The Tunas of Banda

By Peter Mous.

Yesterday, Bas called me. Bas works for a fish trading company, and he was wondering about the recent drop in the supply of high-quality yellowfin tuna Thunnus albacares, known in Indonesia as madidihang. As a fisheries expert, I am always both flattered and embarrassed by such questions: Flattered, because the trade seems to value the insights gained from the sustainable fisheries program that I work for. And embarrassed because it’s often so difficult to provide a straightforward answer (and beware those experts who do provide a straightforward answer!). Bas’s predicament is that he promised his clients boxes of high-quality tuna, but that the fishers don’t catch any. The fishers say that there seem to be few tuna around… what’s going on here, Dr Mous?

Well, the tuna trade is all about securing supply from an ecosystem that is highly variable. Tuna supply lines are of mind-boggling complexity, and I am amazed that it all results in continuous presence of sashimi at restaurants and of canned tuna at supermarkets worldwide. Consumers worldwide take tuna for granted, but I certainly don’t. The continuous presence of tuna on the global market is probably the only aspect of a tuna that is “continuous”—everything else is a whirlpool of events and processes.

First, let’s have a look at the habitat of yellowfin tuna: The open waters of the world’s tropical oceans, where depth is measured in kilometers rather than meters. The Banda Sea, where the Seven Seas took us, is rather exceptional, since it features deep seas as well as isolated, often volcanic islands rising up from the deep. My theory is that yellowfin tuna have an innate drive to orient themselves to a fixed reference point, and that the small islands of Banda serve that purpose for tuna. Fixed reference points are a scarce asset in the open ocean, as the bottom is too deep to keep an eye on. As a result, yellowfin tuna deal with current, sometimes strong current, 24 hours per day usually without noticing it! As a SCUBA diver, when you descent in blue water, you can experience this first-hand: If the bottom is not yet in sight, then the universe moves along with you and you loose any reference to a “fixed” location. That’s pretty annoying if you are diving a deep wreck and you know there is a current but you forgot to take a bearing at the surface. You want to swim hard against the current while descending, but you cannot tell any more what way it is going!

It is likely that the isolated, small islands of the Banda Sea result in a “stickiness” that keep tuna longer in the area than they would be in plain open ocean. Still, for the fishers of the Banda Sea those islands are not sticky enough, and chasing fast-swimming tuna in a large open sea gets tiring and unprofitable quickly. Hence, Indonesia’s highly skilled fishers found a way to artificially increase the stickiness of their favorite fishing grounds: Enter Fish Aggregating Devices, or FADs. Known in Indonesia as rumpon, FADs are anchored rafts. This sounds like a low-tech, low-cost contraption, but keep in mind that deployment involves kilometers of polypropylene rope, heavy anchors (often various oil drums filled with concrete, tied together), and steel cable. FADs are modern fishing gear, and they cost thousands of dollars each. The fishers tie palm fronds to these rafts, but if you ask me the rope and the raft itself is already enough to make it work. FADs also make excellent dive sites, though some might find it a bit unnerving to hang out in a bottomless ocean, without the comfort of a reef within eyesight.

On our “Epic” trips with Seven Seas, we dived and snorkeled a FAD near Kumba, a small volcanic island, and the underwater life was amazing—see accompanying video! The fish hanging out directly around the FAD are actually not tuna. We saw mahi mahi, wahoo, rainbow runners, drummers, some shark, sergeant majors, and of course fusiliers. For some reason, there is nearly always one or two tripletail Lobotes surinamensis hanging out directly on the raft or among the palm fronds. So where were the tuna? Well, they are usually at some distance from the FAD, up to a few kilometers away, cruising around at great speed, just occasionally “checking in” with the FAD. While snorkeling around that FAD, I was lucky enough to see passing by a large school of small ones, each about 3 kg. I think I was the only one in our group who saw them while we were diving that FAD. For a moment, the world seemed to be made of tuna, and just a minute later it is as quiet as before. A short, but unforgettable moment that is easy to miss.

Even though FADs help to concentrate tuna, this does not guarantee a successful catch. A canoe with two people may bring back about five mid–sized tuna (5-10 kg) per trip or a couple of big ones, but trips with no catch are not rare. Add to that the variability due to weather, and the variability caused by operational mishaps such as engine failure, shortage of ice or fuel, sick crew, and so on. Moreover, some FADs do not seem to “work” at all, while other FADs result in good catches month after month. It goes without saying that one can make a living of fishing, but it’s certainly not a steady income. The reason that Indonesia still produces a steady supply of yellowfin tuna is that the variability averages out higher up in the supply line, which means that the cans on the shelves of the world’s supermarkets rarely, if ever, run out. Individual fishers, however, must live with uncertainty, and traders in the middle of the supply line must constantly adjust their sources of tuna.

Besides moving fast, tuna also grow fast. Growth is not something one can perceive as a SCUBA diver, so to get an understanding of this process we must take off our dive masks and put on our reading glasses. Scientists report that yellowfin tuna reach a length of about 60 cm and a weight of 4.5 kg in their first year. Then, in their second year they almost quadruple their weight to 16 kg, and halfway into their third year they will have reached adulthood at a body weight of about 25 kg. The accompanying video shows how fishers are catching tuna. These fish look quite impressive, but at about 2 years old they are early tuna teenagers at best. There is a market for these small ones, and it’s understandable that the fishers are catching them, but one wonders whether it would not have been better to let them swim (and grow!) for even a short while longer…

To answer that last question, one needs to get an understanding of a more unfortunate aspect of a tuna’s life-history: Death. After all, if these fishers are catching a fish that would have died quickly anyway of natural causes, one might argue that we are better off catching it before it perishes. Mortality is even more difficult to study than growth, but scientists have found an answer: “about one per year, depending on length”. That’s not very enlightening, but let me translate that for you: It means that after one year, only one third of the individuals are left. The rest, two-thirds, has been eaten by other sea creatures. For small tuna (about 30 cm long), survival is even worse: 5%. That is a very low survival indeed, even for fish. Now, the only way that yellowfin tuna can survive as a species is to outgrow this mayhem of mortality and then produce eggs by the millions. If a female tuna manages to escape death and reach adulthood (which is unlikely, unfortunately) she can produce over 2.5 million eggs per year, thereby moving the balance in the favor of the species again.

Tuna makes up for high natural mortality by fast growth and fast reproduction, and this enables tuna also to support a substantial fishery. The ecosystem is producing and consuming an enormous flow of yellowfin tuna, and for the survival of the species it does not matter if mankind is among the many predators out there to get a piece of the action. It gets problematic, of course, once mankind becomes the most important predator of them all. In Indonesia, this is already so for the largest tuna, the big ones of over 50 kg each. For very large tuna, mankind consumes more than all other large predators in the ocean combined! This begs the question what “sustainability” really means.

A rule of thumb often applied by fishery scientists is that a fishery is “sustainable” if it exploits the population until about half of its pristine biomass is left. That is also the point where fishing harder would result in a lower catch. Well, I think that under the “sustainable” scenario mankind still claims an awfully large part of the pie for its own use. I might add that we do so not only for tuna but also for all kinds of other “seafood”. If mankind exploits each of the key species of an ecosystem down to 50% of its pristine level, I would say we are still looking at a pretty degraded ecosystem, even though we may label it “sustainable” for each of these species individually.

What is even worse is that 50% is often consider the gold standard of objective-setting in fishery management, suggesting that objectives that allow more deterioration are justifiable. For some other tuna species, for example southern bluefin tuna Thunnus maccoyii, decision makers must accept degradation of the stock to 20% or even less of its pristine biomass. What is particularly sad is that 20% does not mean that we are enjoying a particularly large piece of the pie—it just means that we have degraded the stock to such an extent that it’s capacity to regenerate is almost, but not quite, gone for ever.

Well, yellowfin tuna is still there, but the writing is on the wall, and it is clear that there are far fewer yellowfin tuna out there than there used to be. During our trip on board Seven Seas I kept an eye out for anything of interest in the open ocean. Surface-feeding yellowfin tuna should have been a common sight… I spotted quite a few whales, even two blue whales among them. Pods of dolphins were common. I also counted 112 Fish Aggregating Devices. I did not spot any surface-feeding yellowfin tuna! I figure that the fishers that Bas talked with noticed the same.

There may be many reasons for the absence of tuna. We have had a very wet rainy season that started early and that is still not quite over, so some fishers may be hesitant to go out. This weather may also have affected the spatial distribution of tuna. Covid-19 resulted in a drop of prices, and perhaps fishers don’t want to take risks, which may also depress supply. So it’s difficult to give a definitive answer about the current shortage. Nevertheless, putting the usual academic caution aside, I am pretty sure that we are hearing the first scrapes on the bottom of the barrel. Once again, it is time to demonstrate some stewardship over the oceans and limit our take. We have the science to do so, and we can achieve sustainability without hurting fishers and fish workers in Indonesia and beyond. Society just needs to put its mind to it.

A final thought about yellowfin tuna. For many in the fish trade and for discerning consumers, “sustainability” is a vacuum-packed tuna loin or a can of tuna chunks with the blue label of Marine Stewardship Council on it. This label indicates that the tuna stocks where the product came from is at least at 50% of its pristine biomass, that the fishery does not harm other species, and that the fishery does not cause damage to the habitat. There is nothing wrong with that perspective. Nevertheless, perhaps it is time that we approach it from the other side, and that we start appreciating yellowfin tuna for what they are in their natural environment.

It’s one of the most exhilarating moments in the career of the diving naturalist get up and close with a surface-feeding school, even just watching the action from the tender. In some areas, FADs might attract sufficient yellowfin tuna and other fish to make them into a reliable dive destination, enriching eco-tourism with an entire new habitat to explore. A new appreciation for wild tuna in their natural environment should not keep anybody from enjoying the occasional sushi, tuna rica-rica, or (my favorite) ahi poke, and it may go a long way towards encouraging government and fishing communities to keep wild yellowfin tuna at least at 50% of its pristine biomass.

Peter Mous
April 2021

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