A medium to large, fish with a body that becomes deeper and more laterally compressed with age. Maximum length 610 mm and maximum weight 7.7 kg; usually 350 mm and 2 kg. The single dorsal fin has a higher, spinous anterior section and a lower, rayed section at the rear. The body colour is grey to grey-brown with a lighter belly. The scales are much smaller than those on Golden or Macquarie perch and on some fish dark edges to the scales create a checkered appearance on their sides. The head and mouth are small, and the tail is weakly forked.
Silver perch are currently found in similar broadscale habitats to Murray cod and Golden perch, i.e. lowland regions of the MDB. Historically the species was also known from more upland environments in NSW (e.g. the upper Murrumbidgee and upper Macquarie rivers) where fish were recorded up to 700 m elevation. Within lowland rivers, the species is often associated with faster water, often around riffles or runs, but historically they were also regularly encountered in lagoons in a number of lowland rivers in the 19th century. The maximum known age for this species is 27 years, from a reservoir population. Most riverine fish would be <10 years old. Individuals mature at 3–5 years—males earlier than females. They spawn in spring and summer after an upstream migration, when large schools often form. Spawning occurs at or near the surface possibly at night, just after dusk. Spawning has been recorded at a range of water temperatures from 16–28°C, and in the mid-Murray spawning occurs when temperature is above ~ 20°C. Females produce 200,000–700,000 eggs, with eggs being round, 2.5–3 mm in diameter, non-adhesive, semi-buoyant and hatch in 30–31 hours at temperatures of 26–27°C. Larvae are free swimming after five days and commence feeding by day six.
Whilst spawning can occur across a range of flow conditions, the strongest recruitment is associated with a combination of low to average river discharge (within channel) and high-water temperatures over the peak spawning period, followed by extended high flows and widespread flooding in the following year that allows juveniles to access productive feeding habitat and promotes large scale dispersal. Immature individuals have been recorded moving through fishways in the Murray, Loddon and Murrumbidgee rivers with fish as small as 68 mm TL moving during afternoon and dusk. A recent tracking study showed that juveniles, subadults, and adults occupy extended river reaches in the Murray and its tributaries, with fish movements associated with flow components such as small to moderate rises in mainstem flows and elevated flows in tributaries. Upstream movements were less during large mainstem flows. The majority (73%) of individuals examined in the Darling-Baaka fish kills had emigrated from the Murray River, indicating substantial inter-regional movements.
Silver perch are omnivorous. The diet contains aquatic plants, snails, shrimps and aquatic insect larvae. Reports that the species becomes mainly herbivorous once it reaches lengths of 250 mm are incorrect, at least for reservoir populations, as diet in Googong Reservoir near Canberra shows little change with fish size.
Formerly widespread over much of the Murray–Darling Basin excluding the most upper reaches, Silver perch has declined over most of its range. Numbers moving through a fishway at Euston Weir on the Murray River declined by 93% between 1940 and 1990. In 1940–45 Silver perch comprised 65.4% of all fish moving through Euston fishway, but in 1987–92 it was only 9.8%. Only nine Silver perch were recorded in a two-year survey of 40 randomly selected sites in the NSW MDB in the mid-1990s. Similarly, only 102 individuals in total were recorded across 13 river valleys in the Sustainable Rivers Audit (2004–2013), with 47 and 15 individuals from the Central and Lower Murray valleys respectively. No other valley had >7 individuals. 95 of the Silver perch captured were in lowland sites (0-200 m elevation), with 2 in the slopes and 5 in the upland. A total of only 47 have been recorded in the MDB Fish Survey (2014/15–2021/22) with only the Central Murray recording more than 10 individuals in total. It is still patchily abundant in the lower and mid-Murray, and there have been some local management successes in achieving improved outcomes for spawning and juvenile dispersal in parts of the southern MDB since the breaking of the Millennium Drought. It was estimated that 10,000 Silver perch moved through the Torrumbarry fishway during three weeks in 2006/07 with >23,000 individuals moving over a five-month period. Another good year at this fishway was in 2009/10 when >17,000 Silver perch were recorded. However, numbers moving through this fishway have not again reached these levels with <5,000 per year recorded since 2011. Silver perch can move extremely long distances over their lifetime, with one fish tagged in the lower Murray recaptured in the Barwon River, a distance of 2,565 km. There are anecdotal reports of the species migrating almost to Cooma on its upstream spawning run in the Murrumbidgee catchment, although this summer spawning run from Lake Burrinjuck is unfortunately a thing of the past. The species is now functionally extinct in the ACT, and populations in the northern MDB (particularly the Paroo, Warrego, Condamine and Macintyre valleys) are at very low levels if not functionally extinct. Recent population modelling also supports the highly imperilled nature of northern MDB populations. In some of these northern valleys the occasional captures of this species may represent stocked fish (but not in the Paroo where stocking does not occur). The 2018-19 fish kills at Menindee on the Darling-Baaka River killed millions of fish including thousands of Silver perch. There has been little to no evidence of local recruitment of Silver perch since the kills, with examination of the Menindee fish mortalities demonstrating most Silver perch killed originated from the Murray River >500 km away. The decline in the southern Basin appears to have halted, but more detailed monitoring and more importantly, analysis of contemporary data sets that captures the spatial distribution of each population is required before the trajectory of this population can be confirmed.
This species is bred artificially in a number of government and commercial hatcheries and widely stocked into farm dams and reservoirs. It has been the subject of considerable interest for its potential as an aquaculture species. Stocking for conservation purposes commenced in the Namoi River in 2016 with 120,000 fish stocked between 2016 and 2018. Stocking ceased in the severe drought years of 2019 & 2020, with another 100,000 stocked across 2021 and 2022.
River regulation and associated infrastructure has severely affected this species through disruption of migration and reproductive behaviour. Coldwater pollution reduces growth rates likely decreasing available prey resources and increasing the period that larvae and juveniles are susceptible to predation. Reduced flooding and access to off-channel habitats may also be affecting the species. Interactions with alien species (Carp and Redfin perch) are also suspected to be a threat. Silver perch were listed as critically endangered under the EPBC Act in 2013.
ACT Government 2018; Baumgartner 2003; Clunie & Koehn 2001c,d; Gilligan et al. 2019c; Kibria et al. 1998; King et al. 2005, 2009, 2010, 2016; Koehn et al. 2020a; Koster et al. 2021b; Llewellyn 2014; Mallen-Cooper 1993; Mallen-Cooper & Stuart 2003; Mallen-Cooper and Brand 2007; Mallen-Cooper et al. 1995; Merrick 1996; Michie et al. 2020a; NSW DPI 2006, 2017; Stocks et al. 2022; Thiem et al. 2022; TSSC 2013; Todd et al. 2022; Tonkin et al. 2007, 2019b; Trueman 2011; TSSC 2013; Zampattii et al. 2018; I. Stuart unpubl. data; M. Lintermans unpubl. data; Z Tonkin unpubl. data.
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