Paramount Importance

Science communicator from dipped below the surface to get an idea of what a few of the locals thought about WA World Ocean Day 2026!

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8 June | Free Entry

Monitoring fish communities and water quality is one of the ways we can assess the health of a river or estuary.

When there has been little or no flow, the water can stop mixing and become stratified. This means layers form in the water column, with lighter surface water sitting above denser, saltier bottom water. Once this happens, oxygen from the surface may not reach the bottom. At the same time, organic matter, algae and plant material can settle and break down, with microbes using up oxygen in the process. Over time, the bottom waters can become deoxygenated, or even anoxic, meaning there is little to no oxygen available.

This can also affect the sediment, because organic material accumulates on the bottom and oxygen is rapidly used up as it decomposes. When the sediment becomes deoxygenated, it can reduce habitat quality for bottom-dwelling animals and influence the release of nutrients and other compounds back into the water.
By monitoring water quality and fish communities, we can better understand how the system is responding to low flow, stratification and changing environmental conditions.

This project is funded by the Australian Government Natural Heritage Trust and delivered by PHCC, a member of the Commonwealth Regional Delivery Partners panel.

All sampling is conducted under permit.

Presenter:

Monitoring fish communities is one of the ways we can assess the health of an estuary.

In shallow areas, we use seine nets to sample the small and juvenile fish that live close to the shore. Larger fish can often swim faster than we can pull the net, so seine netting mainly captures smaller species and early life stages.

These fish are important because they respond quickly to changes in environmental conditions such as salinity, oxygen levels, water quality and habitat condition. By looking at which species are present, how abundant they are, and the functional roles they have in the ecosystem, we can better understand the condition of the estuary.

This approach provides a snapshot of the fish community and helps us assess how estuaries respond to local-scale disturbances, as well as how they recover once those pressures are removed.
This project is funded by the Australian Government Natural Heritage Trust and delivered by PHCC, a member of the Commonwealth Regional Delivery Partners panel.

All fishing is conducted under permit.

Presenter:

Monitoring fish communities is one of the ways we can assess the health of an estuary.

In shallow areas, we use seine nets to sample small and juvenile fish. But larger fish are often too fast, too strong, or too widely spread to be caught effectively this way. That is why we also use gill nets.
Gill nets allow us to sample larger and more mobile fish that use deeper parts of the estuary. These species are important because they can tell us how different parts of the ecosystem are functioning, including habitat quality, food-web structure, connectivity, and overall estuary condition.

Fish respond quickly to changes in environmental conditions such as salinity, oxygen, water quality and habitat disturbance, making them useful indicators of ecosystem health. By sampling both small fish and larger fish, we get a more complete picture of how the estuary is responding to local-scale pressures and whether it is recovering after those pressures are removed.

This project is funded by the Australian Government Natural Heritage Trust and delivered by PHCC, a member of the Commonwealth Regional Delivery Partners panel.

All fishing is conducted under permit.

Presenter:

Gobbleguts might be small, but their parenting strategy is incredible.

Unlike many fish that release eggs into the water and leave them to develop on their own, male gobbleguts are mouth brooders. After spawning, the male carries the fertilised eggs inside his mouth, protecting them from predators while they develop.
During this time, he often cannot feed properly, meaning parental care comes at a real cost. It is a tiny fish with a surprisingly serious commitment to the next generation.

This is one of the reasons estuaries are so fascinating, even the smallest species can reveal complex behaviours, survival strategies, and hidden ecological stories happening just below the surface.

All fishing is under permit.

Presenter:

Estuaries are not uniform habitats, and the water quality helps explain why.

As freshwater from rivers mixes with seawater from the ocean, conditions such as salinity, temperature, oxygen, and turbidity change across the system. In some estuaries, especially during dry periods or when river flow is low, evaporation can remove water faster than freshwater enters the system. This can cause salts to concentrate, making parts of the estuary even saltier than the ocean.

By measuring water quality, we can better understand why certain fish species are present, absent, or restricted to particular areas. Some species can tolerate these extreme salinity changes, while others may move away, become stressed, or disappear from the system entirely.

These patterns help us assess estuary health. Water-quality monitoring allows us to link fish communities to changing environmental conditions, identify important habitats, and understand how pressures such as reduced freshwater flow, prolonged bar closure, and drying climate are reshaping these ecosystems.

In this way, the fish communities help us understand them and what is happening environmentally.

All fishing is under permit.

Presenter:

This project is funded by the Australian Government Natural Heritage Trust and delivered by PHCC, a member of the Commonwealth Regional Delivery Partners panel.

Estuaries are not uniform habitats. As freshwater from rivers mixes with seawater from the ocean, salinity changes across space and time, creating different zones for different fish species.

Some species remain closer to the river where salinity is lower, others dominate near the mouth where marine influence is strongest, and many move between these areas as conditions change. This partitioning reduces competition and allows estuaries to support a diverse mix of freshwater, estuarine, and marine fishes within the same system.

In this way, salinity gradients help structure fish communities, influencing where species live, feed, reproduce, and seek refuge. That is part of what makes estuaries so ecologically important: they are dynamic transition zones where fish assemblages are shaped by the constant interaction between land, river, and sea.

All fishing is under permit

Presenter:

Monitoring the health of the ecosystem by looking at the fish communities.
The nearshore waters were sampled using a 21.5 m seine net that was walked out to a maximum depth of 1.5 m, deployed parallel to the shore, and dragged towards and onto the shore or boat.

Bigger fish can swim faster than we pull the net so we generally catch small and juvenile fish. Fish respond very quickly to different environmental conditions and can give an assessment of the condition of the estuary. This has been subjected to extensive testing and validation over several years and shown to be a robust tool for understanding the ecological health responses to local-scale disturbances and the recovery of the estuary following their removal.

This project is funded by the Australian Government Natural Heritage Trust and delivered by PHCC, a member of the Commonwealth Regional Delivery Partners panel.
All fishing is under permit

Presenter:

When things go out of shape 😉