Seasonal Dynamics of Fish Assemblages on Breakwaters and Natural Rocky Reefs in a Temperate Estuary: Consistent Assemblage Differences Driven by Sub-Adults

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.

Marine dock pilings foster diverse, native cryptobenthic fish assemblages across bioregions

“…Furthermore, orientation, slope, and material of the artificial structure are also cited as important agents for homogenization of invertebrate assemblages (Glasby & Connell, ), with the availability of microtopographic refuges being particularly important for sessile organisms (Brandl & Bellwood, ; Freestone & Osman, ). In contrast, compositional differences among mobile fish assemblages appear to be largely due to different macro‐habitats (sheltered vs. exposed [Clynick et al., ; Burt et al., ; ]), light availability (Able et al., ), or ontogenetic shifts in habitat preferences (Fowler & Booth, ). For the cryptobenthic fish assemblages examined in this study, no invasive species were sampled from docks, and the material with which docks were constructed had a limited effect on community composition.…”

From grey to green: Efficacy of eco‐engineering solutions for nature‐based coastal defence

“…However, created or restored habitats may not be effective at providing the same suite of services as natural ecosystems (Bilkovic & Mitchell, 2013 In particular, ecological trade-offs may occur with hybrid coastal protection that incorporates structural elements to facilitate restored habitats (Bilkovic & Mitchell, 2013). The reviewed restored coastal habitats are commonly evaluated for their effectiveness at maintaining biodiversity ( (Fowler & Booth, 2013). Marsh sills, however, supported epifaunal suspension feeders, such as oysters, which colonized the rock sill and a lower deposit-feeding infaunal biomass than natural marshes (Bilkovic & Mitchell, 2013).…”

Zooplanktivory is a key process for fish production on a coastal artificial reef

“…They are small reef-resident zooplanktivores common to the temperate reefs of southeastern Australia (Glasby & Kingsford 1994, Fowler & Booth 2013 and are one of the most abundant fishes found on this study's artificial reef (NSW Department of Primary Industries 2013, Scott et al 2015). Fiftyfive A. strigatus were sampled from the reef using hook-and-line fishing, frozen on collection, and later dissected with their stomachs stored in 5% formaldehyde.…”