Silver perch
The silver perch (Bidyanus bidyanus) is a medium-sized freshwater fish of the family Terapontidae endemic to the Murray-Darling river system in south-eastern Australia.
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OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
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MigratingAnimal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
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starts withThe silver perch is a large grunter with a small head, small eyes, a small mouth at the end of a pointed 'beak-like' snout. The species is streamlined and laterally compressed, with a spiny dorsal fin of medium height, angular soft dorsal and anal fins and a forked tail. Large specimens become very deep bodied with a large hump behind the head. In terms of colouration, they are dark grey to silvery greyish-brown on the back, silver-grey on the sides, with darker scale margins giving a checkered pattern; the belly is whitish; the dorsal and caudal fins are dark, the pelvic fins white.
Silver perch are schooling mid-water fish with a preference for flowing water. Though nowadays found in the lowland reaches of the Murray-Darling system, they originally had a strong presence in the slope and upland reaches of many Murray-Darling rivers as well. In particular, they had a strong presence in upland reaches of the Murrumbidgee River and were originally found as far upstream as Cooma. As recently as the early 1980s, long summer migrations into the upland reaches of the Murrumbidgee were an annual event. Unfortunately these migrations, and these populations, have now collapsed — silver perch are functionally extinct in the Murrumbidgee River now, as in most parts of their former range.
Silver perch have been introduced into the Lake Eyre Basin in arid central Australia. These releases were not officially sanctioned and pose serious hybridisation risks to closely related species of terapontids endemic to the Lake Eyre system.
A translocated and reproducing population of silver perch exists in Cataract Dam on the Hawkesbury-Nepean system. This population was established by NSW Fisheries translocations of juvenile fish from drying billabongs in the lower Murrumbidgee River in approximately 1915–17. The Cataract Dam population is unique in being the only population of silver perch in an artificial impoundment that regularly and successfully recruits and is self-sustaining. The long established prohibition on fishing, the consequent absence of exotic fish and their diseases, and the pristine nature of the dam, including a largely undisturbed, thickly-forested sandstone-dominated catchment and an abundance of coarse rubble and gravel in many inshore areas, where fertilised eggs can settle and not be smothered by silt, are all likely contributors to this unique situation.
Silver perch are opportunistic feeders, feeding on insect larvae, molluscs, annelid worms and algae. The importance of vegetative matter in the diet of silver perch is still debated. Silver perch appear primarily to be a low-order predator of small aquatic invertebrate prey, with occasional intakes of small fish and vegetative matter. In aquaria, silver perch are reported to take blood worms readily.
As recently as the 1970s, silver perch abounded in the entire Murray-Darling Basin, vast though it is. Since then, however, they have undergone a mysterious, rapid and catastrophic decline. Silver perch have now declined close to the point of extinction in the wild. Based on simple catchment area estimates, the silver perch has disappeared from 87% of its former range. Only one sizeable, clearly viable and self-sustaining population now survives in their natural range, in the central reaches of the Murray River. For these reasons, the Australian federal government has listed wild silver perch as critically endangered under national environmental law. Silver perch are bred extensively in aquaculture but these domesticated strains and captive populations are of little use in ensuring the species' survival in the wild. Such aquacultured silver perch are regularly stocked into numerous artificial impoundments where, without exception, they fail to establish self-sustaining populations.
Reasons for the catastrophic decline of silver perch are only partially understood. Dams, weirs and river regulation and the virtual removal of spring floods appear to have removed the conditions silver perch need to breed and recruit successfully on a large scale. Weirs are also believed to have blocked the migrations of spawning adults and juveniles, which are important to maintain populations over the lengths of rivers. Weirs also kill most drifting silver perch larvae that pass through them, if they are of an undershot design (which, unfortunately, most are). Recent studies that has proven more than 90% of silver perch passing through undershot weirs are killed. And without doubt, weirs trap drifting silver perch eggs (and larvae) as well, where they are either diverted down irrigation offtakes, resulting in eventual death, or sink into fine weir pool sediments and die.
It is not widely appreciated that silver perch eggs sink in still water; silver perch eggs are often inaccurately described as simply being pelagic, or "floating". The eggs may actually settle onto the substrate in the wild and should perhaps be considered benthic in many circumstances rather than pelagic. This may be a factor in their recent serious declines; silver perch may rely on their eggs settling onto clean, well oxygenated substrates of coarse sediments. In this era of flow regulation and flood curtailment by dams, which control the flood events that remove fine sediment, and chronic siltation from poor agricultural practices, the eggs may now frequently land in anoxic fine sediment and organic matter — including in weir pools — and fail to survive. It may be that the section of the central Murray River that supports the last clearly viable natural population of silver perch primarily does so because it supplies a sufficiently long stretch of weir-free river, under standard regulated flows, for eggs to successfully complete their drift and hatch larvae into relatively natural, suitable riverine habitats for survival.
Suspicions are also mounting that there is competition for food between introduced carp and silver perch at larval, juvenile and adult stages. Competition at the larval stage is considered the most serious. Indeed, suspicions are mounting that introduced carp are having very large impacts on a number of native Murray-Darling fish species due to competition at the larval stage, and that these impacts have so far been underestimated.
Exotic pathogens such as EHN virus and possibly similar viruses, introduced via importation of non-native fish, are now strongly suspected of playing pivotal role in the species' decline, and may explain the suspicious, very rapid collapse of some populations (e.g. upper Murrumbidgee).
In a positive development, since 2000, the installation of fishways in many Murray River weirs, so that native fish can pass through them and successfully migrate long distances again, and recent carefully managed environmental flow events, have seen silver perch numbers in the last remaining viable population increase strongly, and seen the population expand slightly in geographic range.
