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 m2 and 3.14 m2. Remote underwater video was used to compare the behavioural 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 1621 ± 702 bites hr-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.
Highlights
Consumers have a profound effect on the structure and function of vegetated communities across a diverse range of ecosystems (Bellwood et al, 2004; Wolf et al, 2007; Poore et al, 2012; Bertness et al, 2014; Bennett et al, 2015)
Kelp biomass losses were similar within the bubble curtains and rigid exclusion cages (Figure 2A)
Consistent with the low kelp biomass loss rates, no bites or entries of herbivorous fish into the bubble curtain were observed at any point during the experiment (Figure 2B)
Summary
Consumers have a profound effect on the structure and function of vegetated communities across a diverse range of ecosystems (Bellwood et al, 2004; Wolf et al, 2007; Poore et al, 2012; Bertness et al, 2014; Bennett et al, 2015). In addition to the direct effects of herbivores, many vegetated marine ecosystems such as seagrass meadows and kelp forests are in decline as a result of anthropogenic disturbances from eutrophication, climatic warming and extreme events, sediment-loading, salinity changes and invasive species (Orth et al, 2006; Waycott et al, 2009; Wernberg et al, 2013, 2016; Bennett et al, 2016; Krumhansl et al, 2016) This has prompted efforts to actively revegetate coastal marine systems to curb losses and restore habitats where the initial cause of the loss has been reduced (Bastyan and Cambridge, 2008; Statton et al, 2013; Campbell et al, 2014). The larger the area that can be restored before being exposed to herbivores, the greater the chance of success ( see Discussion about sustainable patch size for exclusion areas)
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