Mercury Risk: Low
Katsuo is one of those Japanese menu terms that can get the average American sushi lover into a bit of trouble. The word corresponds to two different fish: skipjack tuna (Katsuwonus pelamis; notice the similarity to katsuo) and bonito (Sarda spp.), a tuna relative in a different and distinct genus. The precise term for Sarda species is hagatsuo, but it is rarely employed on menus in the United States.
It’s difficult to tell if the katsuo on the plate in front of you is skipjack or bonito. Both fish are often cooked or otherwise preserved before being served. Bonito does not freeze well and is not served raw due to its strong flavor. Instead it is usually seared and served with scallions and horseradish. For similar reasons, skipjack is often served the same way.
As a general rule, it much more common to encounter skipjack tuna than true bonito in a sushi bar. While there are certainly exceptions to this rule, information is often difficult to acquire because of the translation problem posed by the dual applications of the word katsuo. Unless there is reason to believe otherwise, it should be assumed that the katsuo in front of you is skipjack.
A great deal of bonito caught in Japan is dried and shredded into katsuobushi (bonito flakes), which is a very common ingredient in many Japanese dishes. Once dried and prepared in such a manner, bonito can be shipped anywhere in the world and remain palatable for an extended period of time.
True bonito may be facing trouble in the United States. It has slipped through cracks in the regulations: There are no real limits on the capture of bonito, and populations seem to have dwindled severely since the 1960s and 1970s. That being said, bonito is a migratory fish and supports a “pulse” fishery. Pulse fisheries occur when a species passes through an area only once in a while, attracts heavy fishing pressure when it is nearby, and then disappears altogether. So, some might argue that populations are still strong and the fish are just elsewhere for the time being. However, it has been some time since the last bonito “pulse” in U.S. waters.
There were once commercial boats in California that targeted bonito, but this fleet is now largely defunct. True bonito found in the sushi bars of the United States are probably caught by trolling or incidentally by mackerel trawlers and other fleets.
Bonito populations may also be affected by “decadal oscillation,” a phenomenon in which the ocean climate alternates between two extremes. This pattern affects the sardine and anchovy populations off the California coast. Anchovies are the preferred food of bonito, and the tiny fish are currently in decline due to a climate that favors sardines. If there is a shift toward anchovies in the near future, it is quite possible that it will bolster the bonito population substantially.
True bonito from the US West Coast is from a poorly-understood fishery that may or may not be a sustainable option. The general lack of management is troubling, as is the absence of the fish from local waters in recent years. Californian bonito is now targeted primarily by a recreational fishery, so it is only rarely encountered in sushi bars.
As for skipjack tuna, it matures quickly, and populations are relatively strong—the major issue is how it is caught. Skipjack are targeted by longlines, which are miles-long ropes baited with hooks in series; purse seines, a large net pulled around a school of fish, the ends joined into a circle, and then closed at the bottom; and handlines or trolled lines. Each of these methods varies significantly in its impact on the ecosystem.
Longlined skipjack is known to have high levels of bycatch, unintended species and juveniles that are caught and discarded, including turtles and sharks. The only exception to this rule is the Hawaiian longline fishery, which has strict bycatch regulations.
Purse-seined skipjack fleets are generally less destructive to fish populations than longliners. However, many seiners use fish aggregating devices (FADs) to concentrate the catch. These man-made objects, often buoys or floats, attract many types of fish and are known to cause significant bycatch in these fisheries.
Troll-caught skipjack has almost no bycatch and is an excellent choice at the sushi bar.
So, if your sushi bar offers katsuo, be sure to determine the species. Bonito could be a relatively sustainable option, but the jury is still out. Until there is management in place and we know more about the dynamics of the population, it’s best to be cautious. Skipjack can be a good choice or a poor one depending on how it was landed.