Jared Char, Casey Ching, Rylen Nakama, Paul Riley, Genelle Watkins

Effects of Invasive Algae Removal in Maunalua Bay

The removal of invasive alien algae (IAA) from Maunalua Bay produces numerous cascading effects for local socio-ecological systems. IAA invasions have been correlated with alterations in natural biological processes within the bay, causing concern for the health of this valuable marine environment. Since 2007, Mālama Maunalua has directed a community-based restoration initiative called the Huki Project, an ongoing removal effort that has pulled over 1500 tons of invasive algae from the bay. Thus far, Mālama Maunalua has supported the management and maintenance of over 28 acres of reef-flat habitat, due in large part to extensive community outreach events and overwhelming public investment in the goals of the organization. A decade after the start of the Huki Project, ecological and community responses to this monumental restoration effort have yet to be assessed, much less accumulated in an accessible format for the public.

In collaboration with Mālama Maunalua, for the last several months, our group has worked on assessing the cascading effects of invasive algae removal within Maunalua Bay. Overall, several themes were apparent within the scope of the last few months. This included the fish biomass and invertebrate assemblages within the bay, the perspectives of the individuals who are fishing the bay, and the presence of native seagrasses within each area surveyed. As far as fish biomass and invertebrate assemblages are concerned, within and outside the huki areas, there were virtually no fish observed over a period of about 3 weeks. Invertebrate assemblages need further study as there were not enough details about the effect of the non-native and invasive algae’s presence throughout the scope of this study. Looking into fisher perspectives and historical viewpoints of the bay over time, fishers believe that fish populations were low in abundance over time due to several reasons such as overfishing, laying net, and harvesting juvenile fish. This project was a culmination of several research questions pertaining to the quality of Maunalua Bay and the IAA removal, and though we have collected preliminary data, this is only the start of future and more in-depth research.

Invasive algae, the foundation of these ecosystem shifts, has smothered both native seagrass beds of Halophila hawaiiana and coral reef causing a dominant status within the area. Mudweed, or Avrainvillea amadelpha, traps runoff and sediment from streams resulting in an increased sediment load and anoxic zones decreasing species richness. By 2003, mudweed covered 100% of the areas bordering seagrass meadows. Halophila’s propagation through vegetative growth causes it to produce seeds infrequently, therefore, waiting for it to self-spread following IAA removal is impractical. In order to begin restoring the ecosystem to its previously healthy conditions, transplanting cores naturally grown from another area is most effective, according to a study done by Murphy in 2013. However, even under these conditions, a 50% failure rate was still present due to soft substrate, wave action, and turbidity in areas where transplants were planted. Transplanting efforts should still be made, but avoided in areas within Maunalua Bay that did not support seagrass beds historically.

The removal of IAA from Maunalua Bay directly affects the marine invertebrate communities which inhabit the bay’s benthic environments. Assemblages of macrofauna and other bottom-dwelling invertebrate species vary according to substrate type and vegetation profiles. Previous studies on marine invertebrates in Maunalua Bay have identified distinct community types associated with specific habitats. Communities found within mudweed-dominated sites demonstrated overall reduced diversity, higher numbers of invasive species, and overall greater abundances of all invertebrates compared to several native habitat types. However, there were no significant differences in species compositions between mudweed-dominated and native limu-dominated habitats. The removal of mudweed nonetheless initiates a massive disturbance for communities of benthic organisms, and post-removal and recovery sites demonstrated marked declines in total species richness and abundance. Recovery status is tentatively designated after ~9 months after IAA removal. A greater understanding of predator-prey relationships in Maunalua Bay will be necessary for discerning potential repercussions of IAA removal on significant target fish species. Fish abundances are strongly correlated with the presence and type of invertebrate prey items, and therefore we can expect responses in the regional and temporal abundances of certain predatory fish species.

The next trophic level affected by these ecosystem shifts are the fish populations. Due to residents and visitors utilizing Maunalua Bay for fishing and recreational activities that have degraded the reef habitats, fish biomass is lower than in other reef habitats in Hawai‘i. Though the invasive algae is harmful, it fills a niche that was lost when the reef and seagrass beds declined, allowing a place for juvenile fish to seek shelter. When this invasive algae is removed, these fish become vulnerable to fishers and predators decreasing their presence within the area. A study done by Friedlander and Parrish in 1998 found that shelter for fish was conducive to greater biomass of fish.

The most sympathetic to the health of fish populations are those that interact with this resource most frequently, the fishers of the area. Most of the fishers still present within Maunalua Bay are from the surrounding residential areas and recognize that fish populations have greatly declined over the years. Although not much of a change in species composition has occurred, they are worried about the lessened overall presence of fish. The main reasons they attribute to this are the illegal lay netters that indiscriminately catch small juvenile fish, water recreational activities destroying the habitat, and the invasive algae removal itself. However, it was inspiring to hear that all fishers surveyed in this study practiced and recognized sustainable fishing.

The dauntingly complex challenges faced by Mālama Maunalua and the Maunalua Bay community are representative of broader themes in environmental management in the Hawaiian Islands. Creating effective strategies for sustainable stewardship practices must utilize input from all user groups and stakeholders. It is imperative that future decisions are informed by sound scientific evidence and the consent of the community, both of which should be primary components of any community-based restoration projects conducted in Maunalua Bay. Thorough understandings of marine ecosystems in the bay must include assessments of viable conditions for native seagrass beds and the ecological significance of their presence. Current work being done on seagrasses include analyses of subterranean sediment profiles and their associated benthic invertebrate communities. Further ecological studies that characterize the distribution and abundance of significant invertebrate prey species would be essential for understanding potential responses in regional fish assemblages. Future NREM classes could increase fisher survey sample size, investigate seagrass restoration, take samples of invertebrates using sediment cores, and/or conduct further fish surveys using transects within the Paiko area and in other areas of Maunalua Bay.