sushiNovember 1 is National Sushi Day. If you like sushi, you’ve probably tried to get—if not fought tooth and chopstick over—the last unagi roll on the table. Still warm from the grill and sweetened to perfection with a Japanese barbeque sauce, unagi, or freshwater eel, is a favorite among many sushi fans. But you may think twice about ordering eel the next time you’re at a sushi bar.

American eel, which is typically served as unagi in Japanese restaurants, poses a challenging fisheries management problem that has to do with its life cycle.

Aristotle, who was probably the first to observe and write about freshwater eels, postulated in the 4th century that freshwater eels generated spontaneously from the mud. He thought this because he observed eels in lakes that had previously been dry land.

But what Aristotle first observed was likely related to the American eel being a catadromous fish, meaning that it spawns in the ocean and matures in inland lakes, streams and estuaries. This is opposite of many fishes, like salmon, which spawn in rivers and migrate to the ocean where they spend most of their adult lives.

Maturing in streams and lakes also means that a significant portion of wild-caught eels are captured before they have an opportunity to spawn in the sea. Like many long-lived fishes, American eels may take up to 30 years to reach maturity, at which point they are ready to spawn.  That puts the odds against the eel right away.

If that weren’t enough, American eels spawn in only one known location in the world—the Sargasso Sea, situated northeast of the West Indies. There, the eels will lay 10 million to 20 million eggs—talk about putting all your eggs in one basket!

Although American eel populations are large enough to support a steady commercial fishing industry in the United States, Canada and Europe, significant population declines have been observed over recent decades. Many scientists are concerned that a combination of overfishing, barriers to migration, habitat loss, hydro turbine impacts and pollution may threaten the long-term health of this species.

Luckily, you can help keep American eel populations healthy while enjoying delicious sushi. If you love the sweet and smoky flavor of unagi, here’s a good tip: Ask your sushi chef if you can replace the eel with sablefish (also known as black cod). Caught in the North Pacific, the sablefish is a great alternative to eel—its rich and silky texture grills perfectly, and it’s a sustainably harvested fish. For now this sushi alternative may be the road less travelled, but take it from the American eel—it’s worth it!

Reid Bogert, Great Lakes and sustainability team