Ann Kobsa, Mitzi Messick, David Paul, Richard MacKenzie*, Caitlin Kryss*; Malama O Puna,
PO Box 1520, Pahoa, HI, 96778; * Institute of Pacific Islands Forestry, USDA Forest Service, Hilo, HI.
Abstract
The tidal pools and coastal land adjacent to the reef at Wai ‘Opae Marine Life Conservation District Puna, Hawai’i Island, have been invaded for several decades by the highly invasive red mangrove, Rhizophora mangle, and in the past decade the coral pools have been invaded. The native coastal flora have been almost eliminated from the areas of densest mangroves, and the endangered Hilo beach grass, Ischaemum byrone, has been greatly reduced in the past 6 years as the mangroves have spread. The remaining populations of this grass within the infestation are found in pockets of native vegetation. We are attempting a restoration of Wai ‘Opae based on eradication of the mangroves by treatment with the shoreline-approved, systemic herbicides Habitat and Aquamaster, using the drill-inject method on larger trees and foliar spraying smaller ones. An outreach and education component will involve students and community members in manual cutting of the most makai mangrove keiki. Previously reported mangrove eradication efforts used manual removal or heavy machinery and had much higher budgets. We intend this project as a test-case for island-wide eradication of red mangrove and for restoration efforts on the other Hawaiian islands where this species is still an incipient. We will report on the process of obtaining governmental clearance for coastal restoration, present herbicide methodologies, and discuss our plans for extirpation of mangroves and concomitant monitoring of fish community structure in the tidal pools. We hope to inspire others to initiate mangrove eradication campaigns before populations reach unmanageable sizes.
Introduction
Mangroves in Hawai’i are aggressive aliens that replace coral pool and other coastal habitats, shading out coral, dropping large amounts of organic matter, and resulting in muck-filled pools with little diversity. The Wai ‘Opae Marine Life Conservation District (MLCD), in Puna, Hawai’i Island, was granted protection due to the habitat it provides, serving as a fish nursery for an area supporting the second-largest fishery in the state. This area is an unusual complex of coral pools and reef, and a popular snorkeling spot. A 2006 study (Van Der Veur and Beets, unpublished) showed that in areas of Wai ‘Opae where mangroves are present, there are fewer fish, fewer species of fish, and more alien fish than in areas without mangroves.
Approximately 20 acres within and adjacent to the MLCD are infested with red mangrove (Rhizophora mangle). About a third of this area is mangrove forest with trees up to 30 cm in diameter and 15 m high. Another third or so is covered in dense, highly branched mangrove thicket from 0.5 to 3 meters in height, and the remaining third has more sparse mangrove mostly less than 1 m high. The endangered Hilo beach grass, Ischaemum byrone, was not uncommon here as recently as 2003, but has been largely displaced by mangrove, its population now being limited to a few plants within small areas of native vegetation. This coastline is in one of the oldest kipuka in the area, and therefore supports some of the highest diversity in the area, and a more intact native flora, including ‘ohi’a, lama, ‘akia, ‘ulei, alahe’e, and hala, among others.
Elsewhere, non-herbicidal eradication of red mangrove has been very expensive, costing between $32K and $125K per acre. We hope to offer a much more economical alternative that will allow extirpation of mangrove infestations in many more coastal areas.
Figure 1: Aerial image of Wai ‘Opae, showing fish monitoring sites and mangrove infestation.
Figure 2: Foliar spray trials using Habitat herbicide at 0.5 % and 1 %.
Methods
Our restoration project aims to achieve eradication of the red mangrove at Wai ‘Opae. Figure 1 is an aerial image of the infestation. The initial treatment regime, highly selective for mangroves, consists of injection of the larger trees with a systemic herbicide approved for aquatic applications. This is done by drilling into the tree using a cordless electric drill and filling the hole with undiluted herbicide from a squirt bottle, in an amount dependent on the diameter of the trunk. Trees greater than 6” in diameter receive 2 holes, and trees with multiple trunks receive at least one hole per trunk. Small trees (above waist high) are broken off below the lowest branch, and a few drops of herbicide are applied to the break. Killing of the large trees is being followed by foliar spraying of the shorter-stature mangroves using backpack sprayers. We are using both Habitat and Aquamaster, whose active ingredients are imazapyr and glyphosate, respectively. These are the only two systemic herbicides approved for shoreline use.
Fish community structure in the tidal pools is being monitored before and after mangrove eradication, in order to assess the effects of the mangrove and the treatment regime on the tidal pool ecology. Monitoring consists of visual surveys conducted in the water, and sampling using mini-fyke nets. The nets are positioned in the pools and, as the tide falls, fish are trapped, measured, weighed and counted. Native species are then released back into the pools. There are a total of nine sampling sites (see Figure 1). Three of them are in areas in which mangroves will be eliminated in the first year, and three more where mangroves will be eliminated in the second year, to control for natural changes. Three others are in adjacent areas without mangroves, as additional controls.
We have recruited groups of volunteers, made up of both adults and children, to pull and cut the sparse mangroves at the edges of the infestation, collecting and removing the harvested material. This serves an educational role in the community, as volunteers have fun learning about coral pool ecology and stewardship of a beautiful and ecologically significant place.
Governmental Clearance Required
In order to legally carry out our eradication project, we were required to obtain an SMA Minor Permit from the County of Hawai’i Planning Department, which required an application similar to an Environmental Assessment, and signatures of all landowners. The fish surveys required a permit from Division of Aquatic Resources (DAR)/DLNR, because we are working within an MLCD. In order to kill the mangroves, we were also required, by County Planning and the National Fish and Wildlife Service (NFWS), to notify the Office of Conservation and Coastal Land/DLNR, DAR, Army Corps of Engineers, and Hawai’i Department of Health. None of these agencies required us to obtain permits for our activities, though some placed restrictions. We were required to obtain Pesticide Applicator Certification from the Hawai’i Department of Agriculture/Pesticides Branch for application of Habitat herbicide. Both the Planning Dept. and OCCL forbade the use of power tools (excepting electric drills), and required that any material that we cut must be removed from the site, which is not road-accessible, thus providing a major disincentive to cutting. DAR requested that we not use herbicides within the MLCD, which includes some of the sparsely mangrove-infested area.
Results
Our field trials have shown that both shoreline-approved systemic herbicides are effective at killing red mangrove using the drill/inject method. Aquamaster is effective at 0.6 ml per inch of diameter for most trees, but for the tallest trees we used 1 ml per inch of diameter. In our preliminary field trials, Habitat was effective at 0.1 to 0.4 ml per tree. For foliar spraying, Habitat is highly effective at 0.5 % and 1 % (see Figure 2), with a methylated seed oil (mse) at 0.5 % as surfactant. Aquamaster was not effective in a foliar application, failing to efficiently kill the mangroves at 0.5 %, 1%, 4 % and 6 %, also with 0.5% mse.
Propagules released from dying trees poisoned with Aquamaster were nearly 100% viable. In contrast, when the trees had been poisoned with Habitat, only 3/47 propagules had not turned brown at time of writing, and none had grown.
The first phase of the eradication, killing through direct herbicide injection of the larger mangrove trees, is now complete. We spent 286 hours drilling and injecting 9127 mangrove trees on nearly 7 acres, and used 16.5 liters of Aquamaster herbicide. The value of the herbicide was $264. This works out to 1.9 minutes ($0.48 at $15 per hour) and $0.29 in herbicide per tree killed. The cost per acre was $650. We anticipate that the second phase, foliar spraying of the smaller-stature mangroves, will be at least as costly as the first phase.
Conclusions
Mangroves can be killed efficiently and at significantly lower cost by using herbicide rather than manual removal or heavy machinery. This finding will allow consideration of eradication projects that previously had been considered cost-prohibitive.
Habitat is the most effective herbicide, both for injection of large trees and foliar application to smaller plants. Aquamaster is effective using the injection method but not when used for foliar application. Aquamaster application does not require a Pesticide Applicator’s Certification from Hawai’i DOA, whereas Habitat application does. For foliar applications, Habitat is the only shoreline-approved herbicide that is effective, and it is highly effective. Furthermore, Habitat kills the propagules that are attached to the herbicide-killed trees, whereas Aquamaster does not.
This project aims to restore a beautiful area of major ecological significance. We expect that at Wai ‘Opae we will see a strong recovery of the native coastal flora. Already we see seedlings of hau, milo, and alahe’e coming up amongst the fallen mangrove leaves and propagules in the forest. Naupaka, akulikuli, and possibly Ischaemum byroni, will likely fill in much of the area now covered by mid-sized mangroves. An improvement in water quality is anticipated, and possibly an increase in native fish species diversity and population sizes. In addition to the ecosystem benefits at this site, we are helping to reduce the overall spread of red mangroves, which are expected to cover large areas of the coastline in the future if nothing is done to prevent this. Success in mangrove eradication at Wai ‘Opae, where we have the largest infestation on Hawai’i Island, will likely lead to extirpation of the species on the island.
Acknowledgements
Mahalo to: the Hawai’i Toursim Authority and National Fish and Wildlife Service Coastal Program (especially Chris Swenson) for support; Jennifer Vollmer, BASF Corporation, and John Hardy, Monsanto Corporation, for donations of herbicide; to James Leary of CTAHR and Phil Gordon for technical assistance; to Rene Siracusa for administrative guidance; and to our fearless and acrobatic field crew!
