collapse.

Two simple measures: (1) lower the catch to no more than the growth, i.e. a catch limit of about 10,000 tonnes a year or less, to let the stock recover; (2) protect spawning fish or juveniles, for example by closed areas or seasons, larger net mesh or minimum landing sizes. Monitoring and enforcement are needed for either to work.