Mangrove Removal: Alula Bay

Anchialine Pool Restoration and Mangrove Eradication at
Alula Bay, Kealakehe, Kona
Partners: Malama O Puna, Big Island Invasive Species Committee (BIISC), International Union for the Conservation of Nature (IUCN), Paci
fic Coast Joint Venture (PCJV), Kaloko-Honokohau National Historical Park (K-HNHP)

Alula Bay is home to over 20 anchialine pools and an extensive archaeological complex, but has been badly degraded by invasion of red mangrove (Rhizophora mangle) and pickleweed (Batis maritima). This project will replace the alien vegetation with appropriate native plants, restore the anchialine pool ecosystem, and help to preserve the cultural features.

Anchialine pools support a unique and diverse biota, much of which exists nowhere else. Hawai‘i, and Kona in particular, has the largest concentration of anchialine pools in the world, but still this ecosystem type is extremely limited and particularly threatened. Because of the habitat value of the anchialine pools, this area of North Kona is one of the focus areas in the Hawai‘i Wetland Joint Venture Strategic Plan.

Anchialine Pool
Anthropogenic threats to the pools include development, rising salinities due to increased groundwater use and altered rainfall patterns brought about at least in part by global warming and deforestation, and encroachment by invasive species. All these factors likely contribute to cumulative negative impacts on the anchialine ecosystem.

These pools serve as living laboratories of evolution, as genetic divergence has been occurring at enhanced rates for some species, like the tiny red shrimp, Halocaridina rubra or ‘opae ‘ula, and these systems have value for studies of evolution and biogeography, given their relative, yet incomplete, isolation from each other.

Alula’s 20+ pools, in historic accounts clear and emerald green, are now completely overrun by the alien red mangrove and pickleweed. The mangrove has formed impenetrable thickets and the pickleweed dense mats over most of the area of pools. This project’s broader value is that it is part of an island-wide mangrove-eradication effort, and as such, protects much of the coastline from encroachment by this aggressive alien, for which Alula Bay currently serves as a propagule source.

Anchialine Pool

One goal of this project is to eliminate the effect of the invasive species in excluding native aquatic and terrestrial species in the immediate area and to remove the threat they pose to the archaeology. Another goal is to reduce excess nutrient release from these invasives into the anchialine pool ecosystem and the outflow of those nutrients into the nearshore marine environment. A third goal is to protect other important habitats, including reefs, fishponds and estuaries, from mangrove invasion and pollution by removing this source of mangrove propagules. Anticipated measurable outcomes include: red mangrove and pickleweed eradicated from the site and alien biomass removed; lower nutrient loads in water both in the pools and on the reef immediately offshore; improved water flow and reduced salinity in pools, increased oxygen levels, return of anchialine pool biota, including candidate endangered arthropods; visits and possibly nesting by endangered shorebirds.

For mangrove removal, chain saws, hand saws, and machetes will be used to cut the mangroves into lengths that can be hand-carried to the road and transported to a disposal site. For mangroves and also pickleweed, which will be cut with sickles, cutting will be as low to the ground as practical. Pickleweed resprouts will be cut or burned with a propane torch as many times as it takes. If we find Mexican mollies or other poeciliids in the pools, or tilapia, which were noted in a 1972 survey, we will attempt to eradicate these aggressive alien fish, as they would prevent recovery of the anchialine ecosystem. All cut biomass will be hauled to a nearby facility where it will be converted to compost or biochar for long-term carbon sequestration and use as a soil amendment.

The native, low-growing ‘akulikuli (Sesuvium portulacastrum) will be planted, which will help to exclude pickleweed from the edges of the pools, and, further from the water, plantings will consist of naupaka (Scaevola sericea), kou (Cordia subcordata), milo (Thespesia populnea), pohuehue (Ipomoea pes-caprae), maiapilo (Capparis sandwichiana), ‘ohai (Sesbania tomentosa) and ‘ilima (Sida fallax), in areas where native vegetative cover is needed to prevent encroachment by alien species. We anticipate natural recovery of the anchialine pool ecosystem, but are prepared to reintroduce some of the anchialine species from pools at nearby locations, if necessary.

Monitoring of ecosystem disturbance and recovery at the eradication site will quantify the results. We will measure standard water chemistry components including salinity, pH, temperature, dissolved nutrients, total suspended solids, carbon and nitrogen stable isotopes of particulates and sediments, and oxygen dynamics. We will sample weekly or biweekly for at least two months before, during, and for one year after the eradication. We will also monitor changes in biotic community composition and diversity based on selected invertebrate, fish, plant and bird species. We will compare results from this site with those from another eradication site where we are currently using herbicide to kill the mangroves. This will allow us to directly compare ecosystem response variables between herbicide treatment and manual removal methodologies, to inform future mangrove eradication efforts in Hawai‘i.

Outreach is ongoing. We are in conversation with Kumu Keala Ching, of Na Wai Iwi Ola, the organization and hula halau that uses the heiau; Kumu is supportive and offers help. We are contacting lineal descendants, including the family that has been keeping the mangroves off the heiau. We plan to continue outreach for several months leading up to and overlapping the start of work. We hope to involve school groups in service-learning outings, where they will remove small mangroves, as we have done at our other mangrove eradication sites. Please contact us if you would like to be involved in any way. Mahalo nui loa!