Ecological Engineering

Coral reef restoration requires efficient, effective and scalable techniques and methodologies to counteract the continued decline of coral reefs. Here we tested an in situ method to collect and settle fully developed planulae shed by the brooding coral species (Stylophora pistillata). Three devices called Coral Settlement Boxes (CSB; L × W × H: 50 × 40 × 6 cm; V: 4.5 L) were built from transparent Plexiglas and designed to be easily assembled and disassembled. Each CSB contained two integrated biofilm-covered nets (0.5 × 0.5 cm mesh size), which functioned as settlement substrate. The trap container of a traditional planulae trap was replaced by the CSB, and this new construction was used to collected planulae over 4 consecutive days. The CSBs were then transported to a mid-water coral nursery at 12 m depth. One CSB was disassembled immediately, the two settlement substrates were removed and each was placed in a protective cage (mesh size 4 cm²). The other two CSBs were opened after a 4-month period, leaving the four settlement substrates attached to the Plexiglas plates and covered by protective cages. None of the settlement substrates were cleaned of fouling organisms in the nursery. After 5 months in the mid-water nursery, a total of 120 healthy juvenile coral colonies had resulted from the estimated 2045 planulae initially trapped. This inexpensive and simple approach to producing sexually propagated stocks of colonies entirely in situ may enhance the efficiency, effectiveness and scalability of restoration activities that include brooding coral species.

A B S T R A C T

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivore. 

A B S T R A C T

An increasing number of artificial reefs (ARs) are constructed to compensate for the loss of natural reefs (NRs),
but little is known about their benthic community composition. Here, we compared the densities of coral-
associated fauna (CAF) between a centuries-old manmade structure and the nearest NR at St. Eustatius,
eastern Caribbean. Overall, no significant difference in the density of CAF (coral-dwelling barnacles, crabs,
worms) was found between the NR and the AR, nor between the exposed and sheltered sides of each. Signifi-
cantly different densities of CAF related to host cover were observed among corals on both the AR and the NR.
Per host species, the AR did not show such differences in density between exposed and sheltered sides, although
these differences were observed on the NR. Thus, turbulence and host cover regulate the density of CAF, while
differences also depend on host species composition. Furthermore, from an ecological engineering perspective,
the present AR resembles the NR in overall design, but not in relief rugosity and surface structure, which are also
considered important contributors to the difference in species assemblages of the host corals and their CAF, even
after many decades of community development

Highlights

• Fish abundance and biomass differed per artificial reef type.
• This difference is driven by the availability of small shelters.
• Artificial reef deployment can be more efficient by choosing better performing or cheaper designs.

In this study, we compared the early fish colonization of three types of artificial reefs deployed in the coastal waters of Saba and St Eustatius in the Caribbean: reef balls®, layered cakes and piles of locally obtained basaltic rock. As an indicator of performance, three fish assemblage parameters (abundance, biomass, species richness) were measured using underwater visual censuses at 11 months post-deployment and 4 months after restoration from hurricane damage. All artificial reef plots showed higher values for fish abundance, biomass and species richness than control plots covered by bare sand, which shows that artificial reefs can locally enhance the fish assemblage. However, the effect differed among artificial reef plots. Fish abundance was 3.8 times higher on the layered cake plots compared to the reef ball plots, while fish biomass was 4.6 times higher. Rock pile plots had intermediate values. Species richness did not differ significantly among different artificial reef plots. Three-dimensional modelling revealed that layered cakes had a smaller gross volume, shelter volume and total surface area than reef balls. The availability of multiple small shelters in the layered cake design appeared to be more relevant than other physical parameters, as the layered cake plots had higher fish abundance than the reef balls plots. We concluded that on Saba and St. Eustatius, layered cake plots performed better than reef ball plots after one year of colonization. Rock pile plots, made of local volcanic rock, showed an intermediate performance, and were 4–10 times cheaper to construct. If observed differences are consistent with other locations and persist during further colonization, current efforts to deploy reef balls could better be allocated to deploy artificial reef structures with a higher shelter density. 

Reef ball® , Layered cake, Fish abundance, Shelter availability, Habitat restoration, Artificial reef