Proximity effects of natural and artificial reef walls on fish assemblages

Habitat complexity is recognised as a key driver of fish diversity and abundance. However, the effects of small (10s m) reef structures in otherwise uniform reef areas have received little attention. Here, we assessed proximity effects of small natural and artificial vertical walls on patterns of fish assemblages, at a scale relevant to fish survey transects. We tested whether wall size affected assemblages, and whether assemblages differed between wall types. Fish assemblages were found to change in the immediate vicinity of both natural and artificial walls, with significantly higher species richness and abundance occurring at reef walls than in surrounding, flatter reef areas. The size of the effect generated by walls was found to be proportional to the size of the wall, with species richness and abundance generally increasing with wall height and length. Differences between natural and artificial walls were detected, but these were confounded by differences in size between wall types. The study builds on previous work by showing that, within reefs, local areas of higher species richness and abundance can occur in the vicinity of small but important reef features such as vertical walls. This effect introduces considerable variability into data for reef fish surveys and has implications for the design of such studies. In addition, walls appear to act as localised biodiversity “hotspots” and consideration should be given to inclusion of areas containing such features within marine reserves.