Abstract
Development of infrastructure around cities is rapidly increasing the amount of artificial substrate (termed artificial reef, ‘AR’) in coastal marine habitats. However, effects of ARs on marine communities remain unknown, because it is unclear whether ARs can maintain similar communities to natural reefs. We investigated whether well-established (> 30 years old) breakwaters could consistently approximate fish assemblages on interspersed rocky reefs in a temperate estuary over 6 consecutive seasons using regular visual surveys between June 2009 (winter) and November 2010 (spring). We examined whether assemblage differences between reef types were driven by differences in juvenile recruitment, or were related to differences in older life-stages. Assemblages on both reef types were dominated by juveniles (61% of individuals) and sub-adults (34% of individuals). Seasonal fluctuations in assemblage parameters (species richness, diversity, sub-adult abundance) were similar between reef types, and levels of species diversity and assemblage composition were generally comparable. However, abundance and species richness were consistently higher (1.9-7.6 and 1.3-2.6 times, respectively) on breakwaters. These assemblage differences could not be explained by differences in juvenile recruitment, with seasonal patterns of recruitment and juvenile species found to be similar between reef types. In contrast, abundances of sub-adults were consistently higher (1.1-12 times) at breakwaters, and assemblage differences appeared to be driven by this life-stage. Our results indicate that breakwaters in temperate estuaries are capable of supporting abundant and diverse fish assemblages with similar recruitment process to natural reefs. However, breakwaters may not approximate all aspects of natural assemblage structure, with differences maintained by a single-life stage in some cases.
Highlights
With the global population increasing, the expansion of coastal cities is putting increasing pressure on shallow marine ecosystems
We assessed the ability of breakwaters to approximate fish assemblages on natural rocky reefs over 6 consecutive seasons in a temperate estuary, an ecosystem type that has received little attention regarding the performance of artificial reef (AR) habitat
We found no evidence to suggest that differences in juvenile recruitment drove assemblage differences between the two reef types, despite assemblages being dominated by juvenile stages (61% of total fish)
Summary
With the global population increasing, the expansion of coastal cities is putting increasing pressure on shallow marine ecosystems. ARs have typically been shown to support higher fish abundances than natural reefs [9,10,11,12], assemblage parameters (e.g. species richness, diversity) and species composition were found to be similar or different, depending on the study considered [8,12,13,14]. These discrepancies are likely to have resulted, at least in part, from the wide variety of types of ARs investigated and the wide range of ecosystems in which ARs were deployed. Further research is required to determine the circumstances under which ARs are capable of approximating assemblages on natural reefs
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