Calculating Overfishing for Tuna and Billfish


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:


One thought on “Calculating Overfishing for Tuna and Billfish

  1. Thanks for writing this post. It’s great to see students getting involved in communicating science, and both students and faculty getting involved in blogging, which I believe has a role in science that will only increase in the near future. A couple of quick clarification points about our paper:

    1) “..accurately predicted extinction” Marlins and bigeye tuna in the Pacific aren’t extinct (or anywhere close, thankfully). I don’t think you were trying to say this (nor were we), but worth mentioning for other readers.

    2) The key species is the not the species most likely to be exploited, but is the species most likely to be limiting to the long-term fishing effort in the fishery (because this makes its growth rate a key determinant of how much bigger the fishery can get, if that makes sense). In open-access fisheries this is usually the the species that generates the most revenue. In a fishery where there is management to protect a group of weak stocks, the key species is the weakest of these (i.e. most vulnerable, because these are hardest to protect).

    The main point of the paper is that we can make reasonable a priori guesses about how heavily fisheries will eventually exploit some species (e.g. we tend to fully exploit or somewhat overexploit the big revenue generating species with no management, and when we manage, we tend to limit exploitation to weak species that we care about to sustainable levels), and we can also make reasonably good estimates of the relative impacts multispecies fishing practices are having on different populations. Put these two things together, and we can make reasonable guesses about how heavily we will eventually exploit by-catch species and weak stocks without having to wait for mortality to reach dangerous levels.

    Thanks again, and good luck in your studies and wherever they may lead you!

    – Matt


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