A bloom of extremely small but deadly diatoms is believed to have killed 64,000 silver salmon smolt released into a holding pen at the Nick Dudiak Fishing Lagoon on June 3. The Alaska Department of Fish and Game pathology lab in Anchorage found high numbers of chaetoceros diatoms in fish samples, said Nicky Szarzi, a fishery biologist with the department in Homer.

A chain diatom made of rows of plankton, chaetoceros has sharp corners that can cut fish gills. The cuts cause mucus to be secreted and the fish die of suffocation, Szarzi said.

"We just happened to have the misfortune of timing with a smolt release during a bloom," Szarzi said.

Citizen scientists working with the National Oceanic and Atmospheric Administration and the Kachemak Bay Research Reserve periodically monitor Kachemak Bay for harmful algae blooms. At six sites in the bay, mostly at oyster farms, scientists take samples and then examine them under microscopes to look for suspect plankton. High levels of chaetoceros diatoms have been observed in the bay since late May, said Ingrid Harrald of KBRR.

Chaetoceros is found in the top 3 feet of the ocean. Fish can escape by swimming deeper, but the concentration of smolt in the 8-foot deep holding pen meant some fish couldn't escape. When they came to the surface, the smolt encountered the chaetoceros bloom.

About 29,000 smolt survived of the 93,000 released. Szarzi estimated 1,500 adults will return late next summer from the surviving smolt. The surviving smolt are doing fine, she said.

This release of smolt came from the Cook Inlet Aquaculture Association's hatchery at Trail River near Moose Pass. CIAA may be able to provide more smolt.

Now that biologists understand the danger of releasing smolt during chaetoceros blooms, they'll monitor the bay before next releasing smolt, Szarzi said.

Michael Armstrong can be reached at michaelarmstrong.@homernews.com.