Eco-engineering increases habitat availability and utilisation of seawalls by fish

The replacement of natural marine habitats with less structurally complex human infrastructure has been linked to the homogenisation of epibenthic assemblages and associated changes in fish assemblages. To mitigate these impacts, eco-engineering efforts have focussed on increasing the physical and biogenic complexity of artificial structures, in the form of crevices added to seawalls and the seeding of the substrate with habitat-forming organisms such as oysters. While these studies have assessed how these interventions affect epibenthic assemblages, the effect of these strategies on the behaviour, such as feeding and habitat use, of different functional groups of fish (e.g. cryptobenthic and pelagic) remains uncertain. To do this, we manipulated complexity on seawalls by adding concrete tiles with different physical (flat or structured with crevices and ridges) and biogenic (seeding with two common habitat-forming species or naturally recruited fouling) complexities. We assessed pelagic and cryptobenthic fish species composition, abundance, interaction time with the tiles and number of feeding bites on three occasions 8–12 months after deployment. Cryptobenthic fish interacted more with physically complex tiles than flat tiles, regardless of biogenic complexity. In contrast, cryptobenthic fish fed more from flat tiles compared to physically complex tiles, and also appeared to feed more from tiles seeded with oysters. Pelagic fish interacted and fed more from naturally fouled tiles compared to unfouled control tiles, regardless of physical complexity. This study showed that manipulating complexity at the scales used here affects behaviour of fish, but it does not affect fish community. Increasing physical complexity facilitated fish use of seawalls as habitat by providing refuge, while it also hindered fish feeding by providing refuge for their prey. Cryptobenthic fish are important trophic linkages in their ecosystems and we have shown that by changing habitat complexity, we can change the habitat use and feeding activity of these fish, allowing them to fulfil this essential ecosystem role.