Bubble Curtains: Herbivore Exclusion Devices for Ecology and Restoration of Marine Ecosystems?

Abstract

Herbivorous fishes play a critical role in maintaining or disrupting the ecological resilience of many kelp forests, coral reefs and seagrass ecosystems, worldwide. The increasing rate and scale of benthic habitat loss under global change has magnified the importance of herbivores and highlights the need to study marine herbivory at ecologically relevant scales. Currently, underwater herbivore exclusions (or inclusions) have been restricted to small scale experimental plots, in large part due to the challenges of designing structures that can withstand the physical forces of waves and currents, without drastically altering the physical environment inside the exclusion area. We tested the ability of bubble curtains to deter herbivorous fishes from feeding on seaweeds as an alternative to the use of rigid exclusion cages. Kelps (Ecklonia radiata) were transplanted onto reefs with high browsing herbivore pressure into either unprotected plots, exclusion cages or plots protected by bubble curtains of 0.785 m 2 and 3.14 m 2 . Remote underwater video was used to compare the behavioral response of fishes to kelps protected and unprotected by bubble curtains. Kelp biomass loss was significantly lower inside the bubble curtains compared to unprotected kelps and did not differ from kelp loss rates in traditional exclusion cages. Consistent with this finding, no herbivorous fishes were observed entering into the bubble curtain at any point during the experiment. In contrast, fish bite rates on unprotected kelps were 1,621 ± 702 bites h −1 (mean ± SE). Our study provides initial evidence that bubble curtains can exclude herbivorous fishes, paving the way for future studies to examine their application at larger spatial and temporal scales, beyond what has been previously feasible using traditional exclusion cages.

Temperate Kelp Forest Collapse by Fish Herbivory: A Detailed Demographic Study

“…That the degradation of Atlantic kelp forests on NW Spain is driven by fishes, rather than by sea urchins as typical seen in many other temperate latitudes, is a challenge from a management perspective. Attempts have been made to reduce the impact of herbivorous fish through exclusion devices with some success in short-term experiments, but their longterm implementation and scaling up to management-relevant sizes remains unresolved (Bennett et al, 2017). Alternatively, transplanting adult kelps or seeding very young plants have been explored as restoration tools to combat kelp forest decline with mixed results (Fredriksen et al, 2020).…”

Identifying key factors for transplantation success in the restoration of kelp (Ecklonia radiata) beds

“…This was presumably due to the challenges of ensuring 100% removal and exclusion on a topographically complex reef, noting only one urchin was observed inside the fence at Point Linley. The increased difficulty in applying urchin exclusion devices in complex habitats may mean these habitats will incur greater maintenance and financial costs to restore (Bennett et al 2017). The fences also became heavily fouled with filamentous algae, likely resulting in shading and presumably reduced water flow inside the fences (Miller & Gaylord 2007; Bennett et al 2017), both of which could have contributed to the increased loss in fenced treatments.…”

“…The increased difficulty in applying urchin exclusion devices in complex habitats may mean these habitats will incur greater maintenance and financial costs to restore (Bennett et al 2017). The fences also became heavily fouled with filamentous algae, likely resulting in shading and presumably reduced water flow inside the fences (Miller & Gaylord 2007; Bennett et al 2017), both of which could have contributed to the increased loss in fenced treatments. Enhancing this effect was the fence length to area ratio (4 m/m 2 ) which is far higher than would occur in a larger restoration effort.…”

“…Transplanting may be justified where background drivers for loss have been addressed but where spore supply or suitable settlement substrate remain low (Morris et al 2020). While transplantation has rarely been implemented with kelps at scale, and only with mixed success (Sanderson 2003), the method has been used extensively in research (Fowler‐Walker et al 2005; Bennett et al 2017; Layton et al 2021) and for restoring other canopy‐forming seaweeds such as crayweed (Fucales) (Campbell et al 2014) (Table S5). These studies have used various life stages from microscopic gametophyte stages to adult sporophytes (Reeves 2017; Layton et al 2019); however, which stage is most appropriate to transplant is unclear as smaller kelp tend to acclimate more quickly, whereas larger kelp may survive better postdisturbance (Fowler‐Walker et al 2005; personal observation 2019).…”

Optimizing canopy‐forming algae conservation and restoration with a new herbivorous fish deterrent device

“…However, cages require regular cleaning to avoid fouling by epiphytes, which can completely cover the cages, and strongly reduce light intensity. This level of effort can be supported in small spatial–temporal scale studies, but it would not be feasible in large‐scale restoration where there is the need for cost‐efficient management‐free techniques (Terawaki et al ; Bennett et al ).…”