Over the last week there have been multiple news stories saying that we are harvesting tunas at unprecedented and unstainable rates—some stories have implied that tunas are on track for extinction. This is simply not true. Most of the time, catches are not a reliable indicator of abundance.
The news articles were based on a recent paper that showed that global catches have increased more than 1,000% over the past 60 years. That statistic sounds large and scary, but it is the poor understanding of how fisheries develop that is scariest.
Fisheries develop, just like other products and industries
The increase in tuna catches reflects the classic behavior of any developing fishery. Throughout history, catches increase until they level out or until they start declining because of two potential reasons: 1. abundance of populations goes down and therefore with the same fishing effort catches go down, or 2. fisheries management is put in place to reduce fishing effort and therefore catch goes down. In the case of industrial tuna fisheries, it is not surprising that tuna’s catches increased since 1950, because this is when these fisheries started.
Moreover, recent catches are not increasing very fast. In 2017, total tuna catches were only 18% higher than in 2000. Most of this increase was generated by an increase in skipjack tuna catches, the most productive of all the major tuna species. Further, skipjack are fished at sustainable levels in all Oceans (see figure below). Are we driving skipjack tuna to extinction or even overfishing them? Definitely not.
Catch is not abundance
In some cases, bluefin and bigeye tuna catches are decreasing due to management constraints implemented to rebuild overfished stocks (a.k.a progress!). Only 2 out of 23 major tuna stocks are currently overfished AND experiencing overfishing (according to the FAO definition of each B < 0.8 BMSY and U > UMSY, respectively, see upper left quadrant) and only 3 stocks (southern bluefin, Pacific bluefin and Western Atlantic bluefin tuna) have biomass below the critical level of 0.5 BMSY (U.S. definition for overfished). Most tuna stocks are fished at sustainable levels, meaning the abundance of tuna in each population are at good levels.
* Editor’s note: this is what is known as a Kobe plot. For an explainer on what a Kobe plot shows, see here. That post also explains the difference between overfished and overfishing in more detail.
All bluefin tunas have clearly experienced a large decline in biomass, however all populations, except Pacific bluefin, are showing signs of rebuilding. Good science and fisheries management regulations are making this possible. Below I show as an example, the total trends in relative fishing pressure (U/UMSY) and relative biomass (B/BMSY) for the commercial tuna species in the Atlantic Ocean, the ocean that has experienced the highest exploitations rates through history. Horizontal lines at 1 refers to limits in sustainable levels. Back in the 70s fishing pressure was low (graph on the left) and abundance was high (graph on the right); then as fishing pressure increased, reaching a pick in the 2000, biomass decrease and drops below sustainable levels. Today, as fishing pressure has decrease in the Atlantic due to management measures, biomass is starting to go up again and it is now above the target abundance to produce maximum sustainable yield.
Trend in fishing pressure on the left and biomass on the right. year. The orange line represents the mean trend across all assessed stocks as estimated under a state-space model. Being below the horizontal line at 1 on the graph on the left means no overfishing is occurring and being above the horizontal line on the graph on the right means that stocks are not overfished. Boxplots show distributions of estimates of individual stocks in each year: extent of boxes show 25th and 75th percentiles, whiskers show lower and upper end of the range, and red points show median estimates of individual stocks in each. Source: Hilborn et al. Effective fisheries management instrumental in improving fish stock status. PNAS, under review.
Putting catch and abundance in proper historical context
From one of the news articles mentioned in the introduction:
The way that statement is framed is rather misleading: the increase in catch of other tunas relative to the bluefin is the main reason it has dropped from 36% to 1% of all tuna catch.
However, it is true that Southern bluefin tuna catches and abundance dropped dramatically due to overfishing, but an important reason catches are lower now is because catch limits have been implemented in order to rebuild this stock. The drop in catch was framed as a negative when really it is fishery management reducing catches to make the stock more sustainable. The catch limits seem to be working: Southern bluefin tuna biomass is slowly increasing (B/BMSY in 2006 was 0.29 and today is 0.49), though it might take time for the population to be completely rebuilt since they are a long-lived, slow-grow species. More time is needed to see these overfished stocks move to the preferred lower right quadrant of the Kobe plot, but evidence exists that this is happening:
However, I am not saying that all tuna stocks are fished at sustainable levels. We should definitely be concerned about Pacific bluefin tuna, where biomass is probably below 0.3 BMSY and it is still experiencing overfishing. With such high fishing pressure the chances of seeing this stock rebuilt anytime soon are low. In addition, two stocks have recently showed high levels of fishing mortality. Yellowfin tuna in the Indian Ocean and bigeye tuna in the Atlantic Ocean were both considered overfished in their last stock assessment, though both still show biomass above 0.5 BMSY and yellowfin shows biomass above 0.8 BMSY (FAO definition for being overfished).
I am disappointed in the coverage of tuna abundance based on this recent paper showing increased catches over the past 70 years. Of course catches are significantly higher on fish now that the fisheries have developed. Again, catches are not always a reliable indicator of abundance. Headlines that promote “scary” catch numbers mislead the public about the true status of tuna stocks in the ocean. What I tried to show is that, although there are still some tuna populations experiencing excessive fishing pressure (Pacific bluefin, Atlantic bigeye and yellowfin in the Indian Ocean), overall most tuna stocks are being fished at sustainable levels.