Silver perch (Bidyanus bidyanus) are a medium-sized freshwater fish endemic to the Murray-Darling river system in south-eastern Australia. Their scientific name comes from an aboriginal name for the species — Bidyan — recorded by Major Mitchell on the Barwon River on his 1832 expedition. (Mitchell's original scientific name for the species was Cernua Bidyana.) Silver perch are not a "true" perch of the Perca genus, but are instead a member of Terapontidae or 'grunter' family. They are the largest member of the Terapontidae family, capable of growing in excess of 60 cm and close to 8 kg, but today wild river specimens are typically 30–40 cm and 1.0–1.5 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.
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.
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.
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 and moving waters more generally. They were renowned for being very fast and strong fighting fish for their size.
Spawning and biology
Silver perch spawn in late spring and 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 lower temperatures. Silver perch are moderately fecund, with egg 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 to 36 hours to hatch.
Silver perch continue the trend of native fish of southeast Australia being very 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. The 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 silver perch larvae are killed in the fall as they pass over/through weirs. A recent study that has proven more than 90% of silver perch passing through undershot weirs are killed. Without doubt, weirs trap drifting silver perch eggs as well, almost certainly to their detriment.
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, or "floating". The eggs may actually settle onto the substrate in the wild and should perhaps be considered benthic in most 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 — such as in weir pools — and fail to survive.
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 are now strongly suspected of playing a role in the species' decline.
- Allen, Midgley & Allen. Field Guide to the Freshwater Fishes Of Australia. Museum of Western Australia & CSIRO Publishing. 2002. ISBN 0-7307-5486-3
- Boys CA, Baumgartner L, Robinson W, Giddings G and Lay C (2010). Protecting migrating fish at in-stream structures: downstream mortality at weirs and screening water diversions. 2010 Native Fish Forum — Abstracts. Murray-Darling Basin Authority, Canberra. Online at: http://www.mdba.gov.au/system/files/NFS-2010-fish-forum-abstracts_Final.pdf.
- 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.
- Wager, R. 1996. Bidyanus bidyanus. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. Downloaded on 26 June 2013.