Study shows what stresses seagrass (and the fish that love it)

The more human disturbances, the greater the loss of seagrass.

That was the finding of a recent study conducted in the Pittwater estuary by DPI Fisheries.

The study looked at how Posidonia australis seagrass (or strap weed) loss related to the cumulative effects of human impacts such as foreshore development, poor water quality, vessel traffic and prop scour, and anchor damage.

It found that human disturbances are impacting seagrass beds with consequences for fish productivity and biodiversity.

DPI Fisheries Research Scientist Dr Matt Rees led the study.

He said the study was important as it will help protect Posidonia seagrass beds in the future.

“Preserving Posidonia is just so important for the health and ecology of NSW estuaries like Pittwater. Posidonia provides many benefits, one being acting as a critical nursery habitat for many fish species.”

“In Pittwater, like many estuaries across NSW, there has been significant losses of seagrass species like Posidonia, with populations endangered in most of the major Sydney estuaries.”

“We knew seagrass can be lost due to things like shading from poor jetty design, anchoring from boating and poor water quality. What we wanted to better understand was what the cumulative impact of these factors on seagrass is. This paper helps answer that question,” he said.

“Understanding how these factors combine to impact seagrass will help managers make better decisions to reduce future losses,” he said.

The study mapped the distribution and intensity of human impacts across Pittwater. When this map was compared to areas of mapped seagrass across the estuary, the pattern was clear.

“Areas exposed to high human impacts had the greatest losses of Posidonia seagrass over time,” Dr Rees said.

But it is how this loss of seagrass was impacting fish numbers that local fishers might find most interesting.

To determine this, Dr Rees said his team used baited underwater cameras to survey fish across the estuary.

“We found Posidonia seagrass had a unique assemblage of fishes compared to nearby sand habitat,” he said.

“The number of popular target species such as snapper, tarwhine and bream were highest on the edge of seagrass meadows,” Dr Rees said.

“Ultimately, this study shows that it's important to consider how human impacts combine to impact seagrass beds and this has flow on effects to fish biodiversity and fisheries production.”

“We are hoping these research findings will help managers with their decision-making regarding human activities in NSW estuaries to better protect seagrass and fish into the future.”

This study is part of the Threats to estuarine fish assemblages project which is funded via the Marine Estate Management Strategy.

You can find out more about why seagrass is so important on the DPI Fisheries webpage here.

You can read the full paper Cumulative effects of multiple stressors impact an endangered seagrass population and fish communities here.