August 1 - 31, 2025: Issue 645

 

New Study Shows Floods from Hawkesbury  Causes Abundance of Reef Fish to Drop by 50% 

March 26, 2021: RAAF Pilots flying over Barrenjoey to a Commemorative Service at the Hyde Park Memorial for the RAAF 100th Year  caught some of the brown flood tide from a bird's eye view:


Photo: Pilatus PC-21A aircraft from No. 4 Squadron based at RAAF Base Williamtown turn on final heading to fly in formation over Sydney in support of a Royal Australian Air Force Centenary commemorative service held at the Anzac Memorial in Hyde Park. Photographer: CPL Craig Barrett.

New research published in the journal Ecology has found the impacts of extreme flooding don’t stop at the shoreline, they can reach far offshore, affecting fish communities on rocky reefs.

“We know that extreme flooding events have huge impacts on estuarine and coastal species. Our recent study has also found that this can hold true for fish further offshore,” explained lead author, Dr Matthew Rees, from the NSW Department of Primary Industries and Regional Development (DPIRD).

The research team assessed the abundance and diversity of rocky reef fish before and after severe flood events in 2022, using data collected through the DPIRD statewide fish monitoring program.

In 2022, Sydney experienced its wettest year on record. Four major floods in Sydney’s Hawkesbury-Nepean River system created an unprecedented coastal flood plume that extended 70 km offshore and reached depths of up to 50 metres.

Underwater video cameras placed on rocky reefs offshore from the river mouth captured changes in fish populations before and after the floods. The changes were compared to video analysis from unaffected reference sites.

The results were striking. In areas impacted by flood waters, total reef fish abundance fell by 50%, and species richness dropped by 13%.

The declines were primarily due to a reduction in the abundance of fish like Mado that feed on plankton (−75%), and also a reduction in fish that feed on seafloor invertebrates (−40%) such as Southern Maori Wrasse, Silver Trevally and Port Jackson Shark.

Researchers suspect the declines were primarily due to changes in salinity. However poor water quality, a reduction in prey, and degradation of important fish habitat such as kelp, may also have contributed.

A year on, many species had recovered with Silver Trevally and Port Jackson shark numbers increasing back to pre-flood levels.

However, some species such as Mado and Southern Maori Wrasse numbers remained significantly lower than prior to the flooding. Interestingly some species such as Snapper were not impacted by the flooding suggesting that impacts are related to the biology and ecology of individual species.

Map of the study area outlining: (A) the locations of the stereo-baited remote underwater video system deployments across the impacted region and the northern and southern reference areas, (B) satellite image of a Hawkesbury-Nepean River flood plume on July 8, 2022 (Sentinel 2: Earth observations browser European Space Agency) in relation to baited underwater video sites, and (C) Hawkesbury-Nepean River water level data between 2019 and 2023 at Windsor (sourced: www.waternsw.com.au). Light blue banding indicates the pre-flood survey period, while purple banding the post flood and one-year post-flood survey periods.

Fish assemblages at reference locations unaffected by flooding (at offshore sites and sites distant from large rivers) in the Port Stephens, Jervis Bay and Ulladulla regions remained stable over the study period.

Consequently, the impacts of flooding are likely to be greater on species that form localised populations such as many rocky reef fish but will be lower for species that have broad-scale population structure such as Snapper.

With extreme rainfall and flooding expected to become more frequent and intense due to climate change, the findings raise concerns about the long-term impacts of extreme flooding on the ecosystem functions and services provided by rocky reef fish.

“Our results suggest that future extreme flooding may disrupt temperate reef food webs and reduce the occurrence of some important reef species in flood affected regions,” Dr Rees said.

“Future monitoring in the affected area will be necessary to measure the recovery of fish population and will also help us gauge the long term impacts of severe flood events.”

You can read the full paper here:  Quantifying the effects of extreme flooding on coastal reef fishes - Rees - 2025 - Ecology - Wiley Online Library

This research was funded through the NSW Government’s Marine Estate Management Strategy, and the Australian and NSW Government’s Estuary Asset Protection Program.

Abstract

The ecological impacts of extreme floods to benthic ecosystems in oceanic environments remain unclear, as events are difficult to observe and quantify. Between 2020 and 2022, an unusually long La Niña resulted in substantial flooding across southeastern Australia. Major floods in the Hawkesbury-Nepean River led to an unprecedented flood plume extending ~70 km offshore to depths of ~50 m, exposing coastal biodiversity to rare environmental conditions. Here, we use long-term monitoring data to quantify the impacts of large-scale flooding to coastal rocky reef fishes. Flooding reduced the total abundance and species richness of reef fishes by 50% and 13%, respectively. Declines were primarily due to reductions in the abundance of planktivores (−75%) and benthic invertivores (−40%). Assemblages at reference locations isolated from large rivers were stable across the study period, indicating observed patterns were in response to floods. Surveys 1-year post flooding signaled that benthic invertivores were beginning to recover but not planktivores. Our results demonstrate the detrimental impacts of extreme flooding to some coastal rocky reef fishes. Given projected increases in the severity and frequency of rainfall due to climate change, our findings suggest that extreme flooding may compromise ecosystem functions and services provided by rocky reef fish in a future climate.

Related reports:

March 27, 2021 - brown water off Barrenjoeyu headland at north Palm Beach, heading south. Pic: AJG/PON

Fish in pristine waters at Church Point wharf in 2015 - on Pittwater estuary. Pic: AJG/PON