Hundreds of fish died in Maui's Wailuku River this week while the state was implementing measures to improve their habitat.
The die-off occurred as officials installed a ladder to help fish climb a 22-foot (6.7-meter) man-made wall in the river, The Maui News reported.
Officials had arranged for river flows to be diverted to create safe working conditions for the installation. Officials said the reduced flows and low rainfall contributed to the deaths.
On Thursday, residents who frequent the river mouth scrambled to rescue 'o'opu, a type of goby fish, floundering on dry stream beds and puddles of warm water below the project site. Small piles of dead fish and shrimp were strewn about the riverbed.
Volunteers and Department of Land and Natural Resources staff used nets to scoop struggling fish and other wildlife into buckets and coolers and take them to higher Iao Valley streams in the hope they would survive.
"It is obviously ironic that our project to improve stream habitat for 'o'opu appears to have resulted in loss of hundreds of fish," said Suzanne Case, the department's director and the chairwoman of the state water commission. "We regret this situation and express our sincere apologies to the Wailuku River community for these events."
There are five species of 'o'opu in Hawaii streams, four of them are found nowhere else in the world. 'O'opu swim down to the ocean and back up the river at different stages of life. As adults, they remain in freshwater.
Aheone Kanamu, a student at University of Hawaii Maui College, helped move at least 100 fish over multiple trips.
"What makes Hawaii is our native species ... We have to save that before we have nothing left," she said.
The state Commission on Water Resource Management received funding from the U.S. Fish and Wildlife Service in 2015 to improve the river's habitat after the commission found the 22-foot vertical structure in the flood channel was an obstacle to the upstream migration of 'o'opu.
For the fish ladder installation, the state asked Wailuku Water Co. and Mahi Pono to divert river flows. Installation was completed Tuesday and both companies were notified to resume flows.