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.
A Fish Aggregating Device is a raft anchored in deep waters. The palm fronds under the raft help to attract fish, and the palm fronds on top of the raft help fishers find the FAD. Picture by Foued Kaddachi.