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Technological change, environmental variability, and fish stock collapses. Rögnvaldur Hannesson Norges Handelshøyskole. Points to be raised. Technical change increases catch per unit of effort (cpue) It may also make cpue less sensitive to stock size
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Technological change, environmental variability, and fish stock collapses Rögnvaldur Hannesson Norges Handelshøyskole
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
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
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
b 1 for gill nets and purse seine, b 0 for land seine. a 0 for purse seine.
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.
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
The breakthrough of the purse seine in the winter herring fishery
G surplus growth, S stock left after fishing, X stock before fishing
G observed estimated environmental disturbance random environmental disturbance Further autocorrelation terms insignificant
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.
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
Change in b clearly more dangerous than change in A only, but collapse can also be precipitated by the latter if learning is slow.