Imagine a bustling marine neighborhood where industrial ships, naval vessels, recreational boats, and wildlife all share the same waters. This is Cockburn Sound in Western Australia, one of the most intensively used marine areas in the country. Despite decades of human activity, this temperate bay continues to support a remarkable diversity of life, from tiny plankton to apex predators like sharks and dolphins.

Researchers have now created the most comprehensive picture yet of how this ecosystem functions, revealing surprising connections between species and highlighting the delicate balance that keeps this busy bay alive.

A Bay Under Pressure

Cockburn Sound has endured more than 50 years of intensive development. Since the 1950s, the area has seen the construction of industrial facilities, a naval base built in 1969, and a causeway completed in 1973. These developments, combined with nutrient pollution, contributed to the loss of approximately 80 percent of seagrass meadows between the 1950s and early 2000s.

The impact on marine life has been significant. Pink snapper populations declined, and the blue swimmer crab fishery, once the largest in Western Australia with catches peaking at 363 tonnes in 1996, collapsed and remains closed today. The lack of recovery has been linked to changes in primary productivity, warming waters, and increased predation pressure.

Despite these challenges, Cockburn Sound remains ecologically valuable and highly cherished by the community for recreation, fishing, and its natural beauty. Understanding how the ecosystem continues to function under these pressures became a priority for scientists and managers planning future development.

Mapping the Web of Life

To understand how energy and nutrients flow through Cockburn Sound, scientists created a detailed ecosystem model using specialized software called Ecopath with Ecosim. This approach allowed them to map out who eats whom, how much energy moves through the food web, and which species play crucial roles in maintaining ecosystem health.

The model incorporated 73 functional groups, ranging from microscopic phytoplankton to top predators. The research team drew on extensive biological surveys conducted in 2021 and 2022, historical fishery data, and expert knowledge from marine biologists familiar with the region.

What emerged was a portrait of remarkable complexity. More than 60 percent of the total biomass in the system exists at lower trophic levels, including primary producers like seagrass, filter feeders, and various invertebrates. This foundation supports everything above it, from small fish to apex predators.

The Keystone Players

One of the most valuable insights from the study was identifying keystone species, those organisms that have an outsized influence on ecosystem structure despite not necessarily being the most abundant.

Sharks emerged as particularly important. Both large sharks like white sharks, tiger sharks, and hammerheads, and smaller species such as Port Jackson sharks and wobbegongs, were found to be critical in regulating prey populations. Their presence creates cascading effects throughout the food web, indirectly benefiting some species while controlling others.

Bottlenose dolphins and Australian sea lions also ranked as keystone species. These marine mammals influence the abundance and behavior of fish populations, creating indirect effects that ripple through the ecosystem. Cormorants, the diving seabirds that hunt fish in coastal waters, similarly play a structuring role in the food web.

Interestingly, the analysis revealed that commercial and recreational fisheries currently do not have major direct impacts on the biomass of fished species in Cockburn Sound. However, some indirect effects were detected. For example, the squid jigging fishery may indirectly affect dolphins through competition for shared food resources.

The Foundation Matters

While top predators grab headlines, the study emphasized the crucial importance of habitat forming species. Seagrass, despite its massive historical decline, continues to play a vital role in supporting the food web. The model showed positive effects of seagrass on commercial species like blue swimmer crabs and juvenile snapper, as well as on various bottom dwelling invertebrates.

Seagrass provides more than just food. It offers critical nursery habitat where young fish and crustaceans can hide from predators, and it stabilizes sediments and improves water quality. The loss of 80 percent of this habitat represents a fundamental change to the ecosystem's capacity to support marine life.

Corals and sponges, while less abundant than seagrass, also contribute to ecosystem function by providing refuge and structural complexity. These habitat forming species may not directly feed many animals, but they create the three dimensional structure that allows diverse communities to coexist.

Energy Flow and Ecosystem Maturity

The research revealed fascinating details about how energy moves through Cockburn Sound. The total system throughput, a measure of all the energy and material flowing through the ecosystem, was estimated at 10,517 tonnes per square kilometer per year. Nearly half of this represents consumption by animals, about one third goes to respiration, and the remainder flows to detritus or is exported from the system.

When comparing Cockburn Sound to seven other Australian marine ecosystems, some interesting patterns emerged. Despite being the smallest system studied at just 260 square kilometers, Cockburn Sound showed high productivity relative to its size. The system also displayed high complexity in its trophic connections, indicated by a System Omnivory Index that ranked highest among all the ecosystems compared.

However, the ratio of total primary production to total respiration was relatively low at 0.6, suggesting the system depends significantly on external energy inputs. This could reflect the reduced seagrass coverage and primary production that has characterized the bay in recent decades.

Practical Applications for Management

Beyond academic interest, this ecosystem model serves practical management purposes. The research team developed 35 ecological indicators spanning five categories: biomass based measures, ecosystem level conditions, energy use, fisheries impacts, and conservation status.

These indicators provide a baseline snapshot of Cockburn Sound's health in 2020 to 2022. Managers can now track these metrics over time to detect changes in ecosystem condition and assess how the system responds to various stressors, including planned infrastructure development and climate change.

The indicators cover everything from total fish biomass and the trophic level of catches to the biomass of species with conservation significance and the coverage of key habitats like seagrass. This comprehensive approach allows managers to monitor multiple aspects of ecosystem health simultaneously.

Looking Ahead

Western Australia is planning to build a new container port and freight terminal in Cockburn Sound. This major development will involve dredging shipping channels and potentially constructing breakwaters. Understanding how the current ecosystem functions provides a critical baseline for predicting and managing the impacts of this development.

The ecosystem model can be used to explore different development scenarios and management strategies before they are implemented. For example, managers can simulate the effects of habitat loss, changes in fishing pressure, or restoration efforts to see how the ecosystem might respond.

Climate change adds another layer of complexity. Western Australia's waters are warming faster than the global average, and these temperature changes are already affecting marine species distributions and productivity. The model provides a framework for understanding how these climate impacts might cascade through the food web.

Gaps in Knowledge

While the study represents the most comprehensive analysis of Cockburn Sound's ecosystem to date, it also highlighted important knowledge gaps. Biomass estimates for some groups, including snapper at various life stages, small pelagic fishes, seabirds, corals, and benthic primary producers, had to be estimated by the model rather than measured directly.

Diet information for many species, particularly the role of detritus as a food source, remains incompletely understood. Addressing these gaps through targeted research and monitoring would improve the accuracy of future model predictions and management recommendations.

The study also noted that the model does not fully capture the role of Cockburn Sound as nursery habitat for some species. Developing a spatial version of the model that accounts for how different habitats are used throughout species' life cycles would provide additional insights.

A Blueprint for the Future

This research demonstrates the value of ecosystem based approaches to marine management. Rather than focusing on single species in isolation, the study reveals how species are interconnected and how impacts to one part of the system can affect others.

The 73 functional groups in the model interact through thousands of trophic links, creating a web of dependencies that is impossible to understand through intuition alone. Quantitative modeling allows scientists and managers to trace these connections and predict how the system might respond to change.

For Cockburn Sound, the message is clear. Despite decades of intense human use and significant habitat loss, the ecosystem retains remarkable complexity and continues to support diverse marine life. Top predators like sharks and marine mammals play disproportionate roles in maintaining ecosystem structure. Habitat forming species like seagrass, though greatly reduced, remain critical to ecosystem function.

As Western Australia plans for continued development and faces the challenges of climate change, this detailed understanding of ecosystem function provides an essential tool for making informed decisions. The model offers a way to explore trade offs, evaluate management options, and work toward a future where economic development and ecological health can coexist.

The story of Cockburn Sound is ultimately one of resilience and fragility. The ecosystem has survived substantial changes and continues to function, but its capacity to absorb further impacts is not unlimited. By understanding how the system works today, we gain the knowledge needed to protect it for tomorrow.

Publication Details

Published: 2025 (Online)
Journal: Estuarine, Coastal and Shelf Science
Publisher: Elsevier Ltd.
DOI: https://doi.org/10.1016/j.ecss.2025.109285

Credit and Disclaimer

This article is based on original research published in Estuarine, Coastal and Shelf Science. The content has been adapted for a broader audience while maintaining scientific accuracy. For complete details, comprehensive data, full methodology, and in depth analysis, readers are strongly encouraged to access the original peer reviewed research article through the DOI link provided above. All factual information, data interpretations, and scientific conclusions presented here are derived from the original publication, and full credit goes to the research team and their contributing institutions.