The cause of a fish kill that led to thousands of dead fish washing onto the shores of Ni'ihau in February remains a mystery, but the state has ruled out a poison that was used to control rats.

The owners and residents of the privately owned island had suspected that the use of the rodenticide diphacinone may have led to the kill. The U.S. Fish and Wildlife Service, on behalf of the state, used a helicopter to drop the chemical on Lehua Island, north of Ni'ihau, in January to control an infestation of rats that were threatening native seabirds.

But the state Department of Land and Natural Resources yesterday said tests of samples collected from the kill and freshly caught specimens showed no signs of diphacinone.

"We have conducted two rounds of rigorous testing in independent laboratories of samples taken at two different times, and both reported no rodenticide," said Laura H. Thielen, DLNR chairwoman.

Thielen said the cause of the fish kill remains under investigation.

"Although fish kills occur for a variety of reasons, we will continue to look for other leads on possible causes of the fish kill," she said.