River blackfish facts for kids
Quick facts for kids River blackfish |
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The river blackfish (Gadopsis marmoratus) is a cool freshwater fish. It lives only in the cooler waters of south-eastern Australia. You can find it from southern Queensland down to central Victoria. This includes big river systems like the Murray and Darling rivers. It also lives in some rivers that flow to the eastern and southern coasts.
River blackfish mostly live in "upland" (higher, faster-flowing) and "midland" river areas. But old records show they used to live in "lowland" (flatter, slower-flowing) areas too. In the past, river blackfish shared their homes with fish like Murray cod and golden perch. In higher areas, they lived with trout cod and Macquarie perch. Many people enjoy fishing for river blackfish in some places.
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What Does the River Blackfish Look Like?
River blackfish have a long, rounded body. They have a clear snout and a big mouth. Their eyes are small to medium-sized. Their tail fin, soft top fin (dorsal fin), and bottom fin (anal fin) are all rounded. The spiny part of their top fin is low and blends into the soft part.
Their color can change. They are usually yellow or green with dark green spots. Sometimes, they can be a very dark purple or black.
This fish is a "surprise attacker" (ambush predator). It hides and then quickly catches its food. It eats crayfish, shrimp, small fish, and water insects. River blackfish are most active at night.
Most river blackfish in the Murray-Darling rivers grow to about 30 centimeters (about 1 foot) long. They weigh less than 0.8 kilograms (about 1.7 pounds). But some from coastal rivers can grow much bigger, up to 60 centimeters (about 2 feet) long!
Some people think the southern river blackfish is a great sports fish. It is fast and strong. It can be caught using special fishing flies and lures. It can even be caught with lures on the water's surface. People also catch them with worms. There's even an unofficial world record for a 70-centimeter (2.3-foot) river blackfish caught in Victoria, Australia.
Where Do River Blackfish Live?
River blackfish are found only in south-eastern Australia. Like many native fish in the Murray-Darling area, they have crossed the Great Dividing Range. This happened when rivers naturally connected over time. So, you can find them in rivers that flow south to the coast in eastern Victoria.
These fish are often called southern river blackfish. This group can grow much larger, sometimes over 60 centimeters (2 feet) long. They can weigh about 5.5 kilograms (12 pounds). Fish this big are probably very old, maybe 30 years or more! They are rarely seen today. Scientists think southern river blackfish might be a separate species.
River blackfish are part of a pattern seen in native Murray-Darling fish. Many species have split into those that live in lowland areas and those that live in upland areas. The two-spined blackfish (Gadopsis bispinosus) is a good example. It is a special upland fish. It lives in the fast-flowing, rocky rivers of northeast Victoria, southeast New South Wales, and the Australian Capital Territory. Even though river blackfish are found in many upland places, there is some overlap in where they live.
Scientists also think that river blackfish in the northern and southern parts of the Murray-Darling river system might be two different species or subspecies. They have found genetic differences between them.
The Gadopsis blackfish and the Maccullochella cods (like the Murray cod) are quite similar. Some people believe blackfish might be a smaller version of Murray cod. They might live in similar places that are too small for Murray cod. But originally, they lived in many of the same areas. Some scientists even think blackfish and Murray cod share many common genes. For example, the FishBase website once listed blackfish as part of the temperate perch family, Percichthyidae.
A study of their mitochondrial DNA (a type of genetic material) showed a link between blackfish and the Percichthyidae family. But more study is needed to fully understand this relationship. Also, genetic studies suggest that G. marmoratus is actually a group of five different species. One species lives in the Murray-Darling Basin, and four others live in coastal freshwaters. One of these forms from Victoria has been named Gadopsis gracilis.
River Blackfish Life Cycle
Blackfish reproduce in a way that is similar to Murray cod. But they are more suited to upland river homes. Blackfish lay their eggs in spring. They lay a small number of large, sticky eggs (less than 1000). They attach these eggs to sunken wood (called "snags"). Two-spined blackfish lay their eggs on rocks under the water.
Just like Murray cod, the male blackfish guards the eggs until they hatch. After hatching, the baby blackfish are unique. They stay attached to their empty egg case by a small string. They stay like this until they have used up most of the food from their yolk sac. Then, they are ready to start finding their own food. This special way of hatching probably helps the babies from being washed away in strong river currents.
Protecting the River Blackfish
The number of blackfish has dropped a lot. This is due to several reasons:
- Too much fishing: People caught too many fish.
- River changes: Rivers have filled with dirt (siltation). Also, important sunken wood (snags) have been removed.
- New fish: Fish brought from other places, especially trout, eat blackfish and compete with them for food.
Recent bushfires in south-eastern Australia (from 2003 to 2006) filled many blackfish rivers with a lot of dirt. This dirt filled the small gaps between rocks where blackfish usually hide from trout. This means more trout might eat blackfish. Because of this, the future of blackfish species is now a concern.
Blackfish lay very few eggs. They grow slowly and live for a long time. They also do not travel far. This makes them very easy to overfish. It also makes them very likely to disappear from local areas.
River Blackfish are a protected species in South Australia.
