Striking Gold

Golden perch flows and spawning in the Lachlan River

Author: Lachlan Research Team

As good rain fell in the upper Lachlan River catchment in spring 2020, excitement grew around a potential translucent flow through downstream river bends to benefit native fish. A translucent flow occurs in a regulated river system when a portion of inflows are passed through a regulating structure – in this case Wyangala Dam near the headwaters – to enable a near-natural flow pulse into the river system. A translucent flow can also be protecting large rainfall-runoff events in creeks and rivers to pass through the entire riverine system by not capturing unregulated flow events in storages. The timing of this pulse looks like it ideally matched, in both temperature and time of year, the flow-dependent spawning requirements of a native large-bodied fish, the Golden perch. Although Golden perch can display some flexibility in recruitment strategies, they generally requiretriki a rise in river level to promote spawning during water temperatures of around 20 °C (usually spring / early summer in south-eastern Australia).

Over the past 7 years of Commonwealth environmental water monitoring in the lower Lachlan River, conditions appear to have met these spawning requirements on a couple of occasions (notably in summer 2015), but monitoring did not detect any eggs, larvae or juvenile Golden perch as a result. Furthermore, a study undertaken by PhD student Foyez Shams indicated that over the past decade or so, natural recruitment of Golden perch in the Lachlan River catchment was limited, with the population being largely ‘propped’ up by stocking.

A large adult golden perch captured by boat electrofishing on the Lachlan River, NSW (Photo: Hugh Allan).

With the stage set, and a suitable translucent flow snaking its way down the system, all the University of Canberra larval fish team could do was wait, with larval fish traps at the ready. As the pulse reached the monitoring sites around Hillston in November, light traps and drift nets were set to capture any fish eggs or larvae that may have been produced by spawning adults on this flow pulse. Despite initial fossicking through samples on the riverside, the team failed to find any indication of Golden perch eggs and larvae. Samples were preserved for later processing under a microscope back in the laboratory.

Fast forward nearly four months to March 2021 and the NSW DPI fish community monitoring team was in full swing, when they came across a very welcome surprise. While undertaking boat electrofishing, the team stunned and captured two small Golden perch. Upon measuring them, the team was excited to conclude that, based on size, these fish were less than a year old (termed young-of-year). While Golden perch adults are common, this was the first time young-of-year had been caught in fish community monitoring of the lower Lachlan River between 2015–2021 using the standard method!

The big question was, were these fish spawned naturally, or a result of stocking from a hatchery. To answer this question, a genetic sample (in the form of a small piece of fin) was collected from each individual. Comparing the genetic composition of these fish with that of adult Golden perch used for brood stock in hatcheries will reveal whether these fish were naturally spawned or not. We await the results for confirmation! Following these captures, another young-of-year Golden perch was caught during fyke netting a week later at a site down river.

Young-of-year golden perch collected via boat electrofishing on the Lachlan River in March 2021 (Photo: Ugyen Lhendup).

Almost simultaneously, the larval fish team at University of Canberra were processing their preserved samples collected from spring / summer 2020. For this, each larval fish is identified to species level and measured under a microscope. These fish are typically less than 2 cm in length, so they can be difficult to identify with the naked eye. From a sample collected in late November, Rhian Clear and Ugyen Lhendup found larval fish not seen before in the seven years of monitoring. Using a field guide, and comparing to similar species and photos of hatchery reared individuals, it was concluded that these fish were in fact larval Golden perch! In fact, four of these little fish were found in that single sample. Definitive proof of natural recruitment! Eureka!

Larval golden perch discovered in a fish sample collected form the Lachlan River in late spring 2020 (Photo: Rhian Clear).

Otoliths (tiny ear bones) will be removed from the larval Golden perch and analysed to determine their age. This is possible as daily growth rings form within the otolith, allowing the number of rings to be counted (much like that of a tree trunk – see photo below), which corresponds well to the number of days the fish was alive before capture. With this information, we can back calculate the exact time these individuals were spawned. The date and corresponding river conditions (flow and temperature) that triggered spawning can then be identified, providing valuable information into the spawning behaviour of this species in the Lachlan River, which will in turn help guide targeted flows in future years!

An otolith from a 8.87 mm long Murray cod larvae with an estimated age of 11 days. Yellow + indicates location of daily rings (Photo: Fish aging services).

Feature image: An adult Golden perch captured by boat electrofishing in the Lachlan River near Hillston, NSW (Photo by Rohan Rehwinkel)

Our work in the Lachlan Selected Area

The Lachlan River flows through the lands of the Nari Nari, Ngiyampaa, Waradjuri and Yita Yita Nations, forming part of Songlines and Dreaming tracks. It has provided food, shelter and resources to Aboriginal people for between 40,000 and 65,000 years. The Lachlan supports a diverse range of landscapes and species that vary enormously through extremes of weather conditions. Our work here is focusing on monitoring the outcomes of environmental water in the lower Lachlan river system, from Lake Brewster to the Great Cumbung Swamp.

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