Bidyanus bidyanus

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. 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. (Mitchell's original scientific name for the species was Cernua Bidyana.) Silver perch are not a 'true' perch of the genus Perca, 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. The silver perch is the only major representative of the family Terapontidae 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. Common names for Bidyanus bidyanus include silver perch, black or silver bream and the traditional bidyan. 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. 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 omnivorous, 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. 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.