Abstract
Invasive macroalgae pose a serious threat to coral reef biodiversity by monopolizing reef habitats, competing with native species, and directly overgrowing, and smothering reef corals. Several invasive macroalgae (Eucheuma clade E, Kappaphycus clade A and B, Gracilaria salicornia, and Acanthophora spicifera) are established within Kāne‘ohe Bay (O‘ahu, Hawai‘i, USA), and reducing invasive macroalgae cover is a coral reef conservation and management priority. Invasive macroalgae control techniques, however, are limited and few successful large-scale applications exist. Therefore, a two-tiered invasive macroalgae control approach was designed, where first, divers manually remove invasive macroalgae (Eucheuma and Kappaphycus) aided by an underwater vacuum system (“The Super Sucker”). Second, hatchery-raised juvenile sea urchins (Tripneustes gratilla), were outplanted to graze and control invasive macroalgae regrowth. To test the effectiveness of this approach in a natural reef ecosystem, four discrete patch reefs with high invasive macroalgae cover (15–26%) were selected, and macroalgae removal plus urchin biocontrol (treatment reefs, n = 2), or no treatment (control reefs, n = 2), was applied at the patch reef-scale. In applying the invasive macroalgae treatment, the control effort manually removed ∼19,000 kg of invasive macroalgae and ∼99,000 juvenile sea urchins were outplanted across to two patch reefs, totaling ∼24,000 m2 of reef area. Changes in benthic cover were monitored over 2 years (five sampling periods) before-and-after the treatment was applied. Over the study period, removal and biocontrol reduced invasive macroalgae cover by 85% at treatment reefs. Our results show manual removal in combination with hatchery raised urchin biocontrol to be an effective management approach in controlling invasive macroalgae at reef-wide spatial scales and temporal scales of months to years.
Highlights
Non-native macroalgae have been introduced worldwide (Schaffelke, Smith & Hewitt, 2006) as a result of spread through vectors including biofouling, ballast water, the aquarium trade, and seaweed mariculture (Ruiz et al, 2000; Zemke-White & Smith, 2006; How to cite this article Neilson et al (2018), Herbivore biocontrol and manual removal successfully reduce invasive macroalgae on coral reefs
In this study we evaluate the effectiveness of manual removal combined with urchin biocontrol in sustaining a reduced invasive macroalgae cover (E/K (i.e., Eucheuma, Kappaphycus clade A, Kappaphycus clade B), G. salicornia, A. spicifera) at a reef-wide scale over 2 years using a Before After Control Impact (BACI) experimental design
Invasive macroalgae removal and biocontrol techniques were carried out on four shallow (0.5–2.0 m depth) patch reefs located in central Kane‘ohe Bay, on the windward side of O‘ahu, Hawai‘i (21◦28 0 N, 157◦49 0 W), which is the largest embayment in the Hawaiian Islands and contains over 70 distinct patch reefs surrounded by a barrier reef and fringing reef system (Fig. 1)
Summary
Non-native macroalgae have been introduced worldwide (Schaffelke, Smith & Hewitt, 2006) as a result of spread through vectors including biofouling, ballast water, the aquarium trade, and seaweed mariculture (Ruiz et al, 2000; Zemke-White & Smith, 2006; How to cite this article Neilson et al (2018), Herbivore biocontrol and manual removal successfully reduce invasive macroalgae on coral reefs. Macroalgae production can have inadvertent consequences for tropical reef biodiversity (Smith, Smith & Hunter, 2001; Stimson, Larned & Conklin, 2001; Ballesteros, 2006; Kružić, Žuljević & Nikolić, 2008; Longenecker, Bolick & Kawamoto, 2011; Martinez, Smith & Richmond, 2012; Sellers, Saltonstall & Davidson, 2015), contributing to a suite of anthropogenic pressures that are driving the global decline of live coral (Bruno & Selig, 2007; Gardner et al, 2003; Pandolfi et al, 2003; De’ath et al, 2012). Following the 2014–2015 global coral bleaching events (Van Hooidonk, Maynard & Planes, 2013; Eakin et al, 2017; Hughes et al, 2017) there is a need for immediate action to protect and restore coral reefs worldwide, including the management of invasive macroalgae on coral reefs
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