Offshore Artificial Reefs
In a first for Australia, a purpose-built and carefully designed artificial reef was deployed in the ocean off the coast of Sydney in October 2011. It is located in 38 metres of water, about 2 kilometres south east of Hornby Lighthouse on South Head, near Watson’s Bay.
The reef was funded by fees for recreational fishing licences and is managed by the NSW Department of Primary Industries (Fisheries). Made from 42 tonnes of untreated steel, it was specifically built to be attractive for pelagic and benthic fish, based on the experience of South Korea and Japan. It is 12 by 15 metres in length, has two 12 metre high masts and is anchored at each corner by a chain and a 60 tonne concrete block. It works.
The reef abounds with hundreds of large handsome kingfish, and shoals of their prey – yellow tail scad, or yakka, as well as mado, sweep and the pesky leatherjackets. Lurking amongst the frame on the seabed, one can see on video footage the large dark shapes of mulloway, grey morwong, Port Jackson sharks; around the edge are flathead and banjo rays. We’ve even seen marlin, mako shark, and a seal there. And they stick around.
We have deployed hundreds of acoustic tags, or pingers, into fish including flathead, kingfish, banjo rays, and Port Jackson sharks, to monitor their residency and connection to the nearby natural Dunbar Reef. Visually there appears to be a greater abundance of fish around this fairly recent, artificial structure than around the natural one – but why? And where does the catch come from? We have monitored the number of boats visiting the artificial reef. In the first year more than 700 boat-hours were observed, and in the second year, when the January weather was better, more than 2000 boat-hours were observed. This translates into about 500 kg to 1500 kg of fish being harvested by recreational anglers.
But is it simply a case of fish being attracted to the reef, which makes them more vulnerable? Or is it possible that a tonne of fish could be produced around the reef in a year? At first glance it would seem like the answer is fish attraction.
It is impossible to produce a tonne of frogs in 12 months in a 12 by 15 metre pond. But the key lies in a new view of the food web of a temperate reef and the design of the reef. Driven by eddies of the East Australian Current, water flows through the artificial reef at speeds of up to a metre per second and, on some days, even faster. We can monitor these currents using the three IMOS oceanographic moorings off Sydney. The currents sweep through the structure and over its 120 square metres of surface area, bringing plankton that has grown on the continental shelf. That’s why we call these reefs ecological wind farms - they provide habitat for fish and invertebrates to harvest the plankton growing on the continental shelf. Sydney’s offshore artificial reef is no mere scuttled ship or rubble pile. In fact, is likely to be more productive than a natural reef, due to its open structure and water flow. It is designed to sustain a community and a fishery, and as Sydney grows there may be many such reefs off the coast. They are likely to be where our grandchildren will go to catch a fish.
Primary Researchers: Iain Suthers