Testing novel techniques to manage the invasive green alga Avrainvillea erecta (Berkeley) A.Gepp & E.S.Gepp on Oʻahu south shores

Abstract

Marine algal introductions threaten the biodiversity of Hawaiian reefs. With heavy shipping traffic occurring on the south shore of Oʻahu, plants are transported to near shore habitats. Coastal development and extreme use have impacted the native benthic communities. Psammophytic, or soft bottom habitats occur adjacent to Hawaiʻi fringing reefs, starting at about 15-18 m depth. Sea grass / algal meadows adjacent to coral reefs occupy as much as 50 % of benthos in Hawaiian shallow waters (<30 m). This study focuses on the meadows formed within these habitats. A newly introduced species of green alga called Avrainvillea erecta has been establishing and spreading in these meadows. It is the goal of this research to monitor the growth and spread of A. erecta and investigate management techniques to control its populations. Physical removal treatments were applied in 2020-2022. None of the removal treatments were able to significantly reduce the recovery of the plant. However, initial abundance positively influenced the number of plants that did recover in a 1m2 plot. Chemical management, using 10 ml of 3% hydrogen peroxide, were injected into plants at the Ānuenue Fisheries Research Center (AFRC) mesocosms and in situ off Kewalo Basin Oʻahu. While the mesocosm experiment did not result in a significant outcome, the in-situ experiment did result in a significant negative impact on plant densities. After analyzing these results and monitoring the spread of this plant across the main Hawaiian Islands, it is advised to categorize this plant as invasive. Areas frequented by boat anchors are at risk of transporting fragments of plants that can still be viable and propagate in new areas. Hawaiian shores that do not receive an annual storm surge should also be monitored. These quiet water locations specifically within Maui Nui complex are at high risk for invasion by A. erecta. Early detection via monitoring and rapid response appears to be the most critical action managers should consider.