Abstract
Retrofitting microhabitat features is a common ecological engineering technique for enhancing biodiversity and abundance of small, epilithic organisms on artificial shorelines by providing refuge spaces and/or ameliorating abiotic conditions. These features are typically too small to be utilised as refugia by larger, highly motile consumers such as fish, but they may affect these organisms through other mechanisms. This study sought to determine whether microhabitat enhancement units alter the fish abundance, richness and assemblage composition on tropical seawalls and explores possible underlying trophic mechanisms. We created 12 experimental plots consisting of 6 enhanced plots, each with 20 microhabitat enhancement tiles, and 6 control plots without tiles on intertidal seawalls at Pulau Hantu, an offshore island south of mainland Singapore. Benthic cover and fish assemblage were surveyed within each plot using photoquadrats and underwater video cameras, respectively, from April 2018 to February 2019. We found greater abundance and species richness and distinct assemblages of fish in the enhanced plots compared to the control plots. These differences were driven largely by an increase in both abundance and richness of fish species with epibenthic-feeding strategies and were significantly associated with higher biotic cover in the enhanced plots, especially epilithic algal matrix (EAM). Our results indicate that, in addition to facilitating epilithic organisms, microhabitat enhancement can provide food resources for epibenthic-feeding fishes, increase fish biodiversity, and alter fish assemblages in tropical urbanised shorelines.
Highlights
The global human population is disproportionately concentrated within coastal regions, due in part to the resources found near river mouths and nearshore zones, and because the land−sea interface is a strategic location for trade (Duarte et al 2008, Todd et al 2019)
Since coastal artificial structures heavily alter benthic composition and structural complexity (Firth et al 2016), there is a need to better understand their effects on fish assemblages, as they become ubiquitous features of urbanised shorelines
Biotic benthic cover was dominated by epilithic algal matrix (EAM) (10.1%), whereas crustose coralline algae (CCA), MA (e.g. Sargassum and Padina) and other biota together accounted for ~0.3% of the cover
Summary
The global human population is disproportionately concentrated within coastal regions, due in part to the resources found near river mouths and nearshore zones, and because the land−sea interface is a strategic location for trade (Duarte et al 2008, Todd et al 2019). Since coastal artificial structures heavily alter benthic composition and structural complexity (Firth et al 2016), there is a need to better understand their effects on fish assemblages, as they become ubiquitous features of urbanised shorelines. Much of the work on coastal ecological engineering has focussed on microhabitat enhancement designs that target small, epilithic organisms (Strain et al 2018b) The scale of these designs is often too small to act as refugia for larger motile consumers such as fish, but could affect large organisms through other means, by mediating access and availability of food resources. As microhabitat enhancement units are increasingly deployed on urbanised shorelines (Strain et al 2018a, Morris et al 2019), understanding their effect across organismal scales and on functional groups including fish is essential for assessing broader, system-wide consequences of ecological engineering. The 20 tiles in the enhanced plots followed the spatial configuration
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