Design of the optimal artificial reef tested on Saba

A new study compared the effectiveness of three popular artificial reefs to increase fish densities around Saba and St Eustatius. The researchers found that a complex internal structure, providing more shelter availability, resulted in increased fish abundance, biomass and species richness. The results of this study were used to design an even better working artificial reef, the Moreef, which is now being tested on Saba.

Coral reefs within the Caribbean have been slowly degrading since the 1970s. Initially this degradation was due to diseases that wiped out most of the Diadema antillarum (black sea urchin), the most important herbivores, and Acropora, the most important reef building corals. Combined with additional threats such as hurricanes, increased seawater temperatures and human-led pollution, coral cover around the islands of Saba and St Eustatius has decreased to less than 10%.

An artificial reef is a human-made underwater structure, typically built to locally increase biodiversity and productivity. Artificial reefs are gaining popularity as a way to provide increased substrate for coral recruitment and shelter, foraging areas and nursery grounds for fish and other reef creatures. Although there are many artificial reef designs, little research has been done to compare the effectiveness of different designs.

Researchers from University of applied sciences Van Hall Larenstein, Wageningen Marine Research, Wageningen University, the Caribbean Netherlands Science Institute, STENAPA, the Saba Conservation Foundation and Golden Rock Dive Centre worked together in the AROSSTA (Artificial Reefs on Saba and Statia) project, in which the fish abundance, biomass (weight) and species richness of three different artificial reefs was compared. 

The Study

The three designs were selected based on their popularity and availability within the Caribbean. The Reef Ball® artificial reef has become increasingly popular, with over 600,000 units deployed worldwide. This reef is made of concrete and has a domed shape with many holes, allowing fish to enter and exit the reef easily. The second design is the layered cake artificial reef, this reef has the same outer shape as the reef ball, but includes multiple layers on the inside, providing a more complex habitat for fish. The final design was constructed with local basaltic rock, which was used to create a pile with the same dimensions as three reef balls or layered cakes.

Beginning in May 2017, four locations were selected around Saba and Sint Eustatius. At each location, four different treatments were tested (reef balls, layered cakes, a rock pile and a control consisting of bare sand). On each plot, 10 fish surveys were performed 11 months post deployments and 4 months after restoration from damage caused by Hurricanes Maria and Irma. After the hurricanes, all of the artificial reefs were at least partially buried in the sand and required human intervention to be cleared out.

The Results

This project provides important insight into the effectiveness of artificial reefs. In general, all three reef designs demonstrated an increase in fish abundance compared to areas with bare sand. In total, 2102 fish from 48 different species were seen in and around the artificial reefs. Interestingly, the popular reefs ball design was not the most effective way to increase the fish abundance. Instead, this study showed that  layered cakes, which provide higher complexity and shelter availability, are much more effective and harbored 4 times more fish (both in numbers and in biomass). This study also demonstrated that more economical solutions, such as rock piles, can be effective and up to 10 times cheaper to construct. However, it should be noted that this style of artificial reef is less stable and could be more easily destroyed in high energy waves or hurricanes.

Lessons Learned

The devastating impacts of hurricanes Irma and Maria which hit Saba and St. Eustatius over the course of this study really highlighted the need for artificial reefs which are weather-proof and resilient in high energy situations. Placing artificial reefs in sandy areas leaves them vulnerable to being smothered or sinking, therefore, careful site selection and monitoring is required if these reefs are to be a long-term solution. Artificial reefs with complex internal structures and a high shelter availability should be selected over simpler designs to optimize fish assemblages.

Using the lessons learned from this study, the researchers worked together with Wortel Product Design and developed a new type of artificial reef, the Moreef (Modular Restoration Reef). The Moreef has all the advantages of the three different artificial reef designs tested in AROSSTA, while all disadvantages were removed. The Moreef design is now being produced and tested in Saba and will hopefully set a new standard in artificial reef design.

Caribbean coral reefs are under unprecedented pressures and will require various levels of human intervention to reverse current trends of reef degradation. Artificial reefs can provide a variety of benefits beyond improving local fish populations, such as coastal protection and creating new dive sites, taking some of the pressure off natural reefs. Studies such as these provide crucial insights into artificial reef functioning and will help to guide the development of artificial reef designs in the future.

For more information, you can find the full article here:  A. Hylkema, A.O. Debrot, R. Osinga, et al., (2020) Fish assemblages of three common artificial reef designs during early colonization, Ecological Engineering  https/::doi.org:10.1016:j.ecoena.2020.100021 – Or follow the project here: http://www.hvhl.nl/arossta

For more information, contact project leader Alwin Hylkema at alwin.hylkema@hvhl.nl

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