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Habitat Enhancement

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While government policies focus predominantly on sustainable fishing, habitat loss is one of the major contributing factors to fisheries decline. Subcon are leading the industry in marine habitat restoration.

Dredging, run-off and trawl fishing have contributed to broad acre loss of habitat on every coastline. In addition, over the past few decades, we have lost over 40% of our natural coral reefs. Projections are that 90% of coral reefs will be in danger by 2030, and all of them by 2050.
To manage fish and invertebrate stocks for sustainability reasons, it is essential to restore the supporting habitats of targeted species, as well as implement the more common management practices of quota limits, size limits and closures.

Engineered reef substrates restore the ecosystem services previously provided by natural reefs. Properly designed structures provide the critical substrates to recruit marine flora and sessile fauna. Well designed light and shade, diversity in cryptic spaces, vertical relief, reef field layout and flow modification are essential features of a productive reef.

FAQ’s

Properly designed structures provide the critical stable substrates for marine flora and sessile fauna to recruit onto. Well designed light and shade, diversity in cryptic spaces, vertical relief, reef field layout and flow modification are essential features of a productive reef.

To promote flora settlement its important that surfaces are sloping and in the water column, not flat and at or below the mudline. Flat low profile surfaces silt up and don’t recruit. Tall, sloping surfaces recruit really effectively. Its important to remember that artificial reef productivity isn’t just about more fish. More flora and sessile fauna means a more diverse a productive ecosystem!

A fish’ life cycle is complex. Many species are known to aggregate to spawn in one habitat, mature from juveniles in another and then move again as mature adults onto deeper or larger reefs. Reef productivity is not just about more fish, also bigger and healthier fish. Reef ecosystems that promote the maturation of juvenile fish is a key service that we can engineer into the design.

Modules that create real upwelling recruit fish better than ones that don’t. Especially bait balls love upwellings. Modules create upwelling through a combination of hydrodynamic design and orientation to the current.

The field layout at Subcon’s King Reef, Exmouth is a good example of module orientation considering the prevailing current. Our ReefPyramid is specifically designed to create real upwellings.

Reef module arrangement creates connectivity.

Research by NSW DPI and UNSW has demonstrated how fish assemblages are significantly increased by the presence of enhanced habitat.

We think nature is really good at the fine scale design. Its the macro habitat design that takes thousands of years. At Subcon we’ve focussed on creating large, productive macro seascapes and then we let nature take over and do what she does best!

Artificial reefs mimic the characteristics of natural reefs. By creating new habitats, providing shelter and feeding opportunities, changing water flow dynamics and activating internal reef voids, purpose built reefs have been shown to be many times more productive than natural reef systems. It is common practice to ensure reef are larger than 800m3 in order to ensure they become habitat production devices, rather than simply drawing fish in from other areas. For example the 27,000m3 of new habitat used to create the Exmouth reef is more than 30 times larger than the volume required for a reef to be considered productive. King Reef at Exmouth has recruited multiple populations of juvenile Red Emperor over multiple year classes demonstrating outstanding productivity in a degraded fishery.

Some artificial reefs used to be made from sunken ships, tyres and similar materials. While they do work to some degree, they can have some negative impacts on the marine environment including leaching of pollutants and stability issues. Tyre reefs have also proven to be a poor settlement medium meaning reefs made from tyres show limited growth and do not promote production. Poor positioning and management of these reefs has also resulted on materials becoming separated from the reef and damaging nearby natural reefs.

The attraction-production question was raised as early as the 1970s, when artificial reef research first started to increase in popularity (Bohnsack & Sutherland, 1985) (Silva Lima, Rosental Zalmon, & Love, 2019). The long history of use of artificial reefs has emphasised the need to resolve the attraction-production issue as it has implications in particular for fisheries management (Polovina & Sakai, 1989) (Moon, Otake, & Kim, 2018).

Arguments that reefs are simply aggregation devices that make catching fish easier are disingenuous and ignore the science which conclusively demonstrates that engineered ecosystems are productive habitats that are critically important to creating abundance in our oceans.

In the first instance it must be recognised that fish life cycles typically involve many different habitats (they may spawn in estuaries, mature on shallow reefs and migrate to deeper waters upon adulthood). All reefs, whether natural or engineered are therefore both attractive and productive. Consider that anthropogenic impacts have caused broad acre marine habitat loss and that this is now identified by the UN as one of the key drivers in fish stock decline. We believe we should be focussed how to reintroduce habitat complexity and therefore productivity on a similar scale.

Many fish populations are limited to available habitat (i.e. hard substrata) and therefore an increase in habitat (e.g. via the installation of an artificial reef) will increase the carrying capacity and thus increase the ecological productivity and biomass of the area. If habitats are saturated then artificial reefs will produce new fish (Wilson, Osenberg, St. Mary, Watson, & Lindberg, 2001) (Powers, Grabowski, Peterson, & Lindberg, 2003). Conversely, attraction theory is when a habitat or structure serves only to attract fish from surrounding areas due to behavioural preferences, without increasing the biomass or carrying capacity of the population (Pickering & Whitmarsh, 1997).

Biomass is often used as a metric to quantify attraction and production as it quantifies attraction as well as all facets of production which may include population growth or an increase in size of the existing population. The current state of the argument is that it is likely rare in practice that attraction and production are mutually exclusive and both likely affect the assemblages on artificial reefs (Shin, Cheung, Tsang, & Wai, 2014). Production especially may manifest itself in different ways such as, improved survival, relocation of fishing effort, attraction of a permanent population, or provisioning food to improve survival of juveniles, or optimising zooplanktivorous fish abundance whilst avoiding food limitation (Champion, Suthers, & Smith, 2015) (Fabi, et al., 2015).

Engineered reefs have by shown to support up to six times more biomass than existing, un- damaged natural reefs.

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