Silver perch (Bidyanus bidyanus) is a medium sized freshwater fish endemic to the Murray-Darling river system in south-eastern Australia. The scientific name comes from an aboriginal name for the species recorded by Major Mitchell on his 1832 expedition. It is not a perch, being a grunter in the family Terapontidae. It is the largest of the Australian grunters, capable of growing to 40 cm and 8 kg, more usually around 30 cm and up to 1.5 kg.
The silver perch is the only major representative of the Terapontidae family in the southern Murray-Darling system, compared to northern tropical systems where terapontid species are common. Another small terapontid, the spangled perch (Leiopotherapon unicolor), does occur sporadically in the northern Murray-Darling Basin.
Silver perch are streamlined laterally compressed fish 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.
Silver perch are common at 1–2 kg in size, but have been recorded to as much as 8 kg in the past.
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.
Silver perch are schooling mid-water fish with a preference for flowing water. Though found in the lowland reaches of the Murray-Darling system, they actually had a significant presence in the upland reaches as well. Long summer migrations into the upland reaches of rivers like the Murrumbidgee were once an annual event. Silver perch will eat blood worms without any troubles if fed these types of foods.
Fishermen caught silver perch on unweighted baits such as worms and on small spinning-blade lures in rapids during these migrations, as well as flowing/moving waters more generally. They were renowned as being a very fast and strong fighting fish for their size.
Spawning and biology
Silver perch spawn in late spring/early summer. Originally temperatures of close to 24 degrees Celsius were considered necessary but as with all Murray-Darling fish species it has become apparent that the "required" spawning temperature is flexible and that they can and do spawn at somewhat lower temperatures. Silver perch are moderately fecund (fertile), with eggs counts commonly around 200,000 to 300,000. Spawning occurs at the surface at dusk or the first few hours of night. The female sheds the eggs and the male fertilizes them in a few seconds of vigorous thrashing. The eggs are semi-buoyant and will sink without significant current, and take 24–36 hours to hatch.
Silver perch continue the trend of native fish of southeast Australia being incredibly long-lived. Longevity is a survival strategy in the often challenging Australian environment to ensure that most adults participate in at least one exceptional spawning and recruitment event, which are often linked to unusually wet La Niña years and may occur only every one or two decades. Maximum recorded age is 26 years.
Silver perch have declined close to the point of extinction in the wild. Only one sizeable, viable population remains in the wild in the central reaches of the Murray River. Silver perch are bred extensively in aquaculture but these domesticated strains are of little use in ensuring the species survival in the wild.
Reasons for their catastrophic decline are not clear. Dams, weirs and river regulation and the virtual removal of spring floods does appear to have removed the conditions silver perch need to breed and recruit successfully on a large scale. Weirs are also believed to impact on migrations of spawning adults and juveniles, and it is suspected many drifting eggs and larvae are killed in the fall as they pass over weirs. A recent study that has proven an extremely high mortality rate for golden perch and Murray cod larvae passing through undershot weirs means Silver perch larvae passing through these structures almost certainly experience a high mortality rate as well.
It is not widely appreciated that silver perch eggs sink without significant water current — silver perch eggs are often inaccurately described as simply being pelagic ("floating"). This means that silver perch eggs may often settle onto the substrate in the wild and should perhaps be considered benthic in most circumstances rather than pelagic. This interesting fact 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 remove the flood events that remove fine sediment) and chronic siltation from poor agricultural practices and carp, silver perch eggs may now frequently land in anoxic fine sediment and organic matter and fail to survive.
Suspicions are also mounting that there is serious 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 Murray-Darling native fish species due to competition at the larval stage, and that these impacts have so far been vastly underestimated.
- Allen, Midgley & Allen (2002), Field Guide To The Freshwater Fishes Of Australia, Museum of Western Australia & CSIRO Publishing, ISBN 0-7307-5486-3
- Mallen-Cooper, M. and I. G. Stuart (2003) Age, growth and non-flood recruitment of two potamodromous fishes in a large semi-arid/temperate river system. River Research and Applications 19, 7: 697 – 719.