Technological change, environmental variability, and fish stock collapses

1. Technological change, environmental variability, and fish stock collapses Rögnvaldur Hannesson Norges Handelshøyskole

2. Points to be raised • Technical change increases catch per unit of effort (cpue) • It may also make cpue less sensitive to stock size • If cpue used as stock index, this will make stock assessment overoptimistic • With environmental variability, this may precipitate stock collapses

3. The Norwegian winter herring fishery as a case in point • Two major technical changes in the 1960s • Power block • Mechanical hauling of nets • Bigger nets, bigger boats • Sonar • Possible to see fish below the surface • CPUE increased and may have become less dependent on stock

4. b insignificant for land seine, 0 < b < 1 for gill nets & purse seine. Lopping off the years after 1964 raises b and the t-value, but lopping off still more years in the 1950s & 60s lowers b and makes it insignificant. Shaky evidence that b fell after 1964, may have been low all along. a  1

5. b 1 for gill nets and purse seine, b  0 for land seine. a  0 for purse seine.

6. There does not seem to be a strong correlation between stock and cpue for purse seine even before 1964, but more so for gill nets.

7. b = 1, x constant Technical progress, A rises S was falling after ’65, falling b may have contributed to rising x Define Technical progress & fall in b: first term > 0

8. The breakthrough of the purse seine in the winter herring fishery

9. G surplus growth, S stock left after fishing, X stock before fishing

10. G observed estimated environmental disturbance random environmental disturbance Further autocorrelation terms insignificant

11. Transformed V lognormally distributed

12. Sample paths of stock with no fishing, Note possibilities of trendless minor fluctuations, quasi-regular major cycles, and total collapse. The Bohuslän herring collapsed in the 1500s, the Baltic herring a bit earlier.

13. Stock development with fishing • Stock managed by target escapement with S = Smsy • X forecast by cpue as index (implies b = 1) • Technical change happens in year 10 • Once and for all rise in A • Once and for all fall in b and simultaneous rise in A • These effects never discovered • Managers continue using cpue as if nothing happened

14. Change in b clearly more dangerous than change in A only, but collapse can also be precipitated by the latter if learning is slow.