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 historical decrease in southern bluefin tuna populations is eye-popping. In the 1960s, southern bluefin tuna from the Indian Ocean accounted for 36% of tuna catches there. Today, it has fallen to less than 1% of the catch
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.
Dr. Maite Pons
Dr. Maite Pons is a postdoctoral researcher at the University of Washington. A major focus of her PhD was tracking the status of commercially important tuna and billfish species.
4 Responses
This is a nice article and helpful (I think!) for students in my Masters level Fisheries Management class. We discussed Kobe plots a couple of weeks ago. I think there are a couple of key messages (in addition to the fact that catch is not an adequate indicator of abundance). Firstly, we can’t generically say much about “tuna” – skipjack and bluefin (not to mention yellowfin, bigeye, etc) are obviously very different – in life history characteristics, in fisheries pressure historically, in catches, in market pressures, etc. Secondly, what seems to be missing is an acknowledgement of the fact that reported fishing pressure isn’t the only thing impacting the status of the stock. And so even with overfished stocks where overfishing is not occurring (i.e,. rebuilding MAY be taking place) there are other things that may reduce the extent to which rebuilding to the green quadrant is possible (IUU catches and climate change for example). I love the fisheries pressure and biomass box plots that you show here. But while you acknowledge it is a model and show the range around each estimate, there is a suggestion in the graph that one year we are ok (1989 was great!) and the next year we aren’t (bad, 1990, bad)…and that seems disingenuous. Some kind of probability distribution around the categorizations might be helpful – we may have been overfishing years before 1990. I raise this, and I believe I have raised it before with Trevor perhaps on twitter. These are not knife-edge classifications. So to be fully exploited and at the intersection between F/Fmsy=1 (I have never used U…), and B/Bmsy=0.8 may by technically ok, it is not an ideal spot to hang out….on the tipping point three other quadrants that we don’t want to enter.
Good blog, Maite.
Overall, I welcome the Coulter et al. paper, which advances previous efforts to create a global database of tuna catches in a spatially-finer resolution than the stock-wide level.
However, I am frustrated about the PR around the paper, as it is very misleading and not helpful to sound fisheries management. I think that you covered all of the points pretty well. On the subject of sustainability, there are many nuances and it is bad when everything is put into one –simplistic– basket. For a start, not all species have the same resilience to exploitation.
I found the statement about the dramatic 1000% increase in catches over 60 years to be funny, and yet sad coming from people familiar with maths. When you start close to zero, it’s easy to detect large percentage changes … Expressed as a percentage, I am infinitely older than when I was born.
Keep up the good work.
You all complicate simple things and people who follow get confused.
The Pauly and Hilborn groups have to find consensus by reviewing all their studies and the impacts they have on the global practice of fisheries school of thought and fisheries management to give a non divided direction to future fisheries science and management.
Now for instance, “expressed as a percentage, I am infinitely older than when I was born”. True but where are you going towards? How far are you from the life expectancy of your country? You should realize that your are fast moving nearer the end of your life than your beginning. Off-course birth and death rates are easy to count for people and your sure f when your life will end. It is difficult to know how many fish are down there and you may be lying to your self that it is an endless ditch which you can exploit endlessly relative to when you began.
You have got to be cautious.
Interesting article, however, it does not mention the whole situation. Some stocks may not be overfished but the fish being caught does not have to proper size to keep stocks healthy. If all of the stakeholders understood the importance of different species in the oceans, tuna would not be overfished in some places as it happens with the Atlantic BET.