The use of gabions as an eco-engineering approach to enhance estuarine fish habitats in urbanised waterways

Anthropogenic activities are often concentrated around coastal ecosystems. Harbours and marinas comprising artificial structures with steeper orientations and reduced topographic complexities than natural ecosystems contribute to habitat alteration and biodiversity loss. Ecological engineering aims to mitigate these impacts by integrating ecological principles into coastal development to enhance habitat potential and improve biodiversity. This study investigated the potential of gabions (rock-filled wire mesh baskets) to enhance fish abundance and biodiversity by comparing these structures to conventional seawalls in an urbanised marina in Knysna, South Africa. Remote underwater video systems were used to determine fish diversity, abundance and composition over a 12-month period from August 2020 to August 2021. Overall, a significantly greater abundance and diversity of fish were associated with gabions compared to seawalls. Importantly, the gabions supported an endangered species, the Knysna seahorse (Hippocampus capensis), and two near threatened species recorded only on the gabions. This study highlights the efficacy of using structurally complex gabions as a viable ecoengineering alternative to less complex seawalls to enhance fish habitat.