The Alenaio canal next to the Wailoa Visitor Center in Hilo is getting a little help from the Division of Aquatic Resources and State Parks to fight off an invasion of Salvinia molesta, an aquatic fern that has recently taken over Lake Wilson on Oahu.
Wayne Okamura, a fish farmer, discovered the salvinia in October. Okamura notified aquatic biologist Bob Nishimoto of the DAR, who contacted the Division of State Parks. According to the DAR’s education specialist, John Kahiapo, State Parks was already working on some weed control in another area, so they offered to do a test spray of the herbicide Rodeo on the fern. The test spray, applied a couple of weeks after the discovery of the invasive plant, was effective in killing the 8-by-24-foot patch of salvinia. However, a thick growth of California grass around the salvinia allowed the fern to rest on top of the water instead of sinking after it died. This allowed it to grow back more easily. Kahiapo said he noticed the regrowth around the end of November.
DAR personnel are hopeful that the Hilo invasion can be treated more successfully than that in Lake Wilson in central O`ahu. For one thing, the invasion was caught at an earlier stage in its development. For another, the Alenaio is part of an open water system close to the coast, whereas Lake Wilson is more closed.
Nishimoto says if there is enough rain to flood the canal, then the salvinia should be washed out to the ocean where it will die, but the DAR is not waiting for this to happen. Because the plant’s growth rate is so fast (it can double its size in just two days) the division plans to remove the weed manually, possibly with the help of prison labor from the county correctional facility or Kulani Prison.
According to Nishimoto, there’s no way of really knowing how the salvinia got into the canal, but he suspects it originated from someone dumping a home aquarium into the Alenaio. Although salvinia is federally list of noxious weeds, it is often used as an aquarium plant.
The salvinia invasion thus becomes another example of how the human tendency to be compassionate toward animals can backfire when it results in the release of animals and plants into places they don’t belong, Nishimoto explains.
Another aquatic biologist for DAR, Glenn Higashi, suspects that the salvinia in Lake Wilson got there the same way it got to the Alenaio canal. Although the weed had been in Lake Wilson for a couple of years, recently conditions became right and it “just went crazy,” Higashi said. The agricultural and effluent run-off that reach the lake make it a nutrient-rich haven for the plant.
Because Lake Wilson is used by recreational fishers, herbicide use there is not possible. Nishimoto pointed out that the effect of the herbicide on the fish was not the only reason poisoning the weed might not be the best option.
“I think the biggest concern is it [salvinia] sinking to the bottom and then you’ll have oxygen depletion,” Nishimoto explained. “When it dies, basically you’ll have a degradation and decomposition. Then you’ll have depleted oxygen and the fish will die.”
At present, the only option is physical removal. The DAR is using oil booms and front loaders to scoop the salvinia out of the lake. Higashi says he hopes to borrow excavators from the City and County of Honolulu to speed up the process. The manual removal is being used in conjunction with the herbicide Rodeo, which is used in areas the machinery cannot reach, such as under trees or in thick patches of California grass. Higashi says the task could take more than a year.
— Miranda Watson
Volume 13, Number 7 January 2003