On May 17 I wrote about two fin whales approaching my sailboat, noting that I could smell their nose-wrinkling breath after a blow. I wrote, "The odor isn’t what you’d call refreshing. It’s the essence of dead fish."

That day, a Big Island boat captain and marine naturalist e-mailed me that "whale breath … does sometimes smell, but it’s not because you’re smelling dead or decomposing fish from their dinner. … Whales with bad breath actually have a form of diphtheria."

The captain thoughtfully included the source of this information, a website called WhaleNet, sponsored by Boston’s Wheelock College, specializing in early childhood education.

In 1994 a Boston-area dentist, Tom Ford, posted on WhaleNet his theory that the lousy breath in humpback whales does not come from the food they eat, but rather from bacteria in their respiratory tract. He based this premise on the fact that a whale’s respiratory system has no connection to its digestive system, as it does in humans. Therefore, Ford reasoned, the foul odor on a whale’s breath can’t come from its food.

In 1985, Ford cultured bacteria from a humpback whale’s respiratory "goop." His conclusion: Humpback whales with bad breath are between 1 and 3 years old and have a nonfatal form of diphtheria.

When I e-mailed Ford with questions about this unpublished investigation, he replied, "The whales in Hawaii are most likely afflicted with bad breath due to ketosis." Ketones are chemicals produced when a fasting person or animal lives off its body fat.

Tom’s hypotheses are interesting but I’m not convinced. That the odor of dead marine animals in whales’ blows is caused by either diphtheria or ketosis seems a leap.

I think this because I’ve smelled the foul exhalations of humpback whales in Antarctic waters while watching them eat. My fin whales also were feeding, as evidenced by the enormous poop one deposited off the side of my boat.

In addition, some food odors are carried in the bloodstream. The smell on a person who ate garlic the day before comes from sulfur compounds that enter the bloodstream and gradually exit through the lungs and skin pores. Alcohol is another "food" we swallow that’s exhaled (and quantified) in a person’s breath.

Perhaps some chemical in fish or krill also makes it into whales’ bloodstreams, giving their exhalations fishy odors.

Whale species all over the world have fish breath. The term "stinky minke" is a nickname minke whales earned for their odor of rotten fish. Right whales reek, too. Philip Hoare, author of "The Whale, In Search of the Giants of the Sea" (Ecco/HarperCollins, 2010), writes that the smell of right whales is "somewhere between a cow’s fart and a fishy wharf."

Gray whales have their own odor issues. Some individuals smell so "medicinal" that neither Chukotkan subsistence hunters (in Russia) nor their sled dogs can eat the meat. In 2008, Russian and U.S. scientists examined grays for industrial pollutants. The tissues of the stinkies and the nonstinkies were the same. The cause of the unpleasant smell remains unknown.

Odors in grays may have changed over recent years (or not), but their foul smell is nothing new. In the 1700s, Capt. Jean-Francois La Perouse wrote that gray whales were so abundant in Monterey Bay that "their breath caused a most annoying stench."

It seems unlikely that every time we smell fishy whale breath, the animal has diphtheria or is starving. I look forward to seeing more research on the subject.

There is one significant bonus to whale halitosis: If you can smell their breath, it means those glorious creatures are close to your boat.

Susan Scott can be reached at www.susanscott.net.