by Hannah Tannenbaum
Threatened fish stocks have generally only been identified as being overfished and potentially reaching extinction once they have already been exploited and are in population decline. This makes remediation efforts for overfished stocks much more difficult, because much of the damage has already been done. Burgess et al .(2013) defined the “eventual threat index” (T), taking into consideration primary effects on overfishing potential. The variables they used to understand exploitation potential include: vulnerability, population size, average catch-per-unit effort, maximum per-capita growth rate, as well as profitability. In particular, this study examined multispecies fisheries, referring to fisheries in which there is a target species, but often the fishery affects many other species as well. Therefore they identified the “key” species as the most commercially valued, or most environmentally sensitive, and therefore the most likely to be exploited. The authors then applied the eventual threat index for historical information on four Pacific tuna and billfish populations, and were successfully able to retroactively predict the eventual declines these fisheries face today. It is intended that this metric can be used to predict the declines of fisheries in the future, before exploitation has occurred.
Many of the most threatened species of fish are caught by multispecies fisheries which catch more species than just the intended catch. Multispecies fisheries are often exploitative because of the fishing techniques used to catch the “key,” or commercially desirable species. Burgess et al examined historical catch data for four species of tuna and billfish with the eventual threat index they developed, to predict current populations of the species. The eventual threat index for multispecies fisheries was developed based on the assumption that the fates of all species affected can be linked to one “key” species, either the most vulnerable, or the most economically desirable, and therefore the most likely individual species to face exploitation.
Vulnerability of a species was measured using population size, average catch-per-unit effort, and maximum per-capita growth rate. The eventual threat index, T, was calculated by the ratio of the vulnerability of an individual species to the vulnerability of the key species of the fishery in question. Through a series of equations and derivations, the authors were able to generalize the meanings of eventual threat index values: T>2 poses a high threat of eventual extinction, 1<T<2 poses a high threat of overfishing, 0.5<T<1 poses a possible threat of overfishing, and T<0.5 poses a low threat of overfishing. The authors emphasize that changes in technology, equipment, markets and politics can all have an affect on the commercial demand and amount of effort affecting these calculations.
The eventual threat indices were applied to tuna and billfish data from the 1950s to present to see if the current population declines could have been predicted before they occurred. The authors found that T values accurately predicted extinction and overfishing from data as early as the 1950s for marlin and bigeye tuna populations, before the population declines actually started to occur. The purpose of developing the index was to use relatively simple population metrics to predict eventual fisheries declines before they occur, so as to improve overall management while maximizing profits.
Burgess, M.G., Polasky, S., Tilman, D., 2013. Predicting overfishing and extinction threats in multispecies fisheries. Proceedings of the National Academy of Sciences 110, 15943-15948. Full Paper: http://www.pnas.org/content/110/40/15943.full