Does species diversity stabilise national marine catches? Portfolio effect, synchrony among taxa and catch variability in 168 countries, 1950–2024
Tür çeşitliliği ulusal deniz avını istikrarlı kılıyor mu? 168 ülkede portföy etkisi, türler arası eş zamanlılık ve av değişkenliği, 1950–2024
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Abstract
Year-to-year variability in marine catches directly affects food security and livelihoods in countries that depend on fisheries. It is widely expected that a catch relying on many species will be more stable than one relying on a single species, yet the size and source of this portfolio effect at national scale have not been measured systematically. Using the global capture database of the Food and Agriculture Organization of the United Nations, this study examines the marine catch of 168 countries over three twenty-five-year periods between 1950 and 2024 (403 country-periods). The variability of the total catch is decomposed into the product of the weighted mean variability of taxa and the synchrony among taxa; a second portfolio measure that accounts for mean–variance scaling is computed, and the determinants of variability are examined by regression. In the median country the variability of the total catch is 61% lower than the weighted mean variability of its taxa. The source of this effect is not compensation among species: the median of the mean correlation among taxa is 0.003 and indistinguishable from zero; the portfolio effect arises from the statistical averaging of independently fluctuating taxa. A doubling of the effective number of taxa reduces synchrony by 24% but raises weighted variability by 13%, because small taxa are relatively more variable; the net effect is a 13% reduction in total variability. The effect of catch size is stronger: a catch ten times larger is 29% less variable. The share of small pelagic fish raises variability; a single species, the Peruvian anchoveta, accounts for about 71% of the variance of annual changes in the world total catch. Following the same countries, the portfolio effect rose from 57% in 1950–1974 to 69% in 2000–2024 and the variability of total catch declined. The findings indicate that diversification provides stability, but that its effect is limited and the decisive factors are the scale and composition of the catch.
Keywords
Portfolio effect; Catch variability; Species diversity; Synchrony; Taylor's law; Small pelagic fish; Food security
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