Like many other important native freshwater fish species within the Murray Darling Basin, Golden perch (Macquaria ambigua) were immensely impacted by the 2018-19 fish kills in the lower Darling River.  A 2021 study involving 18 researchers (by Zampatti et.al) looked into the species’ historical and current population demographics in the Darling River. The study provided guidance on how best to manage our waterways to support Golden perch rehabilitation following fish kills.

Step 1: Understanding population demographics and habits 

Understanding population demographics (i.e. the age, place of birth, habitats, etc) and the life cycles of Golden perch is critical to helping the rehabilitation of the species after fish kills. Fish kills are a natural process that Australian rivers periodically undergo. They are often triggered by large sudden changes in the environment. 

What are fish kills exactly?

Fish kills are a natural occurrence in Australia and worldwide, and can arise from both natural and manmade causes. Fish kills are not uncommon in the Murray Darling Basin, where over 60 fish kills were reported during the 2019-20 summer.  

Most fish kills occur as a result of fluctuations in the natural environment. A common cause of fish kills is algal blooms, which cause water quality issues like low oxygen or toxins that make fish unable to survive in the river. Fish kills can be caused by single factors such as poison in the waterway or an unexpected temperature shock, or by a combination of stressors. These stressors could include hot temperatures, low water flows and low dissolved oxygen in the waterway.  

The 2018-19 fish kills in the Lower Darling-Baaka River were caused by low water flow creating stagnant water, which is susceptible to blue-green algae infestation. The blue-green algae uses the oxygen in the water as it dies and decomposes. Fish cannot survive in deoxygenated water. 

Fish kills can cause significant damage to native fish populations that are already endangered from other threats. Fish kills can make the existing status of threatened fishes worse by limiting their recovery progress and reducing the availability of habitat.  

• Collecting and analysing otoliths

Otoliths are the ear bones of fish, and can be used to determine a fish’s history, including its age, where it was born, what sort of habitat it has lived in and potentially even its diet. This critical information enables scientists and fish and water managers to understand what conditions can best facilitate population growth.

• Analysing age structure

Using the information from the Golden perch otoliths, the researchers were able to determine the age structure of Golden perch in Darling River. Once the age structure was established, the study found that many of the fish, whose otoliths were collected in 2014 and 2019, were predominantly born in 2009 or 2011. However, fish older than 10 years old were not present. It’s likely that the lack of older fish may be because of the drier conditions experienced during the Millennium Drought which permeated throughout the Murray-Darling Basin roughly between 1997-2009. Increased rainfall and flow in south-east Australia in 2009-10 saw a boom in numbers of many native fish, including Golden perch. This information helps water managers, as it shows what conditions Golden perch respond to most effectively, in this case high river flows.    

• Analysing natal origin

In addition to fish age, otoliths also can reveal where a  fish was born and has spent its life. The study found that almost all the Golden perch found in the Darling River were spawned in the Darling. Based on this, the study found that it could be more effective to restore conditions that support recruitment (fish reproduction) than stocking Golden perch continually. Restoration projects could also be more effective when they connect tributary recruitment sources to the main channel of the river. Some tributaries of the Barwon-Darling River retain flow patterns and habitat that are conducive to Golden perch reproduction. Connecting these tributaries to the main channel will also help rehabilitate Golden perch populations throughout the Barwon-Darling river system.  

Step 2: Understanding current threats and barriers

Movement and migration are a key part of a Golden perch’s life, however, barriers to their migration are common in the Darling River. Golden perch require different habitat types throughout the course of their life cycle. The study of Golden perch otoliths demonstrated that Golden perch spawned in the Darling River often migrate to the Murray River as juveniles and then move back to the Darling later in their life. These essential migration patterns are inhibited by dams, weirs and other barriers in waterways that prevent fish from entering new reaches of a river or creek. Water managers around Australia have been exploring options to improve fish migration, including building fishways, which are structures that help fish move around barriers.  

Ultimately, it’s important to understand what threats exist for Golden perch and other native fish, on top of major environmental fluctuations like droughts and floods, which are being exacerbated by climate change. Some of these threats could be from invasive species like Carp or from human-caused issues like river regulation and cold water pollution which disrupts not only migration, but also spawning conditions.  

Step 3: Managing for rehabilitation

The study’s results found that having a greater diversity in ages of Golden perch will help to safeguard Golden perch populations against environmental fluctuations and changes, such as fish kills, floods or droughts. Managing for a more diverse age structure will help improve spawning outcomes, recruitment and population growth.

To rehabilitate Golden perch populations, we need to understand their population dynamics, including age structure, where they live and migrate and what conditions they thrive in, to better manage water flow allocations. Using this information, we can maximise water flows to enhance Golden perch and improve their reproduction and migration chances, especially following dramatic changes in the environment such as fish kills.

Resources

• Population demographics of golden perch (Macquaria ambigua) in the Darling River prior to a major fish kill: a guide for rehabilitation
• Golden Perch in the Murray Darling Basin Facts

Main photo: Native fish species, Golden Perch (Macquaria ambigua).

Source: Gunther Schmida

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