Bidyanus bidyanus facts for kids
Quick facts for kids Bidyanus bidyanus |
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The silver perch (Bidyanus bidyanus) is a medium-sized freshwater fish. It belongs to the Terapontidae family. This fish is special because it only lives in the Murray-Darling river system. This river system is located in south-eastern Australia.
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What's in a Name?
The scientific name for the silver perch, Bidyanus bidyanus, comes from an Aboriginal word. The word is "bidyan." Major Mitchell heard this name on the Barwon River in 1832. He first called the fish Cernua Bidyana.
Even though it's called a "perch," the silver perch is not a "true" perch. It's part of the Terapontidae family, also known as 'grunters'. Silver perch are the biggest fish in the Terapontidae family. They can grow to be over 60 centimeters long. They can also weigh close to 8 kilograms. Wild fish today are usually 30–40 centimeters long. They weigh about 1.0–1.5 kilograms.
The silver perch is the main grunter fish found in the southern Murray-Darling system. In northern tropical areas, there are many different grunter species. Another small grunter, the spangled perch (Leiopotherapon unicolor), is sometimes found in the northern Murray-Darling Basin.
Other common names for Bidyanus bidyanus include black bream, silver bream, and its traditional name, bidyan.
How to Spot a Silver Perch
The silver perch is a large grunter fish. It has a small head and small eyes. Its mouth is small and at the end of a pointed snout, like a beak. The fish has a smooth, streamlined body. It is also flat from side to side.
It has a spiny fin on its back that is medium height. Its soft dorsal and anal fins are angled. Its tail is forked. Very large silver perch can have a deep body with a big hump behind their head.
Their color is dark grey to silvery grey-brown on their back. Their sides are silver-grey. The edges of their scales are darker, making a checkered pattern. Their belly is whitish. The fins on their back and tail are dark, but their bottom fins are white.
What Silver Perch Eat
Silver perch are omnivores, meaning they eat both plants and animals. They eat insect larvae, snails, worms, and algae. Scientists still discuss how much plant matter they eat.
Silver perch mainly hunt small water bugs. Sometimes, they eat small fish and plants. In fish tanks, silver perch are known to eat bloodworms easily.
Where Silver Perch Live
Silver perch are fish that swim in groups in the middle of the water. They like water that is flowing. They are now mostly found in the lower parts of the Murray-Darling system. However, they used to live in the higher, sloped parts of many Murray-Darling rivers too.
They were once common in the upper parts of the Murrumbidgee River. They were found as far upstream as Cooma. In the early 1980s, they would migrate long distances in summer. These migrations went into the higher parts of the Murrumbidgee. Sadly, these migrations and populations have now disappeared. Silver perch are almost gone from the Murrumbidgee River and most of their old homes.
Silver perch have been moved to the Lake Eyre Basin in central Australia. These moves were not approved. They could cause problems for other grunter species that naturally live there.
There is a group of silver perch living and reproducing in Cataract Dam. This dam is part of the Hawkesbury-Nepean system. This group was started by NSW Fisheries around 1915–1917. They moved young fish from drying waterholes in the lower Murrumbidgee River. The Cataract Dam population is special. It's the only group of silver perch in a man-made dam that regularly reproduces and keeps itself going. This is likely because fishing is not allowed there. Also, there are no foreign fish or diseases. The dam is clean, with lots of rocks and gravel. This allows their eggs to settle without getting covered in mud.
Fishing for Silver Perch
Fishermen used to catch silver perch with light baits like worms. They also used small lures in fast-flowing water during migrations. Silver perch were known for being very fast and strong fighters.
One old account says: "The [fishing] rod is … used amongst the bream [silver perch] which run up to six pounds, and fight every inch of their way from the time they are struck till they are safely landed. … It is as easy to land a fifteen pound cod as it is a five pound bream, as the latter is notoriously the hardest fighter in our rivers, only being even nearly approached by the catfish."
Silver Perch Life Cycle and Reproduction
Male silver perch can start having babies when they are three years old. Female silver perch can start when they are five years old. They lay their eggs in late spring and early summer.
It was once thought that water temperatures of about 24 degrees Celsius were needed for them to lay eggs. But like other Murray-Darling fish, they can lay eggs at lower temperatures too. Researchers have found silver perch eggs in water from 17.2 to 28.5 degrees Celsius. This was between early November and mid-February. Eggs were most often found when the water was above 20 degrees.
Silver perch lay a good number of eggs. They often lay between 200,000 and 300,000 eggs. They lay eggs at the water's surface at dusk or in the first few hours of night. The female releases her eggs. The male fertilizes them in a few seconds of splashing. The eggs are semi-floating. They will sink if there isn't enough current. They hatch in 24 to 36 hours.
An account from 1914 describes silver perch laying eggs in the Murrumbidgee River: "Between 50 and 70 silver perch were playing—some feeding at the surface and others swimming about apparently aimlessly—in a series of eddies under a precipitous bank of the Murrumbidgee River, at a spot where the water was 10 or 12 foot deep. A section of the shoal, mostly the largest fish, remained in a central position. Suddenly, as though preconcerted, all the fish swam rapidly into a centre, splashing the water, in all directions, and becoming for an instant invisible owing to the agitation of the surface. Next moment the water all around and below the fish had assumed a whitish, opaque tinge, as though a bucket of milk had been thrown in; clearly caused by the extrusion of the milt of the male fish, and its contact with the colourless ova thrown out by the female fish. The operation was repeated five or six times at intervals of about 20 to 30 minutes. Soon after sundown the fish disappeared."
Silver perch can live for a long time. This helps them survive in Australia's tough environment. It means most adults will get to be part of at least one big spawning event. These events often happen during very wet years and might only occur every 10 or 20 years. The oldest silver perch found was 27 years old. It was from Cataract Dam. Fish from the Murray River have been found to be 17 years old.
However, recent studies (2017) found that most silver perch in the Murray River were younger than seven years old. It's not clear why. It could be due to past environmental events. It might also be a natural part of their life. If it's natural, it means they have a short time to breed. This is because females only start breeding at five years old. This finding has raised new worries about their survival.
Saving the Silver Perch
In the 1970s, silver perch were very common throughout the huge Murray-Darling Basin. But since then, their numbers have dropped quickly and mysteriously. Silver perch are now almost extinct in the wild. They have disappeared from 87% of the areas where they used to live.
Only one large, healthy group of silver perch still lives in its natural home. This group is in the central parts of the Murray River. Because of this, the Australian government has listed wild silver perch as critically endangered.
Silver perch are raised a lot in fish farms. But these farm-raised fish are not very helpful for saving the wild species. Farm-raised silver perch are often put into man-made dams. But they never manage to create groups that can keep themselves going.
Scientists don't fully understand why silver perch numbers have dropped so much. Dams, weirs, and river changes seem to have removed the conditions silver perch need to breed well. Weirs are also thought to block adult and young fish from migrating. These migrations are important for keeping populations healthy along rivers. Weirs can also kill most young silver perch that pass through them. This is especially true for weirs that let water flow from the bottom. Studies show that over 90% of silver perch die when passing through these types of weirs. Weirs also trap silver perch eggs and young fish. They might be sent into irrigation channels and die. Or they might sink into muddy areas in weir pools and die.
Many people don't know that silver perch eggs sink in still water. The eggs might settle on clean, oxygen-rich surfaces like coarse sand or gravel. With dams controlling floods and poor farming causing mud to build up, the eggs might now often land in muddy areas without oxygen. This can happen in weir pools, causing them to die. The last healthy group of silver perch in the central Murray River might survive because that part of the river is free of weirs for a long distance. This allows eggs to drift and hatch in natural river habitats.
Scientists also suspect that introduced carp compete with silver perch for food. This competition happens when fish are very young, as well as when they are older. Competition when they are young is thought to be the most serious. It's believed that carp are having a big impact on many native Murray-Darling fish species. This is due to competition when the fish are young, and these impacts might have been underestimated.
Diseases from foreign fish are also strongly suspected of causing the species' decline. These diseases might explain why some groups of fish disappeared very quickly.
There's good news too. Since 2000, many weirs on the Murray River have had fishways installed. These allow native fish to pass through and migrate long distances again. Also, careful management of environmental water flows has helped. These efforts have led to a strong increase in silver perch numbers in the last remaining healthy group. The population has also spread slightly to new areas.
