Natural rocky reefs and coral communities host a diverse range of fish, invertebrate and algal species, as they provide habitat for shelter and food. Artificial reefs, like natural reefs, provide habitat and add to the structural complexity of the seabed around them. Popular in the imagination are the artificial reefs of sunken ships and aircraft. However, artificial reefs can also include natural objects like logs, large boulders and rocks. In essence, artificial reefs are anything placed in the water, accidentally or purposefully, which increase the surface area of the seabed and are structurally stable, allowing for the colonisation and attraction of organisms over time.
Natural and artificial reefs appear similar in functionality, so the next logical step is to ask, if a reef, artificial or natural, tends to increase biodiversity in the area, could we make artificial reefs to promote biodiversity and sustainable use of the resources in our own Port Phillip Bay? To find out more about this question, Wild Melbourne asked PhD student Kade Mills of Deakin University about his research into how artificial reefs may influence fish assemblages and biodiversity in Port Phillip Bay.
Wild Melbourne (WM): Hi Kade! Artificial reefs look like a promising way to enhance biodiversity in the bay. Are artificial reefs a relatively recent concept?
Kade Mills (KM):
They’ve been used for thousands of years; one of the first examples was in India where they would sink tree logs and then fish around them after a certain period of time and they would always catch their fish.
WM: In Port Phillip Bay, what has been the purpose of placing artificial reefs?
Artificial reefs were placed in the bay to provide additional locations for recreational anglers to go fishing.
WM: How long have artificial reefs been purposely placed in Port Phillip Bay?
In the 60s, they sank barges and put concrete pipes and natural rocks in a few different locations. In the 70s, they tried tires, as there was a trend around the world in their use as artificial reefs. It appears to have been driven by tire companies as a cheap way to dispose of unwanted used tires. Some of the reefs in the bay are still intact and you can dive the barge and tire reefs. Tire reefs are really interesting, you get some really cool assemblages like sponges, seahorses and particularly octopi, which really like the structure of the inside rims.
WM: What is used now for artificial reefs? What is the forefront of reef technology?
Reefs are now purpose-built to attract certain species and so they don’t subside into the seabed. They’re usually concrete with the surface being a similar pH to seawater, and they are ‘roughed up’ for texture to improve recruitment of spores and provide a solid base of attachment for things like sponges and seaweed. We use reef balls which are currently used around the world. In principle, they have a large void on the inside and several holes throughout that allow water to flow through them and provide a range of microhabitats.
WM: Has biodiversity increased since the artificial reefs have been deployed?
Not surprisingly the biodiversity has increased, as you have increased the surface area of the seabed and provided habitat for many new plant and animal species to occupy. The reefs tended to be populated by spores in the water column that hit the substrate, settle onto the surface and then develop. At first we found epithitic growth like algae and hydroids, then over time barnacles, ascidians and sponge growth.
WM: Do fish appear on the reefs straight away? Or does it take time?
Fish turn up straight away! Researchers in New South Wales found fish appeared only a few hours after instalment. I imagine the presence of fish will change over time though; the reef would have a carrying capacity and if a reef was there for a food purpose (for fish), then it can’t support many because the area isn’t much, but there’s also shelter provided and space to rest and hide from predators.
WM: What species have you found at the artificial reefs?
The reefs were put in to attract snapper, and it worked. Although you’ll be lucky to see them diving due to diver avoidance more than the fact they’re not there. [Kade dives on the reefs to assess the species present over time]. There’s goat fish all over them! They like being on the structure and feeding in the sand around it. You’ll also find toadfish and leather jackets.
WM: From an environmental perspective, are the artificial reefs bringing in more species? Is our bay becoming richer?
They will not bring more species into the bay, but they have increased biodiversity in a small area, although as far as making the bay richer it’s hard to tell just yet. As far as habitat goes, it’s not going to compete with a natural reef that’s been there for a long time - it’ll take time to find out. But other research has shown that even shipwrecks that have been there for over a hundred years have assemblages that are completely different to a nearby natural reef, but it still does support a diverse array of species. [Artificial reefs] worked for snapper and other fish, and recreational fishers do catch fish there.
WM: Do you find that recreational fishers are happy to work with fisheries and scientists?
Yes, they have been quite helpful. Anglers have provided ‘angler diaries’ which list various things, including the species they’ve caught and how many. The concept and implementation of artificial reefs are popular amongst recreational fishers and part of the research looks at the public perception of artificial reefs and how they use them. We asked whether they knew about the reefs, whether people fished off the reefs, and what their success rate was. We found the general public, including recreational fishers, were happy with the implementation of reefs.
WM: Were there any initial concerns regarding the implementation of artificial reefs?
The EPA (Environmental Protection Agency) and the former DSE (Department of Sustainability and Environment) were concerned about introduced species in that the reefs may be a stepping stone for them to move into other areas of the bay and that the reefs may become a ‘hub’ for colonisation. There was also a concern that the reefs may draw fish off natural reefs, making them easier to catch.
WM: You mentioned that snapper use the reefs. How exactly do they use both the artificial and natural reefs?
The ones tagged on natural reefs didn’t really move off the natural reefs, but those on artificial reefs tended to move amongst all types of habitats. This meant that the artificial reefs being a vacuum, drawing in fish where fishers at the top can easily catch them, isn’t the case. The catch rates above the artificial reefs were similar to natural reefs after the reefs went in. They’re catching a similar number of species, but it seems that it’s mainly smaller, sub-adult juveniles.
WM: What is the production value of the artificial reefs as far as new recruitments year after year are concerned? Or is it yet to be established because of the time required to assess this?
It’s not just a matter of time, but more size. Fish spawning and production is such a large scale thing that three reefs are not going to have a discernible impact. There may be some species you could do it with: the more philopatric species (animals that tend to reside or return to specific areas), like the southern hulafish, which are residents of a given reef and recruit in particular areas of the bay.
WM: As far as artificial reefs being a hub or spawning point for introduced/invasive species, has this occurred?
No, not really. The starfish asterias has turned up at some of the artificial reefs. However, they are known to move around in large numbers depending where the food is. There was one time we saw quite a few of them, but there’d been a whole lot of drift algae and bivalves that had built up [on the artificial reef which they were feeding on. The next time we went, [the starfish] were all gone and so it didn’t seem like they were settling and staying on the reef.
WM: What’s involved in a typical day of assessing the reefs?
Some days we dive and count the fish, look at the invertebrates and assess the reef structure. You also dive on soft sediment and natural reef areas to compare with the artificial reefs. The other work also includes baited video recordings where you can see the snapper move in and out and you place these on the soft sediment and both reef types to compare.
WM: Can we effectively use artificial reefs to assist conservation and sustainability?
You’ve got to have a reason - you need an objective. Are reefs being put in to catch fish off? Are they being there to enhance recruitment? I think we’re under-utilising artificial reefs in some ways. There are ideas to use them to create marine parks; get an area people aren’t interested in fishing in - an area fishers are happy to be excluded from - and try to create habitats and subsequently protect those areas. It’s not an ideal way to do [conservation], but it’s another option to look at. I think there may be specific species which will benefit from artificial reefs if they were made the right way.
WM: What knowledge to we need to make and use them ‘the right way’?
The biggest problem with a lot of artificial reef projects is that we don’t know enough about the basic biology or ecology of many fish: their diet, their life stages, where they move to and from. Understanding their life history and the crucial stages where, if at all possible, artificial reefs could be utilised to improve survival.
Artificial reefs look like a promising option to help improve the biodiversity of Port Phillip Bay and also to assist the development of the sustainable use of our bay’s resources. Although artificial reefs seem like a simple option, much more research is required into certain aspects, such as the reef’s physical structure, understanding reef dynamics between and among species, and, as Kade mentioned, understanding to a greater extent the biology and behaviours of the organisms themselves. If you would like to learn more about the Port Phillip Bay environment, contact both Parks Victoria and the Victorian Department of Environment and Sustainability.