Webber, Kelsey, Fabricius, Katharina, Wilson, Shaun K., Konovalov, Dmitry A., Winding, Nicoline, and Hoey, Andrew S. (2025) Canapes in the canopy: Canopy macroalgae are dining hotspots for small and juvenile fishes at Orpheus Island, Great Barrier Reef. Marine Environmental Research, 212. 107490.

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Abstract

Tropical macroalgal habitats are often overlooked or perceived as degraded reef states, yet they contribute to reef habitat diversity and provide important ecological functions including supply of nutritional resources for a diverse suite of reef fishes. Understanding how reef fishes use macroalgal habitats is critical for appreciating their importance in tropical seascapes. We investigated which components of natural macroalgal habitats are selectively used by reef fishes in natural macroalgal beds across three sites on an inshore reef on Orpheus Island, Great Barrier Reef. We quantified benthic composition and feeding behaviour of reef fishes within 1 m² quadrats across the three sites, with feeding behaviour quantification based on remote underwater video. Small bodied and/or juvenile herbivorous fish dominated the foraging assemblage, particularly juvenile Siganus doliatus and parrotfishes. Notably, seven of the eight most commonly observed fish taxa exhibited strong and consistent positive feeding electivity for canopy macroalgae over all other microhabitats (i.e., understorey algae, turfs and sand). This clear pattern of selective feeding highlights the important functional role of canopy macroalgae in structuring reef trophic interactions and indicates non-random microhabitat use by foraging fishes. In contrast, multivariate and univariate analyses indicated that benthic composition and macroalgal structure were poor predictors of feeding rates of the dominant feeding fishes. These results indicate that although small-bodied fishes selectively targeted macroalgal canopies, feeding rates were not simply limited by the availability and physical structure, inviting further investigation of additional ecological drivers. Our findings underscore the importance of macroalgal habitats in tropical seascapes and challenge the perception that they are ecologically impoverished or merely symptomatic of degradation.

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