The science of sustainable seafood, explained

What is the Status of Commercially-Fished Tuna Worldwide?

A recent article in the Guardian discusses the future of seafood through the lens of the UN Fish and Agriculture Organization (FAO)’s latest report on the State of World’s Fisheries. The article has some interesting thoughts on the future of fish consumption (especially concerning the rise of aquaculture), but misrepresents some data on the status of tuna stocks worldwide. We called on Maite Pons, of the University of Washington to clarify.

Comment by Maite Pons, University of Washington

The term sustainability varies among countries, fisheries organizations and even more among scientists, but, in general, scientists define sustainability in terms of maximum sustainable yield (MSY) – the maximum catch that can be obtained on a sustained basis. A stock is sustainable if their abundance (B) is above the levels that produce MSY (BMSY).

Figure 1. Stock status relative of commercial tunas to target reference points (dashed lines) for fishing mortality (FMSY) and biomass (BMSY). Horizontal and vertical dashed lines show MSY target reference points. The area of circles is proportional to MSY (mt). Figure from Pons et al. (2016).

Pons et al. (2016) found that, up to 2014, only 8 stocks, out of 22 commercial tuna stocks (36%), had abundance below that which would produce maximum yield. Only four stocks are now fished at rates that are higher than would produce maximum yield, and only 2 (Pacific Bluefin tuna and Bigeye tuna in the Western Central Pacific) are fished so hard the stocks are threatened with collapsing to very low abundance.

But, how much have tunas declined?

Considering the mean trajectory of abundance relative to the target abundance (that which produces maximum sustainable yield), for all commercial tuna stocks (N=22) and billfishes (including swordfish and marlins, N=18), we can see that abundance has been declining since 1950, but with a slight increase in the last 5 years (Figure 2). Today, abundance values are very close to the level that produces maximum sustainable yield. The relative abundance was around 60% in 2008 and it is now half of what is was in 1950. There has been an increase in abundance of about 10% in the past few years due to an improvement in management regulations and enforcement (Pons et al. 2016).


Figure 2. Mean time series of B/BMSY for all 40 stocks considered in Pons et al. (2016).
Figure 2. Mean time series of B/BMSY for all 40 stocks considered in Pons et al. (2016).

This increase in abundance is associated to a reduction in fishing pressure in the last ten years (Figure 3). The fishing pressure reached a maximum in the year 2000 that was above sustainable levels but dropped below the optimum in 2008. It is still below the MSY target today (averaged) for all 40 stock analyzed.

Figure 3. Mean time series of F/FMSY for all 40 stocks considered in Pons et al. (2016).

In summary, the abundance of tunas and their relatives has declined from pre-industrial levels but in general, they are at sustainable levels. There are exceptions and urgent management actions are needed for some stocks. In particular, for Pacific bluefin, catch limits have been implemented since 2013 so, we expect to see an increase in abundance for this stock in the near future. As an example, due to catch limits and other management measures, Southern bluefin tuna and Atlantic bluefin tuna are showing signs of rebuilding.

Maite Pons is a PhD student at the University of Washington School of Aquatic and Fishery Science. Read her latest paper here.

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