A project undertaken at Macquarie University, and supervised by A/Prof Melanie Bishop.
Along the southeast coast of Australia, once abundant oyster reefs have been reduced by historic overharvest to 8% of their former abundance. With the loss of oyster reefs, has been the loss of the critical services they provide such as fisheries productivity, water filtration and shoreline protection. Attempts to restore oyster reefs are increasing, but success has been variable. Besides the availability of oyster larvae and a hard substrate on which reefs can form, consumption of juvenile oysters by marine predators such as fishes can limit oyster reef re-establishment.
Substrates with complex surface geometries can provide protection to oysters from predators and environmental stressors. However, until this study, it was unclear what specific geometries were most effective. Most studies had used qualitative measures (e.g. flat vs complex) of substrate geometry that lack precision. Moreover, little was known about how the importance of surface geometry is dependent on supply of colonising oysters. In areas where there is frequent and high supply of colonising oysters, establishment of oysters may be less dependent on the availability of substrate geometries that protect them from predators.
Remnant oyster reefs. Credit: F Martinez Baeana.
This project determined the optimal roughness, irregularity and vertical height of protective habitats for oyster re-establishment, and determined how levels of predation and larval supply influence the optimum solution. The optimum geometries of substrates for oyster re-establishment and reef formation were addressed through field experiments in estuaries of New South Wales, Australia. Concrete panels of the same planar area but of 16 different geometries were generated by crossing four levels of surface irregularity (fractal dimension) with four levels of height range. Panels of each design were deployed at three sites with known supply of oyster larvae. So as to ascertain effects of habitat geometry arising from predator mitigation, habitat units were deployed both caged and uncaged. After one year, oyster densities on each of the panels was censused.
Oyster densities (i.e. number per unit of surface area) varied little across the 16 geometries when panels were caged. This indicated that in the absence of predation, effects of habitat geometry were largely driven by surface area for colonisation. However, when panels were uncaged and open to predators, oyster densities peaked at intermediate levels of rugosity – indicative of area-independent effects of habitat geometry arising from predator mitigation (Fig. 2). When exposed to predators, the substrate fractal dimensions and height ranges at which oyster densities peaked matched that of natural remnant oyster reefs. This suggests that the three-dimensional geometries formed by oyster reefs are benefiting the oysters themselves – with the structural complexity observed in nature maximising predator mitigation.
For plates open to predators, oyster colonisation was maximised at intermediate fractal dimensions and height ranges. Credit: J Esquivel Muelbert.
Sampling of uncaged oysters at 2 years confirmed that effects on oyster density of substrate geometry persisted through time, and also extended to a greater richness and abundance of associated invertebrates. Moreover, complex geometries of substrates provided cooler substrate temperatures that allowed persistence of species over hot summer months.
In separate experiments simultaneously manipulating oyster supply and habitat geometry, we found that effects of oyster supply were more important at low levels of substrate structural complexity. At low complexities, survival (%) of colonising oysters increased with increasing density of arriving oysters. At high complexities, the survival of recruits was independent of the number of oysters arriving.
Field experiment examining how survival of oysters is influenced by oyster supply, habitat complexity, and caging.
Credit: J Esquivel Muelbert.
Our study provides a template for developing targets of habitat structural complexity metrics that can be incorporated into oyster reef restoration guidelines to improve restoration success. Here, the best performing geometry resulted in 1,369 oysters per square meter one year after substrate deployment, while the worst performing resulted in complete recruitment failure. Our results highlight that both fractal dimension and height are required but not sufficient alone to maximise oyster recruitment. The optimal solution converged on what is found in nature. Combined, our results suggest that as marine habitat restoration continues to scale up globally, projects should look to nature in guiding the selection and design of restoration substrates.