Bidyanus bidyanus

Bidyanus bidyanus
Conservation status
Vulnerable  (IUCN 2.3)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Terapontidae
Genus:	Bidyanus
Species:	B. bidyanus
Binomial name
Bidyanus bidyanus (Mitchell, 1838)

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.^[2][3][4]

Taxonomy

The silver perch's scientific name comes from an aboriginal name for the species — Bidyan — recorded by Major Mitchell on the Barwon River on his 1832 expedition.^[5] (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, 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.^{[2][3][4][5][6]}

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.^[2]

Description

The silver perch is a large grunter with a small head, small eyes, a small mouth at the end of a pointed 'beak-like' snout.^[2][3][4][7] 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.^[2][3]

Diet

Silver perch are omnivorous, feeding on insect larvae, molluscs, annelid worms and algae.^[2][3][4][7] 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.^[2][3] In aquaria, silver perch are reported to take blood worms readily.

Distribution

Silver perch are schooling mid-water fish with a preference for flowing water.^[2][3][4] Though found in the lowland reaches of the Murray-Darling system, they formerly had a very significant presence in the slope and upland reaches as well.^[2] In particular, they had a very strong presence in upland reaches of the Murrumbidgee River and originally were found as far upstream as Cooma. As recently as the early 1980s, long summer migrations into the upland reaches of rivers like the Murrumbidgee were once an annual event.^[2][4] Unfortunately silver perch are functionally extinct in the Murrumbidgee River now, as in most parts of their former range.^[2]

Silver perch have also been introduced into the Lake Eyre Basin in arid central Australia.

Fishing

Fishermen caught silver perch on unweighted baits such as worms and on small spinning-blade lures in rapids during migrations into upland rivers, as well as flowing and moving waters more generally. They were renowned for being very fast and strong fighting fish.^[2]

Spawning and biology

Silver perch spawn in late spring and early summer. Originally water temperatures of close to 24 degrees Celsius were considered necessary for spawning to occur^[8] 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.^[2] Researchers in the Barmah Forest region of the Murray River have collected drifting fertilised silver perch eggs in water temperatures as low as 17.2 degrees and as high as 28.5 degrees C, between early November and mid-February. Eggs were consistently collected in water temperatures above 20 degrees.^[9]

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.^{[2][3][4][6][10]}

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.^[2][3][4] Silver perch are relatively long-lived; the oldest individual aged so far was sampled from Cataract Dam, NSW (where a vitally important, self-sustaining, translocated population survives) and calculated to be 27 years old through otolith examination.^[6][7]

Conservation

As recently as the 1970s, silver perch abounded in entire Murray-Darling Basin, vast though it is. Since then, however, they have undergone a mysterious, rapid and catatrophic 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.^[2] 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.^[2] 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 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.^[11][12] 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.^[2][3][4]

It is not widely appreciated that silver perch eggs sink in still water;^[10] 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.^[2][3]

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.^[2][3][4]

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).^[2]

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.^[2]

References