AZoCleantech speaks to Dr. Matthew Holland, currently at the University of Plymouth, UK, about his UNSW team's recent research on artificial reefs and how they can improve fishing sustainability. Holland also discusses why we must preserve natural reefs and the effectiveness of marine protected areas (MPAs).
How did you begin your research into coral reefs?
Although some species of corals grow around Sydney, Australia, the reefs we studied are considered “temperate rocky reefs”. These reefs typically have kelp, rather than coral, as the dominant habitat formers.
Over the past ten years, there has been a big push across New South Wales to install artificial reefs for enhancing recreational fisheries. This provided interesting opportunities for research, particularly in understanding how these reefs differ to the natural reefs found nearby. In 2017, our lab acquired a brand new multibeam echosounder which presented novel opportunities for studying these complex coastal systems and led to this work.
Can you explain what an artificial reef is and the main differences that set it apart from natural reefs?
Artificial reefs are structures built on the seafloor as a means of providing additional habitat for marine organisms, namely fish. In the past, artificial reefs have been created opportunistically from car tires, decommissioned vessels, and various materials which had varying levels of success at attracting and producing fish. Many of these designs and materials have now been found to be ineffective and wasteful. The legalities of dumping materials at sea have also become a lot stricter, as they should be.
In recent years, artificial reefs are increasingly being purpose-built from materials such as concrete and steel, with specific design features to be more attractive to marine organisms and to increase the productivity of coastal waters. They are often built to provide new opportunities for recreational fishers and tend to attract fish from the surrounding area and provide a place for larval fish to settle and grow.
They mainly differ from natural reefs in terms of their footprint and vertical relief. Artificial reefs are typically much taller and rise more abruptly from the seafloor, which is very different structurally from the low-lying rocky boulder and shelf reefs typically found around Sydney. Artificial reefs are also usually restricted in size, due to the cost of building them, whereas natural reefs are often spread over much wider areas. Finally, artificial reefs are bare habitats when they are initially deployed, and it takes years for them to go through the successional processes of marine organisms settling and growing on or around these reefs. Because of this, there are often differences in the diversity and abundance of species, which can change as artificial reefs age.
Image Credit: Subphoto.com/Shutterstock.com
Can you explain your team’s research into artificial reefs and what key findings came out of the study?
We surveyed natural and artificial reefs around Sydney at night using a small boat and a multibeam echosounder, by driving over the reefs with repeated parallel transects. We then returned to the same reefs after sunrise to survey them again and see how the distribution of fish had changed.
The key finding to come out of our research was that man-made structures such as artificial reefs allow schooling planktivorous fish to use space quite differently than they do in nearby natural habitats. They fed much higher in the water column at the artificial reefs than they did at natural reefs.
Across temperate and subtropical coastal areas, such as the coast of Sydney, most of the seafloor is relatively flat and featureless. Rocky reefs or any form of hard structure on the seafloor is rare overall and fish tend to congregate on these structures or “reefs”. These artificial reefs typically rise very abruptly from the seafloor over short distances. Picture a group of tall buildings across a flat plain, such as the downtown of a major city. These structures allow humans to use this vertical space while keeping their feet on solid ground. By building skyscrapers rather than single-story buildings, we can gain more usable space for human habitat from the same spatial footprint on the ground. Similarly, while fish can travel up and down in the water column, many prefer to remain close to structures and seafloor for protection from predators. Tall vertical habitats allow them to access a greater proportion of the water column while remaining close to the safety afforded by physical structures.
Why is it beneficial to build artificial reefs and why is your research helping to improve knowledge surrounding this area? Why do artificial reefs help to drive forward coastal fishing sustainability?
When done well, artificial reefs can help to reduce the fishing pressure around popular fishing spots, improving fishery sustainability. They can also increase the total productivity of coastal areas, by providing additional habitat for reef organisms where there was previously only sand. However, artificial reef projects must be carefully thought out so that they do not simply attract all the fish from the natural habitat surrounding the reef, concentrating them, and thus making them easier to catch. Ideally, artificial reefs should increase the total productivity of an area, rather than simply changing the way fish are distributed.
Our research has helped demonstrate aspects of fish behavior that can be taken advantage of to improve the productivity of artificial reefs. We observed a difference in the vertical distribution of fish at natural versus artificial reefs. By providing tall vertical structures which allow fish to feed across a greater height above the seafloor, small schooling fish feeding at artificial reefs can capture additional energy from the plankton that would normally drift overhead.
What technology was used within your research and how was it used?
The WASSP multibeam echosounder which we used to survey the reefs is an instrument produced by a company in New Zealand called FURUNO ENL. This instrument generates sound waves that reflect off the seafloor and objects in the water column, such as fish. These reflected sound waves are detected by a receiver. However, unlike ordinary echosounders which use a single beam of sound waves, multibeam echosounders use constructive and destructive interference patterns to separate the beam into hundreds of separate channels. So, while normal echosounders produce a 2D profile of the water column, multibeam echosounders can be used to generate a three-dimensional profile of the seafloor and everything in the water column. This allowed us to study the distribution of fish around artificial reefs in three dimensions.
Are there any challenges to building artificial reefs on a larger scale and how could these be overcome?
The biggest challenges are cost and regulations. The first artificial reef built off Sydney had to be very overbuilt with four massive concrete blocks to anchor it in place due to its proximity to a busy shipping lane. There was initial concern that the reef could become dislodged in a very large storm and cause a hazard to shipping traffic. There was a very complex regulatory process involved with getting this reef approved and deployed.
Over the past ten years, there have been a lot of improvements to the designs being used in New South Wales. Some of the earlier reefs were built from concrete and had to be transported to their deployment site using a specialized barge with a crane and they could only be deployed under very flat calm conditions. Newer designs now incorporate ballast tanks which are filled with air, allowing reef structures to be towed through the water. These tanks are then opened, and they fill with seawater, causing the structure to sink. As reef designers continue to innovate and find ways to reduce the cost of deployment, it should become more feasible to deploy them on a larger scale.
Why do artificial reefs allow fish to use space differently than in natural habitats and why is this an advantage?
All life on Earth obtains energy from sunlight. In the ocean, solar energy is captured by benthic algae and seagrass (attached to the seafloor), and by phytoplankton.
One pathway for predatory fish (the ones we typically like to eat) to obtain energy is by feeding on organisms that themselves feed directly on benthic algae, but many reefs are nearly devoid of benthic algae (e.g., urchin barrens commonly found in this region), while still producing lots of fish.
The reason these habitats can support high numbers of fish, despite having very little benthic production, is because they receive a “meal delivery service” or a conveyor belt of food. The always-present East Australian Current delivers passively drifting plankton produced over huge areas of the ocean surface to coastal regions along eastern Australia.
As this plankton drifts over reef habitats, much of it is consumed by resident planktivorous fish, which then grow, reproduce, and get eaten by predators themselves.
These fish are attracted to novel habitat or habitat that is in some way different from the large areas of flat, sandy seafloor which is typical off our coast.
Artificial reefs provide new habitat in areas where these fish would typically not be present, giving them a structure to remain close to escape predators while they feed on this drifting plankton. Building taller reefs that rise higher off the seafloor, therefore, allows these fish to feed over a greater vertical extent of the water column (e.g. up to 12 m above the seafloor) while still remaining close to the safety of the artificial reef structure. This means that more of the plankton which would typically drift above these fish can now be consumed by them, which could increase the net productivity of a coastal area.
Despite the positives of building artificial reefs, why is it important that we preserve natural reefs as much as possible?
Through studying the differences in where fish were distributed in artificial habitats at night, versus during the day, we have observed how they might use vertical habitats differently to less vertical natural habitats to facilitate this increased production. Essentially, we demonstrated a mechanism for why artificial habitats can be more productive than natural ones. However, this does not mean that artificial reefs are a replacement for natural reefs, as natural reefs often have higher biodiversity.
The artificial reefs we studied exhibited much greater abundances of small planktivorous prey fish than nearby natural reefs, but these were primarily only two species of fish. Nearby natural reefs exhibited greater biodiversity or species richness, but with lower fish abundance overall. We still do not know exactly what causes these differences in fish communities, but it has been documented in many studies that the fish communities at artificial reefs are often different to those at natural reefs.
Our study also only looked at fish and did not examine differences in the diverse invertebrate communities which are also very important to coastal habitats. Invertebrate communities that grow on reefs often take decades to establish and when they do, they provide a greater variety of food and microhabitats than would typically be found at artificial reefs.
Do you believe that there are any key steps that need to be taken to maintain or restore natural reefs?
Marine protected areas (MPAs) are probably one of the most effective means of protecting natural reefs. These areas have been shown to provide a spillover effect, with fish maturing unharvested in MPAs and traveling outside to adjacent habitats.
Our colleagues at UNSW are also doing great work through a program called Operation Crayweed, so named because it is preferred habitat by rock lobster or “crayfish”. They are working to restore an important habitat-forming species of macroalgae (crayweed or Phyllospora comosa) which had disappeared from the Sydney area due to historic water pollution issues which have now been resolved. Habitat restoration and reintroductions of native species can have huge benefits for reef ecosystems, with downstream benefits to fishers as well.
What are the next steps for your team’s project?
We are just starting a similar comparison study with the WASSP multibeam echosounder to survey four artificial reefs in New South Wales so that we can get a better understanding of how different artificial reef designs can affect fish behavior and distribution. We are comparing two reefs constructed out of multiple concrete structures spread over a wide spatial footprint, with two reefs consisting of single large steel structures. The concrete structures create a “reef field effect”, allowing fish to travel among modules, while the single steel structures contain more internal spaces for fish to refuge.
We believe that the optimal design would combine features of both reef types, with a single large central structure surrounded by a field of concrete structures to benefit from the advantages of each design, but of course, it ultimately comes down to cost.
Where can readers find more information?
You can find more information about the artificial reef projects in New South Wales by visiting the state government website: https://www.dpi.nsw.gov.au/fishing/recreational/resources/artificial-reef
About Dr. Matthew Holland
I conducted the above research with colleagues at UNSW while at UNSW. Since completing a PhD in Australia, I have moved to pursue a post-doctoral position at the University of Plymouth, UK, but still continue to work with UNSW colleagues.
My current focus is on developing environmental indicators which are used to determine whether pelagic habitats are in good environmental health, or whether they have been negatively affected by human impacts such as climate change and eutrophication. This information is used to create policy that determines how the North Sea is co-managed by the countries which border it.
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