Author: Brad Martin
Martin, Brad, 2025 Temperate Australian Shellfish Reefs: Forms, Functions, and Fishes, Flinders University, College of Science and Engineering
Terms of Use: This electronic version is (or will be) made publicly available by Flinders University in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. You may use this material for uses permitted under the Copyright Act 1968. If you are the owner of any included third party copyright material and/or you believe that any material has been made available without permission of the copyright owner please contact copyright@flinders.edu.au with the details.
Shellfish reefs are biogenic ecosystems characterised by bivalve ‘shellfish’ aggregations, including oysters and pinnids. Shellfish reefs can create, modify, and maintain distinct environmental conditions and structural habitats. Shellfish reefs may benefit fish and nekton assemblages, including fisheries-targeted species. These ecosystems are however understudied due to global shellfish reef declines in recent centuries. The overall aim of this thesis was to investigate fish-shellfish reef habitat interactions with a South Australian focus. This thesis examined how different shellfish, habitat structure and environmental conditions influenced fish assemblages, providing knowledge to inform and monitor shellfish reef management outcomes.
I first conducted an archival review of South Australia’s shellfish reefs with the objective of understanding past reef descriptions and management [Ch. 2]. This identified 140 historical reef sites with >887 km2 of flat oyster (Ostrea angasi) reefs. It revived historic ecological descriptions, and management shifts relevant to modern shellfish reefs. I then summarised global research on shellfish reef fishes using a systematic review of 116 peer-reviewed studies to understand monitoring practices within a global context [Ch. 3]. Studies were concentrated in the United Sates (67%), on oyster reefs (83%) and restoration focused (61%), with no South Australian studies (at the time of review).
With knowledge of fish monitoring methods [Ch. 3], I investigated natural intertidal pinnid (Pinna bicolor) aggregations and mangrove-attached Pacific oysters (Magallana gigas) within the Port River-Barker Inlet estuary [Ch. 4 & 5]. These chapters aimed to assess how intertidal shellfish structured influenced estuarine assemblages. Pinnid density and mortality status demonstrated scale-dependent effects on epibenthic organisms and fishes [Ch. 4]. Pacific oysters supported x2 greater invertebrate richness and x8 greater biomass than mangrove and sediment habitats without oysters. Mangrove habitats with and without oysters supported similar fish communities, but Pacific oysters enhanced fish richness and recorded foraging behaviour [Ch. 5]. Both chapters 4 and 5 indicated that seascape characteristics impacted intertidal habitat use.
Subtidal restoration reefs were then assessed to evaluate temporal [Ch. 6] and spatial [Ch. 7] variability in fish assemblages. Port River nekton demonstrated seasonal variation and were insensitive to initial shellfish reef restoration activities [Ch. 6]. This showed that small-scale restoration efforts within anthropogenic environments may have limited fish enhancement effects. In Chapter 7, fish communities across shellfish reef, seagrass meadow and bare sediment habitats were evaluated. I found that nekton showed habitat dissimilarities in taxonomic assemblages but had functional and behavioural similarities across structured habitat types. This suggests common adaptions of fishes to structured habitats, but species-specific preferences based on the dominant structures and habitat-formers.
The overall aim of this thesis was to assess fish-habitat interactions for understudied temperate Australian shellfish reefs. Collectively, this thesis contributed new ecological baselines for South Australian shellfish ecosystems using a multi-disciplinary and multi-method approach. The findings demonstrated that shellfish reefs generally formed distinct habitat structures, facilitating unique ecological functions and fish communities, but fish-habitat interactions were site-, species-, and context-specific. Shellfish reefs were shown to be complex ecosystems that require adaptive, targeted and nuanced management strategies to maximise ecological and fisheries outcomes.
Keywords: Shellfish reefs, nekton, restoration, marine ecology, fish, South Australia, oyster reefs, Pinnidae, temperate reefs
Subject: Biological Sciences thesis
Thesis type: Doctor of Philosophy
Completed: 2025
School: College of Science and Engineering
Supervisor: Ryan Baring