Does Ni'ihau fish kill cause still lurk?

Nearly two months after they were found, the thousands of dead fish on Ni'ihau's coast are still a mystery.

Fish have stopped dying, and health officials have told residents they can resume eating their nearshore marine life.

(Image: One of the species killed in the January 2009 Ni'ihau fish kill was the humuhumu 'ele'ele. This 1903 image from NOAA's historic fisheries collection is originally from "The Shore Fishes of the Hawaiian Islands, with a General Account of the Fish Fauna", by David Starr Jordan and Barton Warren Evermann. Bulletin of the United States Fish Commission)

But what killed the fish? Can it happen again? Is there some lurking threat? And what about the initial suspect, the use of rat poison on the neighboring islet of Lehua. Was that the culprit?

The answer to the last question, in short, is no. Something killed lots of fish near Ni'ihau, but it's extremely unlikely it was diphacinone, the rat bait that was aerially distributed on nearby Lehua Islet. More on this farther down.

Ni'ihau residents grew understandably concerned when thousands of dead fish washed ashore in January, and they made the logical connection to an aerial application of the rat poison diphacinone on nearby Lehua Islet a couple of weeks earlier.

After weeks of research, the Ni'ihau fish kill continues to baffle. But the research is continuing.

The first suspect was diphacinone, so here's the case on that. It was used nearby—Ni'ihau and Lehua are only half a mile apart—and the time of application, early January, was reasonably close to when large numbers of dead fish were seen, which was in the third week of January. But all the evidence thus far appears to show that diphacinone could not have been the culprit.

A key piece of evidence is a research project done just a year earlier on another steep-sided Hawaiian island, about the same size as Lehua, where precisely the same rat poison was distributed in the same way. The report by Robert W. Gale, Michael Tanner, and Carl E. Orazio is “Determination of Diphacinone in Sea Water, Vertebrates, Invertebrates, and Bait Pellet Formulations Following Aerial Broadcast on Mokapu Island, Molokai, Hawai’i.”

Within days after the broadcast of diphacinone at Mokapu, the researchers tested the water, they tested the fish, they tested the 'opihi clinging to the rocks on the side of the island. They also checked the water at Kalaupapa, which is downwind from Mokapu, as Niihau is (during tradewind weather) downwind from Lehua. Their result: “No detectable concentrations of diphacinone were found in the fish, limpets, or sea-water samples from Mokapu Island or from the reference sites.”

Diphacinone is a powerful anticoagulant. In big enough doses, it inhibits blood clotting, causing internal bleeding and death in target species. It is primarily used to control rodents, like rats, mice and voles, and can harm bats as well. These species are very susceptible to it in quite small doses. It is listed as slightly to moderately toxic in fish, although only freshwater fishes were tested.

We checked diphacinone at Extoxnet, a chemical toxicity site operated by a number of universities in the United States, and at NOAA's toxic chemical site, as well as a label for one of the diphacinone baits. Make no mistake, diphacinone in high enough concentrations is extremely toxic to humans. In its pure form, a teaspoon full is probably enough to kill a human.

Here are some pieces of the puzzle:

The baits are extremely dilute mixtures, with a fraction of a percent diphacinone in the bait pellets.

Diphacinone doesn't survive long in the marine environment. It decomposes quickly in water and sunlight.

The necropsies done on the Ni'ihau fish showed they had no detectable diphacinone.

There were no heavy rains during the period between the distribution of rat bait and the fish kill that would have caused rat bait to wash into the ocean.

And, for the entire period in question, winds and currents in the Lehua were flowing away from Niihau not toward it.

The dominant fish involved in the kill were two species of triggerfish, the humuhumu 'ele'ele and the humuhumu hi'ukole. Also located were a few gray chub, or nenue, and a blue-stripe snapper, ta'ape.

They were of different ages and sizes, and the deterioration suggested they had died at different times.

Some appeared to have distended swim bladders, and many had inflamed gills. Gill inflammation can come from parasites, viruses, use of toxic chemicals for fishing, bacteria and other things. It's not known from diphacinone.

Inflamed gills are a fairly common symptom in fish kills across the oceans, and their cause often remains mysterious—as in this case, where no cause could be readily identified.

State aquatic biologist Don Heacock, who collected dead fish samples on Ni'ihau, said the options are endless. It could be a natural event, a man-made event at sea, a man-made event on land that affected the marine coast.

The questions are endless, too. Why did it affect only a few species, not all of them related to each other? Why both young and old, large and small specimens? Why did it continue to kill over time rather than all at once? Why only Ni'ihau and not other islands? Why were most of the dead fish found at the uninhabited southern end of Ni'ihau? What factor links inflamed gills and distended swim bladders?

It's a classic mystery. Heacock said state and federal officials continue to work on it. They're looking desperately for the clue, the link, the thing that brings the evidence into focus.

“What is it we're not seeing here?” Heacock said.

©2009 Jan TenBruggencate