Colonists dredged away Sydney’s natural oyster reefs. Now science knows how best to restore them.

(Press-News.org) New research has identified optimal design for artificial habitats to support restoration of oyster reefs, based on a detailed understanding of natural oyster reef geometry.

Published in the global journal Nature, the Sydney-based study shows the complex shapes of natural oyster reefs are not random – their structure and arrangement optimise the establishment and survival of developing oysters and their protection from predators.

Oysters are really “ecosystem engineers”, building their own reefs made up of living oysters and the discarded shells of previous generations, explains lead author of the study, Dr Juan Esquivel-Muelbert of Macquarie University.

“But reefs aren’t just piles of shells or skeletons,” says Dr Esquivel-Muelbert. “Reefs are finely tuned 3D systems. Their shape controls who lives, who dies and how fast the reef grows.”

Dr Esquivel-Muelbert – and colleagues from Macquarie, University of New South Wales, The University of Sydney and the University of Hawai’i – studied and measured surviving natural Sydney rock oyster (Saccostrea glomerata) reefs in the Sydney area in detail using high-resolution 3D photogrammetry to record and map the full range of complex geometry.

Next, using computer modelling, the team engineered 16 concrete “tiles” with different numbers and heights of ridges that replicated the range of complexity seen in natural reefs.

Multiples of these tiles were deployed, with and without predator-proof cages, in three estuaries in the greater Sydney area – Brisbane Water, the Hawkesbury River and Port Hacking – adjacent to existing Sydney rock oyster reefs where there is a supply of oyster larvae.

The concrete habitat units were then monitored and compared over time for recruitment, growth and survival of juvenile oysters.

Research showed that the settling and survival of young oysters was maximized not by the most complex or tallest habitats, but by specific combinations of geometric attributes matching those found in natural oyster reefs.

“Our experiment showed the optimal configuration for both establishment and long-term survival was one that provided multiple small spaces for baby oysters to grow in with minimal exposure to predators or harmful environmental stress,” says Dr Esquivel-Muelbert.

“While total surface area is important, juvenile oysters are very small and highly susceptible to predators like fish and crabs and to overheating and drying out. That’s ultimately what you need to form a reef. There's no point in having lots of oyster larvae turning up if they don't survive.”

The researchers say the study provides critical information to guide effective, nature-based reef restoration, not only in Sydney but elsewhere in the world where shellfish and coral reefs have been depleted and destroyed.

“An estimated 85 per cent of the oyster reefs that were present along the coastline of Australia at the time of European settlement have been lost,” says senior author Professor Melanie Bishop, a coastal ecologist in Macquarie’s School of Natural Sciences.

“Not only were oysters harvested for food from the earliest days of colonisation, but the reefs themselves were dredged and the shells crushed and burned to make lime for cement and mortar,” says Professor Bishop. “Many of Sydney’s early colonial buildings are held together with oyster shell.”

Oyster reefs not only support the survival of oysters – they provide habitat for hundreds of animal and plant species and protect shorelines from erosion.

“This work shows that there are universal architectural rules for reef persistence,” says Professor Joshua Madin of the Hawaiʻi Institute of Marine Biology (HIMB), co-senior author who collaborated on the design of the study. “Nature has already solved the design problem. Our job is to read that blueprint and scale it up to help reefs grow faster and survive longer.”

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