Epibenthic ecology on artificial reefs: community structure in response to material, orientation and predation

Abstract

Fisheries are an important economic resource globally. The sustainability of fisheries must be maintained to ensure the viability of this resource. Artificial reefs have become increasingly common for use in aiding fisheries as a management strategy. The scientific literature on artificial reefs has debated the effectiveness of artificial reefs in promoting sustainability. Most studies thus far have concentrated on fish populations that are of economic value. It is recognised that more focus is needed on the epibenthic assemblages that grow on the artificial reefs to better understand ecosystem productivity. This thesis examines three basic aspects of epibenthic communities on artificial reefs that are yet to be studied in detail: the effect of reef surface material, exposure to fish predation and surface orientation on epibenthic assemblage development.

A new Offshore Artificial Reef (OAR) was deployed 2 km southeast of the south head of Sydney Harbour, Australia in October 2011. Settlement plates made of four different materials (sandstone, Perspex, turpentine wood and steel) were deployed in three orientations (upward facing, downward facing and vertical surfaces) on the OAR. The settlement plates were retrieved after three months and the effects of surface material and orientation were assessed by census of the epibenthic assemblages. Steel was identified as a less desirable material for the recruitment of sessile invertebrates. Turpentine wood, Perspex and Hawkesbury sandstone surfaces had similar communities and surfaces facing downwards had higher abundances of barnacles.

A fish predator exclusion study was also deployed in three orientations on the OAR to determine the effects of fish predation on epibenthic assemblages. There were no caging artefacts detected during the study. Barnacles were more abundant in the presence of fish predation, suggesting an intermediate trophic interaction potentially involving mesopredators. Barnacles also had rapid recruitment and mortality in the presence of fish predation. This suggests nutrient transfer to higher trophic levels. They were also found to be more abundant on downward facing surfaces and there was greater evidence that they were consumed by mesopredators on vertical surfaces. By incorporating more vertical and downward facing surfaces in artificial reef design, it may be possible to increase productivity but this effect may change with seasons.

More work is required to comprehend the function of epibenthic assemblages in artificial reef ecology. It is important that they are incorporated in a whole-ecosystem approach necessary for assessing the productivity of artificial reefs. This thesis provides evidence that reef construction material and surface orientation will strongly influence the development of epibenthic assemblages and that this may have cascading effects for predatory fish. For artificial reefs to continue evolving as an aid to sustainable fisheries, interdisciplinary approaches are necessary to optimise their design for productivity and the support of native biodiversity.