Bringin’ back the ‘o‘opu

  • Gordon Smith, U.S. Fish and Wildlife Service / Special to The Garden Island

    The o’opu, or Hawaiian goby fish, is seen in a stream, above, and in a biologist’s hand, below.

LIHUE — For three years, hydrologist and water resources engineer Matt Rosener has been heading up a stream-restoration project in the North Shore Waipa watershed.

His goal: to bring more native ‘o‘opu back to the stream, and to create a forest of native and food-producing plants to keep invasive species at bay and the Waipa Stream clear.

‘O‘opu fish were an important food source for ancient Hawaiians. They’re still also used in the aquaponics component of taro farming.

Adults live and spawn in fresh water, but their eggs float out to sea and the fish reach the juvenile stage in salt water before they swim back upstream to restart the cycle.

According to the state Department of Land and Natural Resources, a “substantial fish kill” of native ‘o‘opu” was documented in late October in the Wailuku River on Maui. The department cited stream flows “insufficient to reach the stream mouth and sustain fish life” as the cause.

Rosener said the Waipa Stream was facing the same issue, though it was invasive hau bush that was clogging up the stream and reducing flow.

“(That) created isolated ponds that trapped the fish. They’d dry up and the fish would die,” Rosener said. “That was the main reason for the project.”

In 2016, Rosener and his team started hacking away at the hau and monitoring the results. As the hau started to disappear, water began to flow. Sunlight filtered into the stream. Fish came back. Limu (seaweed) came back. Bacteria counts lowered.

Rosener said Wednesday he’s been working on finishing up a report that looks at results in the stream. They’re also working with Kamehameha Schools and the Waipa Foundation to monitor ‘o‘opu larvae numbers in the Waipa Stream.

“We’ve seen some interesting results of that study so far, too,” Rosener said.

According to the data Rosener’s compiled, there’s been a nearly 400% increase in native ‘o‘opu numbers between the 2016 and 2019 surveys of Waipa Stream.

And that’s not all.

According to a May 2019 report on the fish numbers in the stream, “The noteworthy improvement in the overall biotic integrity in Waipa Stream was the result of a 75.7% increase in the total numbers of fish and macroinvertebrates censused by divers in 2019 (compared to 2016) with increased population densities of native ‘o‘opu in all study sites.”

On Maui, officials with the state Commission on Water Resource Management point to low rainfall and declining stream flows in the summer months as the reasons behind some of the dead fish in the Wailuku River. They also point to the intentional reduction of stream flow in connection with the fish ladder project that “further exacerbated” the problem.

Officials on Maui recently installed a fiberglass fish ladder to help improve fish mobility and population numbers of the native ‘o‘opu (gobies) in the Wailuku River.


Jessica Else, environment reporter, can be reached at 245-0452 or


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